Two Rates for the Same Elevation
Sixty metres by thirty, eight to the eave, six degrees on the roof. That is about 1,810 square metres of roof off the slope and, before you take a single door out, 1,440 square metres of wall. Two envelope quotes come back against it, both expressed as a rate per square metre, and the composite one is the higher number. On a spreadsheet with 3,250 square metres in the quantity column, the difference between those two rates is the whole conversation, and it is very nearly the wrong conversation.
The rates are not measuring the same scope. One buys a single factory-made item that arrives on a wagon and is fixed once. The other buys a liner sheet, a spacer system, a quilt, an outer sheet, two separate families of fastener, a bag of filler blocks and the site time to install all of it in sequence on the same access, in whatever weather the programme lands in. Both quotes leave the flashings and the rainwater goods somewhere they can be argued about later, and both assume a U-value neither has shown its working for.
What actually decides it is a short list, and price is on it once: how much depth the build-up can have, how many weeks the envelope has, what the insurer will accept in the core, how much frame tolerance the system has to swallow, and what the spacer arrangement does to the thermal performance already promised in the compliance submission. The last of those gets discovered late, because it is the only item on the list that is invisible on site.
One Delivery, or Five
Sequence a built-up roof and you get five distinct operations on the same steel. The liner goes on first, screwed to the purlin and lapped and sealed to itself, and from that moment it is the air barrier and the vapour control layer whether anybody has called it that or not. Then the spacer system: a bracket or a halter fixed through the liner into the purlin flange, carrying a bar or rail that stands the outer sheet off. Then the quilt, laid out between and over the spacers, in the thickness the U-value calculation assumed. Then the outer sheet, fixed to the bar. Then the fillers, closures and flashings that make the whole thing weathertight.
A composite panel collapses those five into one. The liner face, the core and the weather face are laminated in a factory under a declared process, the panel arrives at its finished thickness with its joint profile already rolled into both edges, and the fixing operation is a single long fastener driven through the whole depth into the purlin. Fewer operations means fewer interfaces, and fewer interfaces on an envelope is worth real money — but it also means the panel is a single indivisible item, so damaging the weather face means replacing the insulation with it, where damaging an outer sheet on a built-up means replacing a sheet. The same trade shows up in the programme: built-up work is five passes over the same MEWP position, each exposed to weather, and a quilt that gets wet before the outer sheet closes over it comes out again — while composite is one pass that needs a lift, a spreader or a vacuum lifter for the longer panels, and a wind limit that stops the job dead more abruptly than a sheeting gang stops.
What a site-assembled twin skin is made of, purlin upward
- Profiled outer sheet — the only weather layer, counted across the elevation at net cover after the side lap rather than at its out-to-out width Corrugated Metal Panel Side Lap Overlap Calculator
- Spacer bar and bracket — steel crossing the whole insulation depth, and the single item that decides how much of the quilt the finished assembly actually keeps Thermal Bridging Effective R-Value Calculator
- Mineral wool quilt — laid loose to the thickness the U-value calculation assumed, and compressed wherever a bracket or a bar crosses it Insulation Batt Calculator
- Liner sheet — fixed first, walked on next, and sealed at every lap because it is the air barrier and the vapour control layer as well as a sheet Corrugated Metal Decking Span Deflection Checker
- Z purlin — the line every bracket has to hit and every sheet lap has to land on, spaced off a span table that belongs to one of the two skins Steel Z/C Purlin Spacing Calculator
Which Skin Owns the Span
A built-up assembly has two span tables in it and neither describes the finished thing. The liner is the thinner gauge, it goes on first, and at construction stage it spans purlin to purlin with a crew and a pallet of quilt on it. The outer sheet is heavier and carries the snow and wind in service, but it spans bracket to bracket, and its supports are only where the spacer system put them. Read the wrong table and you get a roof that either sags on day one or fails in the first winter, and the two errors look nothing like each other.
A composite panel behaves as a structural sandwich — faces in bending, core in shear between them — and spans further than either face alone. That is why a composite specification will often clear a purlin spacing a built-up system of the same thickness will not, and why the same building can come out with one fewer purlin row per slope. That saving lands in the steel package, not the cladding package, which is exactly why it goes missing when the frame is bought by one person in week two and the envelope by somebody else in week nine.
