Warm Deck or Cold Deck on a Flat Roof Over a Room

The roof you are stripping is a cold deck, and it is why you are up here

Lift the felt on a flat roof built any time from the sixties to the nineties and the section is nearly always the same. Joists, mineral wool or a shallow board pushed between them, a gap above the insulation, boarding across the joist tops, felt over that. Sometimes a sheet of polythene under the plasterboard, more often a foil-backed board, frequently nothing. The gap is the load-bearing idea in that assembly: it is meant to be cross-ventilated from one outside edge to the other, so that whatever moisture gets past the ceiling is carried away before it meets the cold underside of the deck.

On the buildings this guide is about, that gap has no chance. A single-storey extension has joists running from the house wall to the outer wall, which means one end of every void terminates in the solid masonry of the original building — there is nothing to ventilate to. A dormer is worse: cheeks on two sides, the window head on the third, the main roof on the fourth. Under both is exactly the room that generates the moisture, a bedroom with an en-suite or a kitchen extension with a hob, and the ceiling between them is punctured by downlights, an extract duct and a loft hatch. Air leakage through those punctures moves far more water into a roof void than vapour diffusion through plasterboard ever does, which is the reason a sealed ceiling matters more than the sheet material behind it. The coldest surface up there is the underside of the boarding, and the boarding is timber.

The standing advice reflects that. BS 6229, Flat roofs with continuously supported flexible waterproof coverings — Code of practice, and BS 5250, Management of moisture in buildings — Code of practice, both set out why the cold deck flat roof is the highest-risk of the flat roof options and steer new work away from it; read the wording in the edition current at the date of your application rather than taking it second hand. In North America the vented route is governed by the International Residential Code, Section R806, Roof Ventilation, which fixes a clear space and a net free ventilating area, and the unvented alternative sits alongside it with its own conditions. None of that forbids the assembly outright. What it does is move rebuilding what you removed from a default into a decision somebody has to defend.

Warm deck: the same materials, the other side of the boarding

Turn the section inside out and the order becomes deck first, vapour control layer bonded to it, insulation on that, waterproofing on top. Nothing goes between the joists. The consequence is worth stating plainly, because it is the whole case for the assembly: the deck and the joists now sit inboard of the insulation and run within a few degrees of the room below them all winter. They stop being condensing surfaces. The coldest plane in the build-up moves to the underside of the membrane, which is outboard of everything, and the layer with the vapour resistance is at the bottom where the warm air arrives rather than at the top where it would trap.

Two habits have to be unlearned on the way. The first is the fascia vents: if the old roof had them, they close. A void ventilated to outside air, under a vapour control layer, beneath a warm deck, is a cold stripe running through the middle of what is supposed to be a heated envelope, and half the failed warm decks in the country are cold decks with the insulation moved and the vents left alone. The second is the leftover wool. Insulation between the joists on a warm deck is not a bonus; it is resistance on the wrong side of the deck, and it cools the deck by exactly the amount it adds. Either it comes out, or the assembly becomes a hybrid, which is a calculation rather than a saving.

The vapour control layer is the item to get right while the deck is bare and easy. It is bonded or fully supported, lapped and sealed at every joint, carried up every upstand and turned over the head of the kerb so the insulation is enclosed rather than merely covered, and terminated into the wall's damp proof course or cavity tray at the abutment. Treat it as the air barrier and not only as the vapour check — that is the substance of what BS 5250 and Approved Document C, Site preparation and resistance to contaminants and moisture, are asking for. The reward is that every downlight, cable and extract duct in the ceiling below is now a hole in the plasterboard rather than a hole in the control layer, because the control layer is a whole deck thickness above the electrician.

Rigid polyisocyanurate under BS EN 13165, Thermal insulation products for buildings — Factory made rigid polyurethane foam (PU) products — Specification, or ASTM C1289, Standard Specification for Faced Rigid Cellular Polyisocyanurate Thermal Insulation Board, is the usual board here for one reason: it buys the most resistance per millimetre, and millimetres are the currency. Lay it in two staggered layers rather than one thick one wherever the vertical allows, because the joint pattern of a single layer is a set of straight lines running from the membrane down to the deck.

Making a fall out of a deck that has none

A flat roof is not flat and never was. UK practice takes its numbers from BS 6229, which pairs a design fall with a smaller finished fall so that deflection, joist settlement and ordinary workmanship still leave water moving — the pairing most detailers carry in their heads is a design fall of one in forty against a finished one in eighty, and the current edition is where to confirm it before it goes on a drawing. Where the building code is the governing document instead, it sets its own minimum for a membrane roof and the adopted amendments decide the figure you have to meet.

Where the fall comes from is the decision. Firrings ripped and fixed along the joists put it under the deck and cost nothing in height above; if the roof you are re-covering already has them, and the survey says the fall still works, a warm deck can be flat board on a sloping deck and you have bought the fall for free. Tapered insulation puts it above the deck instead, which is tidier, more expensive per square metre, and adds every millimetre of the taper to the four pencil marks on the brickwork. Most domestic warm decks end up as flat base board bringing the resistance up to target with a tapered layer over it doing the drainage, because tapered stock is the costly way to buy thermal performance.

Direction is where this job differs from a commercial roof draining to a middle. On an extension the water leaves at the outer edge into a gutter, so the fall runs away from the house and the thick end of the taper lands at the abutment — precisely where the vertical was tightest and where the upstand still has to be found. A dormer usually falls forward to a small gutter at the front, which puts the thick end under the window it was built to serve. Neither direction is negotiable, and both put the deepest part of the build-up against the most sensitive dimension on the survey, which is worth knowing before the boards are ordered rather than on the morning they arrive.

