The gang behind you sets the clock
Foil under a metal roof is not a trade in its own right. It is an hour or two wedged between the steel crew torquing the last purlin bolts and the sheeters starting their first run off the far gable, and all of it happens on a surface that is not yet a roof. Nobody schedules it as a separate visit, nobody prices it as a separate visit, and on a workshop shed it is routinely the only building envelope decision made while standing on the structure rather than sitting in front of a drawing.
That compression is what makes the quantity matter more than the technique. The technique is a person walking a plank with a roll on a spindle. The quantity is a purchase order raised a fortnight earlier against a roof somebody measured off a plan, and if it comes up short the options are a joint landing where you did not want one, a part roll ordered at a premium, or a crew of sheeters standing around at day rate. Membrane suppliers sell fixed roll widths and fixed roll lengths; you do not buy square metres, you buy rectangles, and the leftover of each rectangle is yours whether you use it or not.
Roll out only what the sheeters can cover before you lose the day. Loose foil on open purlins is a sail with a lever arm, and it does not need much wind to lift a bay of it, wrap it round a downpipe and tear it at every fixing. On an exposed site the working rule is that unsheeted foil never stays out overnight, which turns the roll count into a sequencing problem as well as an ordering one: how many bays go down in a shift decides how much foil comes off the truck each morning.
A shiny sheet does nothing on its own
The product is doing one job. It is putting a low-emittance surface in the way of radiant exchange between the hot underside of the roof sheet and everything below it, and that is a different mechanism from the conduction that batts and boards resist. A bare metal sheet in the sun sits well above air temperature and re-radiates downward into the building all afternoon and for some hours after sunset. The barrier does not stop the sheet getting hot. It stops most of that re-radiation reaching the floor.
The whole claim rests on a number the manufacturer publishes: emittance, measured to ASTM C1371, Standard Test Method for Determination of Emittance of Materials Near Room Temperature Using Portable Emissometers, or ASTM E408, Standard Test Methods for Total Normal Emittance of Surfaces Using Inspection-Meter Techniques. Bright aluminium facings report values at the very bottom of that scale, and a roll that does not quote a tested figure against a named method is quoting nothing at all.
The material specification for the product class is ASTM C1313/C1313M, Standard Specification for Sheet Radiant Barriers for Building Construction Applications, and where the same foil is sold as reflective insulation with a system resistance attached it falls under ASTM C1224, Standard Specification for Reflective Insulation for Building Applications. In the United States the claims printed on the packaging are regulated by the Federal Trade Commission's R-value Rule at 16 CFR Part 460, which is the reason a reputable roll carries a number rather than an adjective. None of that documentation follows the roll onto the roof, so the substitution to watch for is the one made at the merchant's counter when the specified product is three days out.
Two site conditions destroy that number, and both are free to avoid. The first is contact: a low-emittance face pressed flat against the steel above it conducts instead of reflecting, so the barrier has to face an air space and keep facing one. The second is dirt. Emittance is a surface property in the top few microns, and a foil that spends a week collecting cement dust and boot marks before the sheets go on is not the material that was tested. The U.S. Department of Energy and Oak Ridge National Laboratory's radiant barrier fact sheet is blunt about dust degradation on upward-facing installations, and although a roof cavity stays cleaner than an attic floor, the foil is still open to the sky for as long as you leave it that way.
Which side faces up is settled by the product, not by preference. Most membranes sold for this position carry a reflective face and an anti-glare face, and they are laid with the reflective side toward the cavity under the sheeting and the anti-glare side looking down into the workshop, so the crew below is not working under a mirror all afternoon. Reversing a run is not a small error to fix once a bay of sheeting is screwed down over it, and it is exactly the sort of thing that happens when a roll is turned end for end to work back across the roof.
What a lined metal roof is made of, above the steel
- Profiled roof sheet — counted across the roof width at the net cover the profile gives once side laps are engaged, never at its out-to-out width Corrugated Metal Panel Side Lap Overlap Calculator
- Thermal break strip — sits on the purlin flange where the specification calls for it, holding the foil clear of the steel it would otherwise touch
- Radiant barrier foil — reflective face toward the cavity under the sheet, bought as rolls of a fixed width against the sloped area plus laps Radiant Barrier Foil Coverage Calculator
- Safety mesh — strained across the purlins before anything else goes on, and the only layer in this drawing that is rated to catch a person
- Z purlin — its row spacing comes from the sheet's load span table, and every foil lap and mesh fixing has to land on one of these lines Steel Z/C Purlin Spacing Calculator
The grid you are rolling across
Everything about the way foil behaves up there is governed by how far apart the purlins are, and that spacing was fixed by somebody else for reasons that had nothing to do with you. It comes out of the roof sheet's load span table for the applied wind and live load at the specified gauge, and the purlin section itself is designed to a cold-formed steel standard, AISI S100, North American Specification for the Design of Cold-Formed Steel Structural Members, or AS/NZS 4600, Cold-formed steel structures, depending on where the shed is going up. What arrives on site is a row count: a run divided into equal bays needs one more row of steel than it has bays, an eave line, a ridge line and every intermediate line between them.