The row arithmetic itself, and the trap where the count comes out one higher than the division suggested, is worked through in the secondary steel guide next door and is not repeated here. What belongs on this page is knowing which number you are typing in. The liner's maximum gives the layout the liner needs; the outer sheet's gives a layout the liner may not survive being walked on; a composite panel's gives the row count that panel earns, which under BS EN 14509 is a tested and declared span carrying a temperature caveat, because a dark weather face on a hot day loses capacity the same panel has in the test house.
Run it once with the liner's rated maximum and once with the composite panel's declared span, and the gap between the two row counts is the tonnage the system choice moves in or out of the steel order.
The total roof run (eave-to-ridge or slope length) the purlins span across.
The maximum allowable spacing between purlin rows, from the roof panel manufacturer's span table.
Purlin rows needed
15 rows
- Bays from eave to ridge
- 14
- Purlin row centres, eave to ridge
- 4.71 ft
They open the calculator with your figures already in it
Steel Z/C Purlin Spacing Calculator: 15 rows — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- The top and bottom rows in this count are not ordinary purlins. At the eaves the member is normally an eave strut or eave beam — a different section, supplied with the frame, carrying the gutter and the head of the side cladding — and at the ridge most systems need a PAIR of purlins, one either side of the apex, to land the sheet end on both slopes and fix the ridge cap. A single row at the top and a purlin priced at the eaves both leave the order wrong.
- Where the sheeting is made up of more than one length up the slope, the end lap has to sit on a purlin and be fixed through it, so the lap line dictates a row position that an even division of the run will not produce. On a long rafter that means either an extra purlin or the whole layout reset from the lap downwards.
- Rows are counted and nothing that holds them straight is. Z and C purlins are slender and torsionally weak, and they need sag rods, bridging or anti-sag ties between rows — commonly one line at mid-span and two over longer spans — to stop them rolling during sheeting and under uplift. The cleats, sleeves and bolts at every rafter sit outside this figure as well.
Two Sheet Counts, or One Module
The counting job is the same shape for both systems and runs a different number of times. A built-up elevation needs it twice: the liner has its own profile and cover width, the outer sheet another, and the two grids will not coincide. Thirty metres of wall against a liner covering a metre net takes thirty sheets; the same thirty metres against an outer sheet covering nine hundred millimetres net takes thirty-four. Neither is the number of brackets, and none of the three lines up with the other two except by coincidence.
That mismatch is only a problem at the ends, and it is always a problem at the ends. The closing liner sheet and the closing outer sheet are different widths, finish at different distances from the corner, and are both ripped to different sizes. Set the two modules out deliberately, from one datum, deciding in advance where each ripped closer lands, and it becomes a detail on a drawing rather than a decision made with a nibbler at height.
Where the count stops being a count of sheets is worth knowing before it is ordered. On the wall the runs are vertical and a single sheet usually reaches base to eave. On the roof, any slope longer than the available sheet length picks up an end lap, a sealed detail landing on a purlin line, and the count becomes sheets per lapped run instead. Composite panels are made to length and generally avoid the lap, at the cost of a delivery that has to be lifted in one piece.
Composite removes one of the two counts and changes the character of the other. Its cover module is fixed, the joint is an engineered interlock rather than an overlap, so there is no lap width to trade against and nothing to shuffle when the building comes out a hundred millimetres over. The count is the length divided by the module, rounded up, and the remainder is a special. Ask for that special early: a made-to-order narrow panel with a proper joint on both edges is a lead-time item, while a site-ripped composite is a cut core that has to be capped and flashed, and cutting one in place fills the building with dust.
Run it once per skin on a built-up wall, using each profile's own out-to-out width and side lap, and note the two closer widths that fall out — those two offcuts are where the setting-out either was thought about or was not.
The total width of roof to be covered, measured across the panel runs.
The panel's full out-to-out width, before accounting for overlap.
How much of each panel's width overlaps the adjacent panel along its side lap.
Panels needed
20 panels
- Net coverage width per panel
- 3.33 ft
They open the calculator with your figures already in it
Corrugated Metal Panel Side Lap Overlap Calculator: 20 panels — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- How many panels is settled here; which panel is not. Gauge and profile depth are chosen against the purlin spacing and the wind and snow the sheet has to carry between supports, so the same count in a thinner or shallower profile deflects between purlins, holds water at the laps and oil-cans in the flats.
