Bay Spacing Was Fixed Before You Opened the File
The frame order went out in week two. Twelve bays at 7.5 m (24 ft 7 in), eave height signed off, tapered rafter webs already cut and tacked at the fabricator. By the time the secondary framing lands on your screen, none of that is negotiable, and the fact that the roof run divides awkwardly by the panel's rated span is not going to buy you a thirteenth bay. Everything you are about to lay out has to fit inside a grid somebody else set.
That grid is not the only constraint arriving with the job. Secondary steel is asked to do four separate things at once: carry the sheeting or the deck back to the primary frames, brace the rafter flange that would otherwise buckle sideways under uplift, hand the roof diaphragm's shear into the braced bays, and give every single fastener on the roof something solid to bite. A purlin row that satisfies the panel span table and misses the flange brace point has only done one of the four.
Two counts run this layout, and both look trivial until you write them down. How many purlin rows fit up the roof run without exceeding the panel's maximum support spacing, and how many supports a run of steel deck needs so no sheet ever spans further than its own table allows. Both are a division and a rounding. Both give an answer that is one larger than the number people expect, and in both cases the number worth writing on the drawing is not the count at all — it is the spacing that falls out the other side.
You Are Counting Rows, Not Gaps
Divide a 20 m (65 ft 7 in) roof run by a 1.5 m (4 ft 11 in) maximum support spacing and you get 13.33. Round up and you have fourteen. Fourteen is the number of gaps between purlins, not the number of purlins: a run split into fourteen intervals needs fifteen rows, because there is a row at each end as well as one at every division. The eave strut is a row. The purlin either side of the ridge is a row. Order fourteen lines of Z section per slope and you are two rows short on a gable before the first sheet goes up.
The spacing you actually build then comes back out of that count. Fifteen rows over 20 m means fourteen equal intervals of 1.429 m (4 ft 8 in) — comfortably inside the 1.5 m the panel table allowed, and that gap between the permitted maximum and the built spacing is normal rather than wasteful. You do not get to choose it. It is whatever the run divides into once the rounding has happened, and it is the figure that goes on the dimension string.
The rounding has a cliff in it, which is why the arithmetic is worth doing before anybody quotes tonnage. At a 1.6 m (5 ft 3 in) maximum, a 22.4 m run divides exactly into fourteen intervals and needs fifteen rows at the full 1.6 m. Stretch the run by 200 mm to 22.6 m and the division goes to 14.125, the rounding takes it to fifteen intervals, and you now need sixteen rows at 1.507 m. Two hundred millimetres of building has bought an extra purlin line — on every slope, in every bay, plus its clips, its bolts, its sag rod connections and its share of the erection sequence.
The temptation at that point is to run fourteen intervals at the full 1.6 m and let the fifteenth take the 200 mm that is left. Resist it on a repetitive roof. Equal spacing is what lets the panel end laps, the sag rod lines, the fastener schedule and the framed openings all be drawn once and repeated, and a single odd interval at one end propagates into four other drawings that then have to carry an exception note. There are roofs where an unequal end bay is the right answer — a monitor, a valley, a change of panel — but it should be a decision with a reason attached, not the residue of a division nobody checked.
| Roof run | Maximum spacing allowed | Intervals | Purlin rows | Spacing actually built |
|---|---|---|---|---|
| 20.0 m (65 ft 7 in) | 1.50 m (4 ft 11 in) | 14 | 15 | 1.429 m (4 ft 8 in) |
| 21.0 m (68 ft 11 in) | 1.50 m (4 ft 11 in) | 14 | 15 | 1.500 m (4 ft 11 in) |
| 22.4 m (73 ft 6 in) | 1.60 m (5 ft 3 in) | 14 | 15 | 1.600 m (5 ft 3 in) |
| 22.6 m (74 ft 2 in) | 1.60 m (5 ft 3 in) | 15 | 16 | 1.507 m (4 ft 11 in) |
| 30.0 m (98 ft 5 in) | 1.25 m (4 ft 1 in) | 24 | 25 | 1.250 m (4 ft 1 in) |
Put the run and the panel's maximum support spacing in together and read the row count, then divide the run by one less than that count to get the spacing the dimension string will carry.