Getting the target and the arithmetic into the same units

Consent is granted against a number in whichever system the paperwork was written in, and the two systems are not a rounding apart. In England the standard is a maximum U-value in W/m²K, from Approved Document L, Conservation of fuel and power, Volume 1: Dwellings, with the row that applies depending on whether this roof counts as new, as a replacement, or as a retained element being upgraded. Across the Atlantic the target arrives as a minimum R-value by climate zone from the International Energy Conservation Code and the IRC. The imperial U-value is about 5.678 times smaller than the metric one for the same assembly, so a specification of 0.15 W/m²K is 0.0264 in BTU/hr·ft²·°F, whose reciprocal is R-37.9 — and the same assembly stated metrically is 6.67 m²K/W. Convert once, at the start, then work in one currency for the rest of the job.

The figures above are arithmetic, not a requirement: what applies at this address comes off the approved documents for the application, and a warm deck on an extension frequently lands in a stricter row than the roof it replaces because the work is new construction rather than a repair.

Why the sum is shorter than you expect

Adding up a warm deck takes three lines, and one of them is nearly the whole answer. There is the insulation, the deck and any overlay above it, and the plasterboard ceiling below — and unlike a wall or a pitched slope, nothing punches through. The insulation runs continuously over the joists, so there is no framing fraction to correct for and no parallel path to average out; the only things crossing the layer are the mechanical fixings, at the density the manufacturer's wind uplift schedule sets. That is the quiet second argument for the warm deck, and it is worth more than the number suggests, because a bridged assembly always underperforms its calculation while an unbridged one roughly meets it.

Working an example through: a hundred and fifty of polyisocyanurate at a declared 0.022 W/m·K is 6.82 m²K/W, which is R-38.7; eighteen millimetres of ply deck is about R-0.8; the ceiling contributes the R-0.45 the calculator already offers as its own third default. Those three plus the air films land the assembly near R-40.8, or 7.19 m²K/W, or a U-value around 0.14 W/m²K. The layer fields are plain imperial R-values that ignore the metric toggle, so a resistance in m²K/W is multiplied by 5.678 on the way in; the air film allowance of R-0.85 is added for you, so do not spend a field on it; and the three fields cap at R-60, R-30 and R-10 in that order, so the board goes in the first one and a passive-standard thickness that passes R-60 has to be split across the first two rather than across any two.

The board between the foam and the waterproofing

Whether an overlay is needed is a membrane question rather than an insulation one. A torch-applied or hot-air-welded system cannot be worked directly onto a foam facer, so it gets a substrate: glass mat gypsum to ASTM C1177/C1177M, Standard Specification for Glass Mat Gypsum Substrate for Use as Sheathing, or a mineral or wood-based board named in the system's own approval. Many fully adhered domestic EPDM systems, by contrast, bond straight to the insulation facer and want no board at all, and adding one there spends height and money for nothing.

The case for one anyway is traffic and point loads. Somebody will go up there — a boiler flue terminal, an aerial, a rooflight that needs cleaning, a window cleaner's ladder foot — and foam boards print their joints through a membrane under a heel long before they fail thermally. Where the roof will be walked at all, an overlay and a walkway are cheaper than the callback, and the fixing pattern for the board is the manufacturer's, not the deck's.

Take the sheet size off the actual data sheet. A 1200 by 2400 board is 2.88 m² and a 4 by 8 is 2.97 m², which is a three per cent difference in count that compounds on a nine-sheet order into whether the last one gets cut or bought.

One sheet, or a roof with seams in it

Single-ply took the domestic extension market because a small roof can be a single piece of rubber with no field seams anywhere on it, which is a claim felt could never make. The limit is the widest sheet the supplier stocks, and above it the roof has seams — so establish that dimension before the roof is designed around it, not after the survey. EPDM sheet is specified under ASTM D4637/D4637M, Standard Specification for EPDM Sheet Used in Single-Ply Roof Membrane, and in Europe under BS EN 13956, Flexible sheets for waterproofing — Plastic and rubber sheets for roof waterproofing — Definitions and characteristics.

Membrane area is never the plan area, and a warm deck makes the gap wider than a cold one did. Every upstand is sheet turned vertical: the height up the kerb on each edge, the return over its head, the dressing down into the gutter, the wrap into each corner. Raising the roof raised those upstands too. Add the perimeter development to the measured field before anything is ordered, and remember that the roll counter has no idea any of it exists.

Seams you did not draw

Count the seams as two populations. Field seams are the ones on a drawing, and on a small warm deck there may be none. Flashing seams are the ones nobody schedules: every internal and external corner of the kerb, every rooflight upstand, every pipe and flue penetration, and each of them is a separate piece with its own perimeter to seal. An extension roof kerbed on three sides with a rooflight in the middle can carry more taped length in details than a much larger open field does in the middle.

Whatever the length, the seam lives or dies on preparation. Cured EPDM has a surface tape will not hold reliably without primer, and a seam taped over an unprimed or under-primed sheet looks perfect on the day and lets go within a couple of seasons. Manufacturers publish a minimum application temperature and it is not advice; the seams that fail first are almost always the last ones made on the coldest afternoon of the job. T-joint patches, where three sheets overlap, are a separate item that no length calculation contains.