The Bar Sets a Ceiling the Quilt Cannot Climb Past
The spacer is structural steel standing between the liner and the outer sheet, and it crosses the entire insulation depth by definition, because that is what it is for. Steel conducts heat on the order of a thousand times better than the mineral wool it passes through. Every bracket, bar and fastener crossing that plane is a route around the insulation, and unlike a stud in a timber wall it is not a modest fraction of a poor conductor — it is a very small area of a very good one.
Put that into the parallel-path arithmetic and something unpleasant falls out, which is that the bridge sets a hard ceiling. If a fraction f of the area bridges the insulation at a local resistance of Rb, then no matter how much insulation the other part of the wall has, the assembly can never do better than Rb divided by f. Take a system where ten per cent of the area is bridged at a local R-2.5. An R-25 quilt gives about R-13 effective, so half the insulation has vanished. Double the quilt to R-50 and you get about R-17 — a thirty per cent gain for a hundred per cent more material — and if you could make the quilt infinitely thick the assembly would still stop at R-25. The bridge, not the insulation, is what you are buying past that point.
The site version of the same problem is compression. A two-hundred-millimetre quilt is only that where nothing is squashing it, and at every bracket it is pinched to whatever gap the system leaves — along a line rather than at a point where a continuous bar sits on it, and everywhere if the sheeter has pulled the outer sheet down hard to close a rattle. None of that appears in a specification that says two hundred millimetres of mineral wool, and all of it appears in the finished building.
So treat the parallel-path number as a shape rather than an answer. It is an isothermal-planes approximation; the real behaviour around a bracket is two-dimensional, assessed by numerical modelling to BS EN ISO 10211, with the conventions in BRE BR 443 and the assembly route in BS EN ISO 6946 — which also carries the correction for mechanical fasteners crossing the insulation. What you want from the spacer supplier is a system U-value for that bar, at that bracket spacing, on that purlin spacing, with that quilt thickness. ANSI/ASHRAE/IES Standard 90.1 reaches the same place from another direction, handling metal building roofs and walls in its Appendix A as whole-assembly U-factors, precisely because adding up the layers does not describe them.
One caution about the calculator below: its framing fraction is floored at ten per cent because it was written for stud and furring walls, and a decent bar-and-bracket system bridges far less. Read it not for a U-value but for the shape of the curve — how flat the effective resistance goes once the bridge dominates — because that is what tells you whether the next fifty millimetres of quilt is worth buying.
Hold the framing fraction and the framing resistance still and push the cavity R-value up, and watch the effective value flatten out — the number it flattens towards is the ceiling the spacer arrangement has set for the whole assembly.
The R-value of the insulation filling the stud cavity.
The R-value of solid wood at the stud's thickness.
The percentage of the wall's area taken up by studs, headers, plates, and other framing rather than insulated cavity.
Effective assembly R-value
8.73 R (effective)
This parallel-path method is a widely used simplification — it doesn't account for more complex heat flow effects like point thermal bridges at intersections, which a full 2D/3D heat transfer model would capture more precisely.
- Nominal cavity-only R-value
- 13 R
They open the calculator with your figures already in it
Thermal Bridging Effective R-Value Calculator: 8.73 R (effective) — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 8.73 R (effective) — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- This is the framed insulation layer only. Drywall, sheathing, cladding, air gaps, interior and exterior air films, and any exterior continuous insulation are not in the arithmetic, so the figure is not the wall's total R-value and should not be read against a target stated for a whole assembly.
- The parallel-path split describes timber. Steel studs, Z-furring, spacer bars and brackets, and masonry ties spread heat sideways into the surrounding material, so the bridged area behaves larger than its geometric fraction and this method will not give you the right penalty. For metal, use a steel-specific method or the system supplier's tested U-value.
- One framing fraction is applied evenly over the whole wall. Corners, headers over openings, rim joists, the wall-to-floor and wall-to-roof junctions, and anything penetrating the envelope lose more heat than a plain stud bay, and none of them are resolved here.
- The cavity is assumed filled to its rated R-value everywhere. Gaps at the edges, batts compressed behind wiring and pipework, settled blown insulation, and air moving through the framed layer all put real performance below this number, and none of them are inputs.
- Not a compliance calculation. An energy code submission or a declared U-value needs a whole-assembly figure produced by the method your code names, with junction losses handled separately. Use this to see how much of the batt's label the framing takes back, not as the document you submit.
