The compact warm deck is the most widely used flat roof construction — and also the one that forgives the least. All its layers are bonded together with no ventilated cavity, so moisture that enters the build-up cannot dry out on its own. This article covers how it differs from other types, how the construction is assembled, when it suits and when another solution is better.
What a compact roof is and how it differs from a pitched one
A pitched roof has a loft — a space between the ceiling and the covering. It is ventilated, so moisture entering the structure has somewhere to go.
A compact roof has no such space. Deck, vapour barrier, insulation and waterproofing are bonded into a single package resting directly on one another. Every characteristic of this type follows from that — the good and the bad.
The practical consequence is simple: because moisture cannot escape, the vapour barrier becomes the critical layer. On a pitched roof an unsealed spot often passes without consequence, because ventilation dries it. On a compact roof the same error saturates the insulation within a few years.
Compact or ventilated: the essential difference
The essential difference — where moisture dries
In a compact roof the layers sit tight together, so moisture has nowhere to ventilate — everything hinges on the right vapour barrier and airtightness. In a ventilated roof an air gap removes part of that risk.
Flat roofs fall into two families, and choosing between them matters more than choosing a membrane manufacturer.
- Compact (warm deck). Layers bonded, no cavity. Thinner, simpler, cheaper. Moisture control depends entirely on the integrity of the vapour barrier.
- Ventilated. An air gap with inlets and outlets between insulation and covering. The gap dries residual moisture, so the system is more forgiving.
The ventilated option is chosen where internal humidity is high — pools, kitchens, production spaces — or where the structure is timber and the moisture risk is greater. In all other cases the compact build-up is the standard and more economical solution.
80
% of leaks begin not on the field, but at the details
6
installation stages from deck to covering
1
decisive factor: the right vapour barrier and airtightness
Types of compact roof
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The compact roof itself has several variants, depending on how the layers are arranged and how the roof is used.
- Classic warm deck. Layer order: deck, vapour barrier, insulation, waterproofing. The most common variant.
- Inverted. Insulation sits above the waterproofing, with ballast on top. The membrane is protected from UV and temperature swings.
- Usable. A terrace, walkway or deck is built over the waterproofing. Requires a protective layer and greater load capacity.
- Green. A drainage layer and substrate over root-resistant waterproofing.
- With solar. Technically a classic warm deck, but designed around array loads and mounting.
The type must be decided at design stage, because structural loading and insulation choice follow from it. Changing later means stripping everything.
The build-up from the bottom up
- Structural deck. Concrete, profiled steel or timber. Carries every load.
- Slope layer (if formed at deck level). A screed or tapered boards.
- Vapour barrier. Laid on the warm side with sealed laps and carried up vertical surfaces.
- Thermal insulation. Boards laid with staggered joints so that joints never form a continuous thermal bridge.
- Waterproofing. Torch-applied bituminous membrane or a welded single-ply sheet.
The order is not a convention — it follows from physics. The vapour barrier always sits beneath the insulation, because warm moist air rises from inside and has to be stopped before it reaches the colder zone.
Advantages and drawbacks
| Advantages | Drawbacks |
|---|---|
| Thinner build-up, lower building height | Moisture has nowhere to escape |
| Simpler and faster to install | Vapour barrier errors cannot be fixed without stripping |
| Lower installation cost | Leaks are harder to locate |
| Suits any roof size | Unsuitable for very humid interiors |
| Can be used — terrace, green roof, solar | Replacing insulation means stripping the whole build-up |
The main drawback is not technical but practical. On a compact roof the cost of an error is disproportionately high, because fixing it means stripping rather than patching. So quality work first time pays back here more than anywhere else.
Waterproofing systems for compact roofs
Waterproofing is the only layer in direct contact with water, so the choice matters.
- Torch-applied SBS bituminous membrane. A two-layer system with offset seams. Reliable in a northern climate, flexible in the cold and easy to repair locally.
- PVC membrane. Single layer, welded seams, fast over large areas. A light colour reduces heat gain.
- TPO membrane. Covered separately below.
TPO membrane: when it suits best
TPO is a thermoplastic sheet with hot-air welded seams. It differs from PVC in several practical respects.
- Chemical resistance. Better against grease, oils and chemicals — hence common on production buildings and wherever extract discharges onto the roof.
- Compatibility with bitumen. Unlike PVC, TPO does not conflict as strongly with bituminous surfaces, which can simplify refurbishment.
- Environmental profile. It contains no plasticisers that migrate over time.
- Light colour. Reflects sunlight and reduces heat gain and cooling demand — noticeable on large commercial buildings.
The drawback is less flexibility when forming complex details, and repairs require welding equipment. On small roofs with many details, bitumen is often more practical.
Fall and drainage
Fall matters especially on a compact roof, because the membrane sits directly on the insulation — and standing water means constant loading and faster degradation.
In practice at least 2 % (about 2 cm per metre) is targeted, and more around outlets. The fall is usually formed with tapered insulation boards, delivering insulation and direction in a single operation.
The drainage system has three elements: outlets at the low points, cross-falls directing water to them, and emergency overflows set slightly higher. The overflow works when the main outlet blocks — without one, water accumulates, and it weighs about a tonne per cubic metre.
Work out the required rise with our slope calculator.
Parapets, penetrations and details: where 80 % of leaks begin
Where most leaks begin
Not on the roof field, but at the details: parapets, small stacks and penetrations. It is the quality of these details, not the membrane itself, that usually decides whether the roof lets water through.