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.
The Maximum Belongs to the Panel, and It Has Conditions Attached
Nothing in the layout is defensible until the maximum spacing has a document behind it. That figure is a span table entry, published by the panel manufacturer for one profile, one gauge, one steel grade and one design load combination, and it moves substantially when any of those change. A 0.55 mm sheet is not a 0.7 mm sheet with a safety margin; a table read for a 0.6 kPa live load says nothing about the same panel under a drift surcharge behind a parapet. The load side comes from ASCE/SEI 7 as the local code adopts it, and the resistance side comes from the panel maker.
Read which span condition the table is quoting, because most tables publish three. A sheet simply supported across two purlins is a single span. The same sheet running continuous over three or more is a double or triple span condition, and it carries meaningfully more because the intermediate supports take negative moment. The catch is that the condition is a property of how the sheet is actually laid, not of the roof: wherever a sheet stops and end laps, the run reverts locally to something closer to a single span. A layout drawn against triple-span numbers and built in short sheets that lap on every third purlin has quietly changed the design.
Underneath the table sits cold-formed steel design — AISI S100, the North American Specification for the Design of Cold-Formed Steel Structural Members, or BS EN 1993-1-3 for supplementary rules on cold-formed members and sheeting, or AS/NZS 4600 for cold-formed steel structures, depending where the building goes. Those documents are also where the failure mode nobody expects lives: web crippling at the support, which is governed by bearing length and not by span at all. A panel that passes bending on a 1.6 m span can still fail over a narrow purlin flange, which is one reason the purlin's own top flange width belongs in the layout conversation. Where a proprietary system carries tested values rather than calculated ones, those come from testing to ASTM E1592, the Standard Test Method for Structural Performance of Sheet Metal Roof and Siding Systems by Uniform Static Air Pressure Difference, and they apply to the assembly tested and to nothing else.
Where Steel Deck Replaces Sheeting
The same layout problem shows up with a different table and a different vocabulary the moment the roof is steel deck rather than screwed-down sheeting — an insulated deck under a membrane, a canopy, an equipment platform, a mezzanine dropped into one end of the building. The deck spans onto joists, secondary beams or purlins, the supports repeat along a run, and the question is again how many of them the run needs so that no sheet ever spans further than its rated maximum.
Run the numbers the same way. A 24 m (78 ft 9 in) building length with a 3.5 m (11 ft 6 in) maximum deck span divides into 6.86, which rounds to seven spans and therefore eight support lines, and those eight lines sit at 3.429 m (11 ft 3 in). A 30 m (98 ft 5 in) run against a 4.0 m (13 ft 1 in) maximum gives 7.5, rounds to eight spans, needs nine lines, and lands at 3.75 m (12 ft 4 in). In both cases the built span is under the maximum by exactly the amount the rounding forced, and in both cases the count is one more than the division suggested.
Deck is delivered in fixed lengths and that constrains the layout from the other direction. A sheet cut for a three-span condition needs four supports at your chosen spacing to sit on, with the bearing length at each one that the ANSI/SDI RD Standard for Steel Roof Deck requires — a sheet that ends 15 mm onto a flange has not got a bearing, it has got a coincidence. Set the support spacing so that the standard sheet length works out to a whole number of spans with the required end bearing at both ends, and check that the sheets can be laid to that pattern in the sequence the erector intends to work in, not just on the plan.
The deck is also the diaphragm, and its shear capacity is read from a table indexed by the deck's profile and gauge and by the fastening pattern together — neither one on its own tells you what the roof can carry, and the pattern is the half that gets treated as a detail. Support fasteners at each bearing line and side-lap connections between adjacent sheets together produce the shear capacity, sized against AISI S310, the North American Standard for the Design of Profiled Steel Diaphragm Panels, or the Steel Deck Institute's Diaphragm Design Manual. Where the connection is a puddle weld through sheet steel, the governing welding code is AWS D1.3, Structural Welding Code — Sheet Steel, not the D1.1 the frame was welded to. A decker who button-punches side laps that were drawn as screws has changed the diaphragm without changing a drawing, and nobody finds out until the building is asked to resist something.