Moving the Bridge Out of the Insulation
Spacer systems come in families and they differ almost entirely in how much steel crosses the insulation. The oldest arrangement is a continuous zed section standing on the liner at every purlin line — a linear bridge running the full length of the roof. A bar-and-bracket system replaces it with discrete brackets at intervals along a rail, turning a line into a series of points. A thermally broken version puts a low-conductivity pad at each bracket's bearing face, so the steel path has an insulating washer in series with it. Each step up that ladder costs more per square metre and each one raises the ceiling from the paragraph above.
The gain is worth quantifying before anyone argues about the rate. Take the same R-25 insulation with a bridge path at R-3 and vary only the bridged fraction. At ten per cent, the assembly comes out around R-14. At three per cent, it comes out around R-20 — the same insulation, the same thickness, the same fixing programme, six points of effective resistance bought purely by rearranging where the steel crosses. The ceilings behind those two figures are R-30 and R-100 respectively, which is another way of saying that on the ten per cent system you have already spent most of what extra thickness can buy you, and on the three per cent system you have not.
Note the unit trap between the two calculators on this page, because it will bite someone. The general framing tool takes its framing fraction as a decimal — a quarter of the wall is 0.25. The continuous-insulation tool takes the same quantity as a percentage — three per cent is 3. Type the decimal into the second one and you are either stopped by its minimum or modelling a wall with almost no bridge in it at all, and the answer will look wonderful.
| Spacer arrangement | What crosses the quilt | What to ask the supplier for |
|---|---|---|
| Continuous zed spacer on the liner | A steel section running the full length of every purlin line | Whether the system still reaches the target U-value at any buildable quilt thickness |
| Bar and bracket | Discrete brackets at intervals, carrying a rail clear of the quilt | The bracket spacing the quoted U-value assumed, not the bracket spacing the fixer prefers |
| Bar and bracket with an isolating pad | The same brackets with a low-conductivity break at the bearing face | A modelled system value with the pad in it, and confirmation the pad carries the design load |
| Two crossed layers of rail | Steel at both levels, but not on the same line, so the paths are in series | The build-up depth it needs, which is usually more than the quilt alone |
| Composite panel (no spacer at all) | Only the fixings and the joint between panels | A declared panel value under BS EN 14509 plus a separate allowance for the joints and fixings |
This one takes the bridged fraction as a percentage, which lets you get down to the three to five per cent a decent bracket system actually occupies — sweep that field alone and the answer is what the upgrade is worth per square metre.
The percentage of the wall's surface area occupied by metal furring or studs that bridge the continuous insulation.
The rated R-value of the continuous insulation layer away from any framing.
The much lower R-value THROUGH THE FURRING OR STUD ITSELF, at the same cross-section as the continuous layer entered above — not the whole assembly. Both paths have to be measured between the same two faces or the parallel-path average is comparing different walls.
Effective R-value through the insulated layer
8.11 R-value
This simplified parallel-path (isothermal planes) method is a common estimating approximation — for code compliance or precise energy modeling, use a 2D/3D thermal bridging calculation (e.g. per ISO 10211 or ASHRAE zone method) specific to your exact furring profile and spacing.
They open the calculator with your figures already in it
Continuous Insulation Thermal Bridging Calculator (Z-Furring): 8.11 R-value — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 8.11 R-value — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- The parallel-path split flatters metal. Treating the girt and the insulation as two independent one-dimensional paths ignores heat funnelling sideways out of the insulation into the steel, which is why the zone and modified-zone methods exist and why they return a lower effective R for the same wall. Read this figure as the optimistic end of the range, not the middle, whenever it lands close to a required value.
- Only the layer the furring passes through is modeled. Sheathing, any cavity insulation, gypsum, the cladding and the inside and outside air films all still have to be added in series afterwards, so this number is not yet the whole-wall R or U that a code table or an energy model is asking for.
- Repeating girts are not the only bridge on the facade. Shelf angles, slab edges, parapets, balcony penetrations and the metal around every window opening carry linear and point heat flow that no area fraction can represent, and on a real building they routinely cost as much again as the furring counted here.
What EN 14509 Declares, and What It Leaves Out
A composite panel arrives with a declared thermal transmittance, and it is a genuinely good number because it describes a laminate made under factory control with nothing crossing it. That declaration sits under BS EN 14509, the product standard for self-supporting double skin metal faced insulating panels, which covers far more than thermal performance — mechanical properties, the wrinkling stress of the faces, creep, and the tests behind the span tables. Behind a declared core conductivity is a guarded hot plate or heat flow meter measurement; behind an assembly value, a hot box test to a standard such as ASTM C1363.