Read that grid as the support the membrane has, because it is the only support the membrane has. Between purlin lines the foil is hanging on its own tensile strength, and how much it drops decides whether the cavity under the sheeting stays open, whether water that condenses on the sheet has somewhere to run, and how much extra length the roll consumes crossing each bay. A membrane strained drum-tight over a wide bay eventually relaxes into contact with the sheet at midspan; one left slack sags into the space the sheeting screws need. Neither extreme is a judgement call to be made per bay by whoever is holding the roll.
Walk the grid before you trust it. Purlins on a longer shed are lapped or sleeved over the internal frames, which leaves a double thickness of steel and a cleat with bolt heads standing proud exactly on the lines your membrane has to cross; a roll dragged over that catches and tears, and a lap landing on it never sits flat. Bracing straps, sag rods and any bridging between purlin rows sit in the same plane and do the same thing. None of it appears on the roof plan you took the area off, and all of it is visible from the ground in ten minutes.
On sheds where roof safety mesh is specified, the mesh goes on first and the foil is laid over it, which changes the problem completely: the mesh carries the drape, the sag is limited by the mesh openings rather than by the purlin spacing, and the foil stops being a membrane spanning three or four metres. AS/NZS 4389, Roof safety mesh, covers the mesh itself. Where there is no mesh, wire strained between purlin lines or a batten over the membrane at the purlin does the same job less thoroughly, and the manufacturer's installation literature, along with AS 4200.2, Pliable building membranes and underlays: Installation, is where the permitted drape for a given support spacing is written down.
The row count is what turns the panel span table into something you can chalk on the steel, and it is also the number of lines your foil laps have to land on, so settle it before the first roll is unwrapped.
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.
Rolls, not square metres
Start from sloped area, not plan area. A shed roof is drawn in plan and quoted in plan, and the surface you are actually covering is longer than that plan by the slope factor. On a low-pitch workshop the difference is small enough to be dismissed and large enough to eat a roll: a shallow fall over a thirty metre building adds real metres, and it adds them in the direction the roll travels, which is where a shortfall shows up.
Then add the things that are not area at all. Every side lap between adjacent runs is width paid for twice, and on a fixed roll width those laps compound across the roof: a run of foil delivers its width minus one lap, so the number of runs is the roof width divided by that net cover, rounded up, exactly as it is for the sheeting above. End laps at purlin lines cost length. Turn-ups at the gables, returns down over the eave purlin or into the gutter line, and the wrap over the ridge all cost material that does not appear on any plan area. Sag across each bay costs a little more length than the straight-line distance suggests. And the offcuts from working round a whirlybird, a translucent sheet or a flue penetration are not reusable in any width that helps you.
The coverage arithmetic itself is unglamorous: area multiplied by a waste allowance, then divided by the coverage of one roll. What deserves care is which side of that multiplication each item goes on. Laps and returns are geometry and belong in the area; cut-offs and rework are waste and belong in the allowance. Loading laps into a waste percentage works right up to the point where the percentage needed exceeds what a sane waste box will hold, at which point the number stops being an allowance and starts being a fiction. If laps and returns are pushing past a tenth of the roof, add them to the measured area and keep the waste allowance for genuine waste. Quantify per roof plane and per run direction while you are at it, because a single figure for a two-plane shed hides the fact that one plane takes seven full runs and the other takes six runs plus a strip four hundred millimetres wide, and that strip is the piece nobody orders.
| Item | Where it belongs in the calculation | What it does to the number |
|---|---|---|
| Roof slope | In the measured area, as the sloped length of each plane | Small on low pitch and always in the direction the roll runs |
| Side laps between runs | In the area, or as net cover per roll width | Compounds across the roof width, one lap per run boundary |
| End laps at purlin lines | In the area, per lap, at the manufacturer's lap width | Only bites on runs longer than one roll length |
| Gable turn-ups and eave returns | In the area, as extra length or width per edge | Frequently omitted entirely from a plan-area takeoff |
| Sag across each bay | In the area, as a small addition per bay crossed | Grows with purlin spacing and disappears where mesh carries the drape |
| Cut-offs around penetrations | In the waste allowance | Rises sharply on a roof with flues, hoods or translucent sheets |
| Damage and rework | In the waste allowance | Wind damage to unsheeted foil is the usual cause |
This is the figure the crew is standing on the slab waiting for: sloped area with the laps and returns already in it, carrying an honest waste allowance, ready to be divided by the coverage of one roll.