The deck's rated maximum span turns into a support count here, and the count turns straight back into the spacing you have to be able to reach with a standard sheet length and a proper end bearing.
The total length of the building run the deck spans across.
The maximum allowable span between support beams, from the deck manufacturer's load/span table.
Support beams needed
8 beams
They open the calculator with your figures already in it
Steel Deck Support Beam Spacing Calculator: 8 beams — 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 single maximum span figure is applied to every bay equally, but deck load/span tables publish separate allowable spans for single-span, double-span and triple-span conditions, and the continuous cases are usually the more generous. An end bay, a bay beside a large opening and any bay where the sheet stops all behave as single spans, so a triple-span figure typed here spaces the supports too far apart at exactly those places.
- This returns a count, not a setting-out. The implied uniform spacing is the run length divided by one less than the beam count — for the default 24 m run at a 3.5 m span that is 3.43 m, not 3.5 m — and the beams still have to land on the column grid, so the buildable layout is rarely the even division this count assumes.
- Nothing on this page tests the maximum span you enter. The field accepts any value up to 8 m (26 ft) and divides by it, whereas a real allowable span is usually set by a deflection limit rather than by strength, commonly quoted as span/180 or span/240 with the governing criterion fixed by the specification and the jurisdiction. For composite deck the wet weight of concrete plus construction loading before it cures often governs instead of the finished service load.
- Spacing driven by gravity span alone omits three things that add steel. The deck normally works as a horizontal diaphragm for wind and seismic load, which can call for perimeter members and a closer fastening pattern that a span-based count says nothing about; every stair, duct and rooflight opening needs its own trimming members; and local concentrations such as a plant unit or snow drifting against a parapet or a taller adjacent building want closer supports than one averaged span figure produces.
Strength and Deflection Are Two Different Questions
A span that passes strength can still be the wrong span. Deflection is a separate check with its own limit, and on secondary framing it decides more layouts than bending does — a roof that sags visibly between purlins, or a deck that dishes under a stack of insulation bales, has failed for the client whatever the numbers say. There are two of these checks and they get confused constantly. The construction stage one is the bare deck under wet concrete or under a loaded crew, capped by the ANSI/SDI standard the deck was supplied to. The in-service one is the finished assembly under design load, against the deflection limit table in the adopted building code, which in the International Building Code is Table 1604.3.
The property that check turns on is not something to derive from the drawn profile. Cold-formed sheet has an effective width that varies with stress level, and the ribs, embossments, gauge and corner radii are all specific to the product, so the effective moment of inertia comes from the manufacturer's published table or the SDI's for that exact profile. A deflection computed from a section modulus somebody worked out off the drawing is arithmetic wearing engineering's clothes, and it will be wrong in the unconservative direction as often as not.
Span carries a fourth power in that calculation, which is the whole reason the rounding in the sections above matters commercially. Stretch a span by ten per cent and deflection goes up by about forty-six per cent for the same load. That is why the difference between fifteen rows and sixteen is rarely worth arguing about once the deflection check is run, and why shaving a row out of the count to save tonnage tends to cost the argument at the first site walk. On a low-slope roof it also feeds back on itself: extra sag holds extra water, extra water is extra load, and ponding is checked rather than eyeballed.
Take the effective moment of inertia straight off the deck maker's span table and put your chosen span against it here, because this is the check that decides whether the row count you just settled survives the first loaded walk across it.
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 Perimeter Does Not Get the Field Spacing
Uplift is not uniform across a roof and a layout that treats it as uniform is wrong at the only places roofs actually come off. Wind separating over an eave, a rake or a corner produces suctions well above the field value, and the components and cladding provisions of ASCE/SEI 7 handle that by dividing the roof into zones with a defined width. That width, written as a in the standard, is the smaller of one tenth of the building's least horizontal dimension and four tenths of the mean roof height, but never less than four hundredths of the least horizontal dimension and never less than 3 ft. It is a real dimension you can draw on the framing plan, and it should be drawn there.