What the declared value does not include is the building. It is a value for the middle of a panel. Every fastener driven through the full depth into the purlin is a steel path from the cold face to the warm one, and BS EN ISO 6946 has a correction for exactly that which somebody has to actually apply. Every longitudinal joint carries some metal-to-metal proximity, and the joint repeats every module across the whole elevation. Neither is a defect; both are simply outside the number printed on the datasheet.
The ends are where the larger loss lives, and they are common to both systems. At every eave, verge, corner, cill, head and base, the insulation stops and a folded steel flashing takes over, and that flashing is a continuous linear bridge running the perimeter of every plane and every opening. On a unit with roller shutter doors, personnel doors, glazing and a dock, that length is substantial. Those junctions belong in the compliance calculation as psi values, and where no calculated value exists for the detail as drawn, the honest route is a modelled junction to BS EN ISO 10211 rather than an optimistic default — which is also where surface condensation risk shows up, assessed under BS EN ISO 13788 and BS 5250.
Suction Picks the Fastener, and It Is Not the Same Fastener
In uplift, a built-up assembly has a load path with three joints in it: the outer sheet is held by a fastener into the spacer bar, the bar by its bracket, the bracket by a fastener through the liner into the purlin flange. The governing one is frequently that last fixing rather than the sheet fixing everyone counts, and it is also the one nobody can inspect once the quilt is in — a good argument for the fixing schedule being a drawing rather than a conversation on the roof. A composite panel has a shorter path and a different failure mode: one long fastener through the whole depth, delivering suction through a washer on the outer face, so what fails first is usually that face dimpling and pulling over the washer, or the core crushing under it, rather than the screw. The fastener also has an unsupported length inside the core, which is one reason panel fixing capacities are established by test on the assembly — by methods such as ASTM E1592 — rather than derived from a screw table.
Zoning turns that into a schedule. Wind separating at a corner, an eave or a verge produces suctions well above the field value, and both code families divide the surfaces into zones with defined widths: the components and cladding provisions of ASCE/SEI 7 use a zone dimension derived from building size and mean roof height, and BS EN 1991-1-4 uses lettered zones on the walls and the roof. The calculator below follows the ASCE route, returning the roof zone width and the pressure for the coefficient you read off your own adopted figure. The wall zones are the same idea turned through ninety degrees, and on a portal-framed unit the corner strips of the elevations get the tightened side-rail fixing pattern for the same reason the eave does.
The zone width it returns is a dimension you can draw in from each edge, and on a built-up roof it is the line inside which the bracket spacing closes up as well as the sheet fixings — a distinction that has to be on the drawing because the brackets are buried before anyone checks.
The mapped basic wind speed for the site, at the risk category of the building.
The terrain roughness upwind of the building, over the distances the code specifies.
Average of eave height and ridge height, measured from grade.
The shorter of the building's two plan dimensions.
The pitch of the roof plane being checked, in degrees.
Read from the components-and-cladding figure in your adopted code, for this zone and effective area.
Design uplift pressure in the selected zone
30.5 psf
The external pressure coefficient is the value you entered, not one this page supplies, so the answer is only as good as the figure you read off. This is a low-slope roof, so the low-slope coefficient figure applies and the corner zone wraps the full perimeter corner. The topographic factor is taken as 1.0, which is wrong on a hill, a ridge or an escarpment.
- Velocity pressure at mean roof height
- 25.82 psf
- Velocity pressure exposure coefficient
- 0.9 (Kz)
- Internal pressure component included
- 4.65 psf
- Corner and edge zone width, measured in from each roof edge
- 3.9 ft
- Corner zone plan area at each corner
- 15.21 ft²
They open the calculator with your figures already in it
Roof Uplift Zone Pressure and Zone Width Calculator: 30.46 psf — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 30.5 psf — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- Enclosed building assumed. A partially enclosed building carries a much larger internal pressure coefficient and a building with a large dominant opening larger still.
- Kzt = 1.0 assumed. Speed-up over a hill or an escarpment can raise the pressure by half again, and that is a separate calculation.
- Gives pressure, not fastener spacing. Turning pressure into a clip or fastener layout needs the tested assembly's own rated resistance and its safety factor.