SettingsSettings for this calculation
Waste is set to 5% by hand. Pick a tier above to replace it, or keep your own figure.
The total attic roof deck area to be covered with radiant barrier foil.
Extra material to allow for cut waste and overlaps.
Radiant barrier material needed
1,690 ft²
They open the calculator with your figures already in it
Radiant Barrier Foil Coverage Calculator: 1,690 ft² — 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
- Foil is a vapour barrier as well as a reflector, and where it lands decides whether that matters. Laid over the loft floor on top of the insulation, an unperforated foil sits on the cold side of the ceiling and traps moisture rising out of the house in the insulation beneath it - perforated products exist precisely so the assembly can still dry, and choosing between them is a climate and construction decision rather than a coverage one. The same job goes wrong at the eaves: foil run tight into them blocks the soffit ventilation the roof space relies on, and the barrier's own benefit assumes that air is still moving.
- It is a sheet of metal in a space full of wiring and fittings. Foil conducts, so it must be kept clear of live parts, junction boxes, old rubber-insulated or knob-and-tube wiring, flues, and the tops of recessed downlights, which carry their own clearance requirements - stapling a continuous conductive sheet across the rafters is not a neutral act. Where it is left exposed in an accessible roof space its surface-burning rating matters as well; this sum counts area and nothing else.
Laps, tape and the plane you are actually building
Decide before the first run goes down whether this foil is a radiant barrier only or whether the design is also asking it to be an air control layer or a vapour control layer. The three are different jobs with different acceptance criteria, and the difference on site is entirely in the laps. A pure radiant barrier tolerates a lap that merely overlaps. A membrane counted on to control air movement has to be lapped to the stated width and taped or clamped along its whole length, and a membrane counted on to control vapour has to be continuous, which means sealed at every edge, every penetration and every junction with the wall membrane below.
Where taping is required, it is a linear-metre item and it is a big one. Every run boundary across the roof is a seam, every end lap is a seam, and every penetration adds a perimeter. On a shed with a lot of narrow runs the tape length can approach the same order of magnitude as the length of foil, which is not what anybody expects from a consumable. The material specification for the membranes themselves is AS/NZS 4200.1, Pliable building membranes and underlays: Materials, and the installation practice, including lap widths and support requirements, sits in AS 4200.2; in the United States the corresponding requirements come from the manufacturer's published instructions read against the adopted building and energy codes.
Then accept what the sheeters are about to do to it. Every roof sheet fixing is a hole through the membrane into the purlin flange, and on a shed there are thousands of them. Sealing washers do their work at the top of the sheet, not at the foil, so a taped and sealed membrane is a sealed membrane with a screw through it every few hundred millimetres. That is normal, and it is the reason a foil layer under sheeting is rarely credited as a strict air barrier on its own. It is also the reason the screw length has to allow for the foil and any thermal break strip: a fastener sized against bare steel and driven through an extra few millimetres of membrane and spacer has that much less thread in the purlin.
If the specification wants sealed laps rather than lapped ones, total the run boundaries, end laps and penetration perimeters here, because tape is ordered by the roll and runs out faster than anyone plans for.
SettingsSettings for this calculation
Waste is set to 5% by hand. Pick a tier above to replace it, or keep your own figure.
The combined length of all sheathing seams and penetrations to be taped.
Extra tape to allow for cut waste, overlaps, and rework.
Seam tape needed
204.7 ft
They open the calculator with your figures already in it
Air Barrier Sealant Tape Linear Footage Calculator: 205 ft — 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
- One length, several different products. A flat sheathing seam takes a straight tape; an inside or outside corner takes a stretch or flexible tape that will turn the angle without tenting; a round penetration or a lapped transition often takes liquid-applied flashing and no tape at all. Those are separate purchases in separate widths sold in fixed roll lengths, and one total silently averages them — buy against it and the corners are the part that comes up short.