What you tighten inside those strips is a choice with cost attached. Most often it is the fastener pattern: the same purlin rows, the same panel, more screws per sheet at the eave and rake, to a schedule the panel manufacturer publishes for the zone pressure. Sometimes the pressure is high enough that the purlin rows themselves close up near the eave, which turns a repetitive layout into two spacing regimes on one plan and demands an unambiguous dimension string for each. Either way the boundary between the regimes has to be a hard line on the drawing, because an erector who cannot see where the tight zone stops will either stop it early or run it everywhere.
Two further cautions belong on the same plan. The eave purlin and the rake purlin carry the worst of it and also carry the least tributary width, so their loading is dominated by suction rather than by gravity and their connections are working in the opposite direction to everything else on the roof. And where the building is insured to a Factory Mutual standard, the securement requirements in FM Global Property Loss Prevention Data Sheet 1-29, Roof Deck Securement and Above-Deck Roof Components, are an additional constraint on top of the code, not an alternative to it.
This is where the zone width stops being an abstraction: it returns a dimension in from each edge, which is the line you draw on the framing plan to separate the tightened perimeter from the field spacing.
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.
Laps, Sag Rods and What Lives Between the Rows
A purlin row on a drawing is a line. On the roof it is a run of Z sections lapped over each frame line, and the lap is doing structural work. Nesting the two sections through the support region gives a doubled cross section exactly where the negative moment peaks, which is what lets a continuous purlin outperform the same section spanning simply from frame to frame. The lap length and the bolt group inside it are part of the manufacturer's design for that system, derived under AISI S100, and they are not a detail to be scaled off the drawing or matched to what the last job used.
Between the frame lines, purlins need holding upright. A Z section under sloping roof load is not being loaded through a principal axis, so it wants to roll and to sag downslope, and it wants to do both most strongly before the sheeting is fixed and able to restrain it. Sag rod or bridging lines answer that, run at the third points or the mid point of the bay depending on the system, and their positions come from the same manufacturer's design rather than from habit. They also have to be reachable and tensionable in the erection sequence, which is a different question from whether they are drawn in the right place.
The purlin rows are simultaneously the rafter's bracing, and this is the constraint that catches people who move a row for a reason unconnected to structure. Under uplift, the rafter's bottom flange goes into compression and its unbraced length is set by the flange braces that run from the purlin down to that flange — bracing that AISC 360 addresses in its member stability bracing appendix, with a required strength and a required stiffness that both have to be satisfied. A purlin row is therefore a brace point, and the spacing you chose off a panel span table is also the spacing the rafter was checked against.
Which means the innocent-looking change is the dangerous one. Sliding a purlin row 400 mm to clear a skylight kerb moves a brace point, and the rafter check that was run on equal spacing no longer describes the frame you are about to build. Frame the opening between rows instead, keep the row where the rafter design put it, and if that genuinely cannot be done, send it back to whoever did the frame design rather than absorbing it in detailing.
- Fix the eave strut and the ridge purlin first — those two rows are set by the geometry, not by the division.
- Divide the remaining run so no interval exceeds the panel's rated maximum for its span condition, and keep the intervals equal.
- Check each row against the rafter's brace point spacing before treating the layout as settled.
- Set the sag rod or bridging lines to the system manufacturer's design, and confirm they can be reached and tensioned in the erection order.
- Draw the lap zones at every frame line, with the bolt group, as a repeated detail rather than a note.
- Mark the uplift zone boundaries and the tightened fastener schedule inside them, with the line dimensioned from the building edge.