The Liner Is Not a Floor, and the Outer Sheet Is Not Flat
The most dangerous half hour on a built-up roof is the one after the liner is down and before anything else is. It looks like a deck. It is a thin steel sheet spanning purlin to purlin, and it is not a working platform. Falls through fragile roof assemblies remain one of the standing killers in this work; HSE guidance HSG33, Health and safety in roof work, covers it, and non-fragility of a large-element assembly is demonstrated by test under ACR[M]001 for the assembly as specified — not inferred from the fact that the sheet held somebody up last Tuesday.
The deflection question is real in service too, and on a low-pitch roof it is not about strength. At six degrees, or the four or three these buildings are often built to, the fall available at the side lap is small and any sag between supports eats into it. Enough sag and water sits at the lap instead of running off it, which is where sealed side laps start earning their keep and where they start failing. The stiffness in that check has to be the real one: an effective moment of inertia from the manufacturer's table for that exact profile and gauge, because cold-formed sheet has an effective width that varies with stress and cannot be worked out from the drawn shape.
None of that keeps people off the roof, because somebody is going on it — to fix the outer sheet, to set a smoke vent, to service a condenser in year four. It is an argument for the specification naming what the assembly was tested as, and for the tested fixing pattern being the pattern installed. Composite never presents a half-built deck at all, but it gets walked on too, and how it behaves underfoot at a joint or a cut end is a different question from the span table it was sold on.
Two versions of that check matter here, and they use different numbers. The construction-stage one is the liner under the loads it sees while the roof is being built. The in-service one is the outer sheet spanning bracket to bracket under wind and snow, against the deflection limit the adopted building code sets. On a composite panel it becomes a single check on the sandwich, against the declared values from the panel maker's own table, and the temperature-dependent part of that table is not optional in a dark colour.
Use it twice with two different stiffnesses — the liner's, against its purlin span at construction stage, and the outer sheet's, against the spacer bar spacing under design load — because those are two different sheets doing two different jobs.
The uniformly distributed load per meter of deck width.
The deck's clear span between supports.
The deck's effective moment of inertia per meter of width, from the manufacturer's or SDI's span table.
The applicable code deflection limit, as a fraction of the span.
Calculated deflection
0.0332 in
The deflection this deck works out to is below the deflection limit for the span entered shown with it — you entered it from the limit ratio you chose. The effective moment of inertia has to come from the deck manufacturer's or the SDI's own span table for this exact profile and gauge, not from a computed section property. Being under one limit is not a design. Nothing else is checked here — not the other limit states, not the connections, not the member the load arrives from.
- Allowable limit
- 0.5 in
They open the calculator with your figures already in it
Corrugated Metal Decking Span Deflection Checker: 0.0332 in — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 0.0332 in — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- The check is deflection alone. Nothing here tests bending capacity, shear, or web crippling where the sheet bears on a narrow support flange, and published deck span tables are frequently governed by crippling or flexure rather than by sag, so a profile can clear this ratio and still fail the check that actually sets its allowable span.
- The 5wL⁴/384EI expression is the single simply supported span, pinned at both ends. A deck sheet lapped continuously over two or three supports deflects only about 40 to 55 per cent as much under the same span and load, so deflection for a continuous layout is overstated here, while cantilevered edge overhangs, unequal adjacent spans and sheets that stop mid-bay are not represented at all.
- The limit you select is applied to whatever load you typed, and the page does not link the two. Codes commonly pair the tighter ratio with live load acting alone and the looser one with the full service load, so entering dead plus live and choosing L/360 tests a combination no code asks for. Only ratio limits are offered, so an absolute cap on deflection, of the kind imposed on composite deck under wet concrete, cannot be checked here at all.
- The effective moment of inertia is treated as one fixed number. For cold-formed decking it is stress-dependent, because the wide compression flange buckles locally and only part of its width stays effective, and composite deck tables publish separate values for the bare sheet under wet concrete and for the finished composite slab. E is likewise fixed at 200,000 MPa (29,000 ksi), so steel at elevated temperature and the long-term creep and shrinkage movement of a concrete topping are outside this.
- The result is the deck's sag between its own two supports, not the movement a floor or ceiling actually sees. The joists or beams carrying the deck deflect as well and the two add at any point below, and where that combined sag collects rainwater or wet concrete the added weight deepens it further, a feedback loop this single-pass calculation does not iterate.