- Taping the perimeter of a rough opening is an AIR seal, not a water detail. Water needs a sloped or pan-formed sill and laps that shingle downward, head over jamb over sill, so anything getting past the window is led back out. Tape run continuously around an opening as an air seal can instead hold water against the sheathing, and that failure shows up as rot at the sill years before anyone opens the wall.
The night the sheet goes colder than the air
The failure that brings people back to a shed roof is not heat. It is water appearing on the underside of the sheeting on a clear night, running down the profile and dripping into the workshop, and it happens because a metal roof radiates to the night sky and can sit several degrees below the surrounding air temperature. Once the underside of that sheet falls below the dew point of the air in the cavity, moisture condenses on it. Nothing has leaked. There is no hole to find.
A foil membrane changes where that water goes, and it can change it for the better or for the worse. An impermeable foil laid tight under the sheet catches whatever forms above it, and if the falls are right that water runs down the membrane and out at the eave. If the membrane is dished between purlins with no drainage path, or if it is tucked into a gutter detail that discharges behind the fascia, the foil becomes a tray. Vapour-permeable reflective membranes exist precisely so that moisture arriving from the building side can pass outward instead of accumulating under an impermeable sheet, and choosing between permeable and impermeable is a design decision about which side the vapour drive comes from, not a preference.
In a heated workshop in a cold climate, an impermeable layer on the cold side of any insulation below it is the classic wrong-side placement, and it will collect interstitial condensation whatever the roof does at night. In a hot, humid climate with a cooled interior the drive reverses and the same layer sits on the correct side. BS 5250, Management of moisture in buildings: Code of practice, and the moisture control material in the ASHRAE Handbook: Fundamentals set out how that gradient is assessed; in Australia the National Construction Code's condensation management provisions cover the same ground and are the reason vapour permeance of roof membranes became a specified property rather than an assumed one.
There is a ventilation half to this that no membrane substitutes for. A cavity between the foil and the sheet that is open at eave and ridge dries; a cavity closed at both ends with foam filler blocks does not. Whichever way the specification goes, it should be a decision on a drawing, because on site the filler blocks get fitted by whoever is closing the ridge, and they will close it.
The calculator is written around a wall, but the gradient it solves is one-dimensional and does not care which way the assembly is turned: feed it the roof's resistances and the temperature at the membrane plane comes out, to be compared against the dew point of the air it is sitting in.
The assumed indoor air temperature.
The outdoor winter design temperature for the site's climate zone.
The full wall assembly's total thermal resistance, interior surface to exterior surface.
The portion of the total R-value between the interior face and where the vapor barrier sits.
The dew point temperature of the interior air, based on its temperature and relative humidity.
Temperature at vapor barrier location
55.9 °F
The temperature at this location stays above the interior dew point shown below, so condensation is not predicted under the design conditions entered. No risk predicted under these conditions is not the same as none. The conditions are the ones you entered, and one surface is not the assembly.
- Interior air dew point
- 50 °F
They open the calculator with your figures already in it
Vapor Barrier Dew-Point Condensation Risk Calculator: 55.85 °F — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
What this calculation does not cover
- Air leakage, not diffusion, is what usually wets a wall, and there is nothing about it here. The gradient gives the temperature at the plane; it cannot say how much moisture arrives there. A wall that passes this check and leaks warm interior air through a top plate, a service penetration or an unsealed electrical box deposits far more water at that plane than vapor diffusion through an intact assembly ever could.
- The R-values entered describe the clear field of the wall, between the framing. The path through every stud, plate and header is colder than this straight line says, and at a steel stud or an uninsulated slab edge much colder — so an assembly that passes in the middle of a bay can be sitting below dew point on the back of the sheathing at every framing member, which is exactly where mold turns up.
- It is one snapshot at one pair of temperatures, and it totals nothing. What damages an assembly is how many hours a year it spends below dew point and whether it dries out in between. A wall that dips below on a few cold nights and recovers is not the same wall as one that stays below for a month, and this returns the identical verdict for both.
- The obvious fix for a failing result can produce a wall that cannot dry. Moving the barrier inboard is right only if the outboard side is open to vapor. Where a low-perm layer already sits outside — exterior foam, a self-adhered membrane, an impermeable sheathing — a second one inside traps whatever gets past either of them, and the assembly then passes this temperature check with no drying path in either direction.