What sits between the roof panel and the rafter
- Roof sheeting — counted across the roof width by net coverage after side lap, and it is this panel's span table that hands you the maximum purlin spacing Corrugated Metal Panel Side Lap Overlap Calculator
- Z-purlin rows — one row at each end of the run plus one at every interval, lapped over each frame line so the doubled section lands where the negative moment peaks Steel Z/C Purlin Spacing Calculator
- Sag rods and flange braces — holds the purlin upright against roll and downslope sag, and carries the rafter flange bracing that fixes the frame's unbraced length Steel Bracing Rod Diagonal Length & Weight Calculator
- Tapered rafter — the primary member every purlin row delivers load into, and whose bottom flange those same rows are bracing under uplift Steel Girder Lateral-Torsional Buckling Limit (Lp) Calculator
Two Grids, and Only One of Them Is Yours
Stand back from the plan and there are two grids on it. Running along the building, frame lines at the bay spacing the primary order fixed — that one is inherited and immovable. Running up the slope, purlin rows at whatever spacing your division produced — that one is yours, within the limits the panel table and the rafter bracing set. Every layout problem on this job is one of those two grids failing to accommodate something that has to sit across both.
The sheeting brings its own module and it does not care about either grid. Panels cover a net width after side lap, and the number of panels across the building comes from that net coverage rather than from the out-to-out sheet width printed on the order. Set the panel module out from a chalked reference at one rake so the closing sheet lands full width instead of ripped to a sliver, and check where the sheet end laps fall — an end lap has to land on a purlin, sealed and fastened through both sheets into the row, and a lap that lands between rows is a leak with a date on it.
Framed openings are where the two grids and the module all collide. A roof-mounted unit, a smoke vent or a skylight wants a curb that sits between purlin rows, carried on a header spanning row to row, so that the row spacing and the rafter bracing both survive the opening intact. That is a constraint to hand back to the mechanical layout early, while unit positions are still cheap to move. Once the frame steel is rolling, an RTU that has landed on top of a purlin row is a problem solved by an expensive local frame rather than by an eraser.
What the Framing Plan Has To Say
Dimension the rows from a datum, never chained one to the next. A chained string on fifteen rows accumulates every rounding in it and finishes at an eave that does not agree with the frame, and an erector reading a chained string measures each dimension from the last piece they set, which turns your 1.429 m into whatever the tape and the last clip decided. One datum, running dimensions off it, and the total stated separately as a check.
Then write down the basis, because in eighteen months somebody will change the panel. The note wants the panel profile and gauge, the design load case it was checked for, the span condition the table entry assumed, the maximum spacing that entry permitted, and the spacing actually built — five lines that let a future detailer see whether the substitute panel they have been handed still works, instead of re-deriving your layout from scratch or, more likely, assuming it. The same note covers the deck: profile, gauge, span condition, bearing length and the diaphragm fastening pattern including side laps.
Last, the sequence. Purlins are erected before they are braced and before the sheeting arrives to restrain them, which is the condition in which they are least stable and most exposed, and the erection of this kind of structure is covered by OSHA 29 CFR 1926 Subpart R, Steel Erection, whose provisions on metal decking and on systems-engineered metal buildings both bear directly on this work. A layout that can only be built by walking on unbraced purlins is not a layout, and the time to find that out is while it is still a drawing.
Settle the layout before the secondary order is placed
Five figures, in the order they constrain each other. The first three come off documents — a frame drawing, a span table and a code figure. The fourth is what the division gives back once those three are fixed. The fifth comes back off the system manufacturer's design, and the fourth has to survive it.
- Bay spacing and roof run, off the primary frame drawing — Both are inherited. Take them from the frame order as built rather than from the architectural set, which is frequently a revision behind.
- Maximum support spacing, from the panel or deck span table — For the actual profile, gauge and design load, and against the span condition the sheets will genuinely be laid in — single, double or triple.
- Uplift zone width in from each edge — Draw the line on the plan. Inside it either the fastener schedule tightens or the rows do, and the drawing has to say which.
- Row and support counts, then the spacing they imply — One more than the number of intervals in both cases. Divide the run by the interval count to get the dimension that goes on the string.
- Lap zones, sag rod lines and flange brace points — All three are set by the system manufacturer's design, and all three move if a row moves — which is why a row does not move for a kerb.
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.