The Insurer Reads a Different Specification
The core is where this decision most often gets taken out of the designer's hands. A built-up system is insulated with mineral wool quilt, which is not the fire risk a combustible foam core is. A composite panel can be cored with mineral wool, rigid urethane or expanded polystyrene, and those three are not interchangeable in the eyes of a property insurer whatever the thermal numbers say. Fire performance of built-up cladding and sandwich panel systems is assessed under standards including the LPCB Loss Prevention Standard LPS 1181 and, in the FM Approvals scheme, Approval Standard 4880. On a food unit, a cold store or anything with a valuable process in it, the insurer's requirement arrives late, is specific about approvals rather than lambda values, and wins.
The statutory side is a separate question again. In England, Approved Document B covers fire safety and Regulation 7 of the Building Regulations restricts combustible materials in the external walls of relevant buildings — a definition that turns on building type and height, so an industrial unit and a residential block ask different questions of the same panel. None of it is decided by the cladding contractor, and all of it has to be settled before the panel order, because a change of core afterwards is a change of thickness, weight, span table and quite possibly purlin layout.
One more difference belongs on the same page because it decides planning conditions rather than insurance. A mineral wool built-up roof is markedly better at rain noise and at sound reduction than a foam-cored composite of the same U-value, because the quilt is a soft absorbent mass and the core is a stiff one. On a unit with residential neighbours and a noise condition attached to the consent, that is not a nicety — it is the constraint that picks the system, and it picks it in the direction the thermal argument may not.
Getting the Two Quotes Onto One Sheet
Almost every comparison that goes wrong goes wrong the same way: the two rates are compared before the two scopes are aligned. A built-up rate that excludes the liner fixings, or a composite rate that excludes the specials at the corners, is not a lower price, it is a smaller scope. Align the scopes first, then argue about the rates, and be explicit that anything the cladding package sheds — a purlin row, a fixing schedule, a flashing run — has landed in someone else's package rather than disappeared.
The items below are the ones that most often differ between the two quotes without being called out. Work down them and the two numbers become comparable; skip them and the cheaper quote wins on the strength of what it left out.
- Get a system U-value from each supplier for the exact arrangement quoted, including the spacer type and bracket spacing on the built-up and the fixing and joint allowance on the composite.
- Confirm which span table each purlin row count came from, and whether the composite quote assumes a purlin layout the steel package has not been told about.
- Price the flashings, closures and filler blocks as a separate measured item in both quotes rather than as a percentage inside a rate.
- Fix the total build-up depth for the built-up option and check it against the eave detail, the door heads and the gutter, because depth is what a spacer upgrade costs you.
- Get the insurer's core requirement in writing before the panel order, and reprice if it moves from foam to mineral wool.
- Count the specials on the composite option — closers, ripped modules, opening surrounds — and put the lead time for them on the programme, not in the rate.
- Agree who owns the liner as the air barrier, how its laps are sealed, and who tests it, since airtightness testing to a standard such as the ATTMA technical standard for non-dwellings will find out either way.
- Record the assumed bracket spacing and quilt thickness on the drawing, because both are things a fixer will change for good site reasons unless told not to.
Six figures before either quote is comparable
Settle these and the choice stops being a fight about a rate per square metre. The first two are inherited, the middle two decide the thermal answer, and the last two decide whether the cheaper system is still cheaper once its scope is complete.
- Envelope area, split roof and wall, off the slope — Roof measured on the plane rather than the plan, walls gross with the openings listed separately, because openings are where the two systems diverge most in cost per square metre.
- Purlin and side rail spacing, and which table it came from — Liner span, outer sheet span and composite panel span are three different maxima. Only one of them is the one your layout was drawn against.
- Bridged area fraction of the spacer system — The single number that sets the ceiling on the assembly. Get it from the spacer manufacturer alongside a modelled system value, not from a rule of thumb.
- Clear-field insulation resistance and the resistance through the bar — Together with the fraction, these are what the parallel-path check needs. They will bracket the answer; they will not replace a two-dimensional model of the bracket.
- Total build-up depth against the eave, head and gutter details — A better spacer usually needs more depth. Check it against the door heads and the gutter sole before pricing the upgrade, not after.
- Perimeter and opening length for flashings and closures — The item most often buried in a rate, the one the two systems price differently, and the one carrying the linear thermal bridges the U-value calculation still has to account for.
Opens the calculators above on one screen with the dimensions from this article already filled in. Quantities only — this site publishes no price list, because local prices vary too much to publish honestly.