Handing the roof over bay by bay
The sheeters and the foil crew are working the same steel at the same time, and the handover is per bay, not per roof. Agree the direction first: sheeting normally starts at the end away from the prevailing weather so the side laps face downwind, and the foil has to be laid in the same sequence or the crews cross each other. Agree the buffer too. Two or three bays of foil ahead of the sheeting line is enough to keep everyone moving and small enough that a gust cannot take the whole roof's worth.
The panel count is what tells you how wide those bays are. A profiled sheet covers less than its out-to-out width once the side lap is engaged, so the number of sheets across the roof comes from the net cover, and that same net cover sets where every screw line falls relative to your foil laps. When the two grids are worked out together, foil laps can be arranged to land on purlin lines where they are clamped by the sheet rather than floating mid-bay.
- Confirm the sheeting start end and lap direction with the sheeting foreman, and lay foil in that same direction.
- Strain and fix the safety mesh across the full bay before any membrane comes out of its wrapper.
- Run the first length square off a chalked reference at the eave, reflective face up, and check the anti-glare face is the one visible from below.
- Set the drape to the membrane manufacturer's allowance for the support spacing, and hold that drape consistently across the run rather than per bay.
- Lap the next run to the stated width, landing end laps on purlin lines, and tape only if the specification asks for a sealed plane.
- Release no more than the agreed number of bays ahead of the sheeting line, and close out every released bay before the crew leaves the roof.
Net cover per sheet is what converts roof width into a sheet count, and it is also what fixes the screw lines your foil laps should be sitting under rather than beside.
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.
What the foil is not
It is not a working platform and it is not fall protection. A taut membrane across purlins looks like a floor from three metres away and will hold nothing. In the United States work on this roof falls under OSHA 29 CFR 1926 Subpart M, Fall Protection, and the protection has to come from mesh rated for the purpose, from harness and anchor systems, or from a physical platform, never from the membrane. The most dangerous moment on the job is the one where a bay is covered and the covering conceals the openings.
It is not thermal resistance either, and the difference matters when someone reads the shed's energy compliance paperwork. A radiant barrier interrupts radiant exchange across an air space and has essentially no resistance of its own; the resistance that gets credited belongs to the system of foil plus cavity plus whatever bulk insulation is in the build, which is why reflective assemblies are rated as systems and why the rated figure depends on cavity depth, heat flow direction and the emittance of the facing. A blanket under the sheeting and a foil under the sheeting are not interchangeable products, and quantities taken off for one do not carry to the other.
It is metal, and that has two consequences people forget. Conductive membrane must be kept clear of live parts, and it is never a bonding or earthing path in its own right; what is required of the installation comes from the wiring rules in force, whether that is NFPA 70, the National Electrical Code, or AS/NZS 3000, Electrical installations. And the facing carries a surface burning classification tested to ASTM E84, Standard Test Method for Surface Burning Characteristics of Building Materials, which is the number to check before anyone assumes an exposed foil soffit is acceptable in a building where the adopted code has something to say about interior finish.
What to write down before the scaffold comes off
Record the product, not the category. Brand, product name, the tested emittance and its test method, permeable or impermeable, roll width and roll length, and the lap width you actually installed. A shed that develops a condensation complaint in its second winter is diagnosed from that record or it is diagnosed by cutting the roof open.
Photograph the membrane in each plane before the sheeting closes it, with something in frame that identifies the bay. Note the purlin spacing you found rather than the one on the drawing, note where the cavity is open and where filler blocks closed it, and note any run laid short or patched. The next person who opens this roof, for a solar array frame or an extraction flue, is going to cut through your work; what they need is a page telling them what they are cutting.
Before the first roll goes up the ladder
Six figures fixed on the ground, in the order they stop the job if they are missing.
- Sloped area per roof plane — Measured off the plane, not the plan, and kept separate per plane so a part run on one side does not vanish into a building total.
- Roll width and net cover per run — Cover is the width minus one side lap; the number of runs is roof width divided by that, rounded up, and the remainder is the strip nobody orders.
- Purlin row count and spacing — Sets the drape between supports, decides where end laps can land, and tells you whether safety mesh is carrying the membrane or the membrane is carrying itself.
- Lap and return allowance, in the area — Side laps, end laps, gable turn-ups and eave returns are geometry; put them in the measured area and keep the waste percentage for cut-offs and rework.
- Seam tape length, if the laps are to be sealed — Run boundaries plus end laps plus penetration perimeters. Only order it once someone has confirmed the membrane is meant to be a sealed plane.
- Fastener length through membrane and spacer — Screws sized against bare purlin flange lose thread engagement when a membrane and a thermal break strip are added under the sheet.
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.