The open field is the easy part of a roof. Problems start where the horizontal plane meets something else.
- Parapets. The membrane is carried up the vertical surface and mechanically secured at the top. An angle fillet is formed at internal corners so the sheet is never bent through a right angle — folded sharply, it cracks along the fold.
- Outlets. The flange is dressed so water runs onto it rather than under it. A reinforcing patch goes beneath the main layer.
- Vent penetrations. Collars must be sealed and reinforced, not smeared with sealant.
- Rooflights and chimneys. Require their own detail with sufficient upstand height.
When assessing a quote, check whether details are mentioned at all. A quote listing only “membrane X €/m²” probably excludes the very part that determines watertightness.
How a compact roof behaves in winter
Winter is when construction errors become visible from outside — before any stain appears on a ceiling.
What to watch for:
- Melt patches in snow. Snow on a well-insulated roof melts evenly. Distinct patches or strips mean heat is escaping more strongly there — usually a thermal bridge or saturated insulation.
- Ice at parapets and outlets. Indicates water melting in the middle of the roof and freezing when it reaches the colder edge. Ice expands precisely at details, where the membrane is bent.
- Icicles at downpipes. The same mechanism — heat from the building melts snow while the edge stays cold.
Snow loading is also assessed separately on a compact roof. Snow does not slide off as it does on a pitch, so it accumulates evenly — and unevenly where wind drifts it, typically behind parapets and against taller parts of the building. That is where the highest local load builds.
A practical recommendation: do not clear snow from the roof without good reason. A shovel or ice pick damages the membrane faster than snow does. If the depth raises genuine load concerns, clearing should be done by people who know the build-up, leaving a protective layer in place.
Refurbishing an existing compact roof
Refurbishment differs from new installation on one essential question: is the existing insulation dry?
Three different routes follow from the answer:
- Insulation dry. A new covering can go over the old, or an additional insulation layer can be added with new waterproofing on top. The cheapest option.
- Insulation wet in places. Affected zones are replaced and the rest left. This requires accurate zone identification, usually by thermography.
- Insulation wet throughout. The whole build-up is stripped to the deck. The most expensive, but the only one that works.
It is also worth checking whether an existing vapour barrier is present at all and whether it is suitable. Older roofs often have none, or a damaged one — in which case new insulation without a new barrier will saturate exactly as the old one did.
When a compact roof is not the right choice
Despite being the standard solution, there are cases where something else suits better.
- Very humid interiors. Pools, laundries, certain production processes. A ventilated build-up is safer there.
- Unclear condition of existing insulation in refurbishment. If dryness cannot be established reliably, sealing it under new layers is a risk.
- Timber structures with limited access. The consequences of an error are more severe in timber than in concrete.
- Buildings where frequent structural changes are planned. Every new penetration is a new risk point.
What drives the cost
Cost follows scope and complexity, not area alone:
- Insulation thickness and type — the single largest factor, especially where boards also form the fall.
- Covering system and number of layers.
- Number of details — parapets, outlets and penetrations are priced separately.
- New build or refurbishment — the latter adds stripping and disposal.
- Access — height, vehicle access, whether the building stays in use.
- Additional loads — a terrace, green roof or solar array changes the structural requirements.
The cost structure is broken down further in our flat roof cost article.
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Compact roof installation: six stages
Compact flat roof installation: 6 stages
Design and layer selection
The roof type, insulation thickness to meet the target U-value, fall scheme and outlet positions are set, and future loads assessed.
Deck preparation
The structure is cleaned and checked for moisture and flatness. Concrete must have dried before the vapour barrier goes down.
Vapour barrier
Laid with sealed laps, carried up vertical surfaces to the top of the insulation, with every penetration sealed.
Insulation and fall
Boards are laid with staggered joints. The fall is formed with tapered boards following the design scheme.
Waterproofing
A two-layer torch-applied system with offset seams, or a welded single-ply membrane, is installed.
Details and leak testing
Parapets, outlets, overflows and penetrations are completed. A test is carried out and warranty and documentation issued.
What to check at handover
Most of a compact roof stays invisible, so the important things must be seen while they are open. Agree interim inspections:
- After the vapour barrier — are laps sealed, is it carried up parapets, is every penetration sealed individually?
- After the insulation — are joints staggered, are there gaps, does the fall run towards the outlets?
- Before final payment — does water pond after rain, are emergency overflows installed?
Ask for photographs from each stage. It costs nothing, and later, if warranty questions arise, documentation of the hidden layers is the only evidence.
Related reading
Frequently asked questions
It is a flat roof construction in which all layers — deck, vapour barrier, insulation and waterproofing — are bonded into one package with no ventilated cavity. Because moisture cannot escape, the vapour barrier becomes the critical layer.
Flat roof is the general term for a shallow-fall roof. Compact, or warm deck, is a variant with no ventilated cavity. There are also ventilated flat roofs, where an air gap is left between insulation and covering.
Always on the warm side, that is BELOW the insulation. Placed above it, it would trap moisture exactly where it must not be left. The exception is an inverted roof, where the waterproofing performs that role.
In a northern climate the most reliable is a two-layer torch-applied SBS bituminous membrane — flexible in the cold and easy to repair locally. For large commercial areas PVC or TPO is often more economical.
In very humid interiors — pools, laundries, certain production processes — a ventilated build-up is safer. Also in refurbishment where the dryness of existing insulation cannot be established reliably.
Yes. A terrace, green roof or solar array can be built on it, but this must be allowed for at design stage — structural loading and the protective layers above the waterproofing follow from it.
