Building envelope · Roof Assemblies
Warm Deck Flat Roof Assembly
A reference account of how a warm deck flat roof is ordered - deck, vapour control, insulation, falls and exposed weathering - why that order is the whole idea rather than a preference, and what an owner should raise before the build-up is fixed.
Educational reference entry. How this system behaves depends on climate, jurisdiction, loading, substrate, the adjacent systems it meets, how the building is used, the manufacturer's instructions and qualified professional design.
Overview
What warm flat roof is
A warm deck flat roof is defined by a sequence rather than by a covering. The structural deck is built first, a vapour control layer is laid over it, the insulation goes above that, and the waterproofing is applied over the insulation and left exposed to the weather. The structure, and the ceiling below it, therefore sit inside the insulated envelope and run close to internal temperature.
The boundary against the inverted roof assembly is settled by asking which side of the insulation the waterproofing is on. Here the membrane is the outermost layer: rain lands on it, sunlight falls on it, and it can be walked over and looked at. In an inverted roof the membrane is buried under the insulation and rainwater reaches it from beneath, so none of it is visible once the roof is finished.
Falls belong to the build-up rather than to the structure. A deck may be built falling, or the fall may be formed within the thermal layer, and that choice decides whether the insulation runs at a constant depth or varies across the roof. Where the fall is formed in the insulation, the outlet layout and the insulation layout become the same drawing.
Because the waterproofing is carried on a compressible layer rather than bonded to a solid deck, it is held down differently from a membrane stuck to a rigid substrate. Wind acts on the outermost surface but has to be resisted through everything beneath it, so how the insulation is restrained and how the membrane is held to the insulation are aspects of one question.
Terminology
Common names and aliases
These names describe the same assembly. The encyclopedia keeps one entry per system so that a trade term or a regional name never becomes a second, thinner page.
- warm deck flat roof
- warm deck roof
- compact flat roof
- unventilated flat roof
- insulation-above-deck flat roof
- warm roof build-up
Purpose
What this system is intended to do
The job the assembly exists to perform, conceptually. An intention is not a guarantee that any particular build achieves it.
- Keep the structural deck and the rooms below it on the warm side of the thermal layer, away from the swing of outside temperature.
- Place a continuous vapour control layer between the internal air and the coldest part of the build-up.
- Present a single exposed weathering surface that sheds water towards designed outlets.
- Carry the fall within the assembly itself where the structure beneath has been built level.
- Hold every layer down against wind action through a restraint path that reaches back to the structure.
Composition
The roles this system is made of
What each part does in the assembly and what it depends on — never a product, a thickness, a fixing or a determination that anything is adequate for the role.
Order is meaningful in this system. The roles below sit in sequence, and moving one relative to another changes what the assembly does. The sequence is conceptual only: it states no thickness, no dimension, no fixing and no order of work on site.
- Layer 1 of 8. Order is meaningful.Primary supportStructural deck
The plate that carries everything above it and hands the load back to the frame or walls. Its material governs how the layers over it can be attached, whether it can be built falling, and how much movement the waterproofing above will have to follow.
- Layer 2 of 8. Order is meaningful.Control layerVapour control layer
The layer laid on the deck, beneath the insulation, that limits how much internal moisture travels up into the build-up. It sits on the warm side of the temperature drop by design, and its worth lies in being unbroken rather than in being heavy.
- Layer 3 of 8. Order is meaningful.Thermal layerInsulation above the deck
The boards that place the whole structure inside the envelope. They also become the substrate the waterproofing is laid on, so their surface, their board edges and how closely they are butted matter to the layer above as much as to the thermal line.
- Layer 4 of 8. Order is meaningful.Drainage planeFalls-forming layer
The tapered arrangement, whether formed in the insulation or in the deck, that gives water a direction. It is what turns a level surface into a drained one, and it is set out from the outlets backwards rather than from the perimeter forwards.
- Layer 5 of 8. Order is meaningful.Finish surfaceExposed weathering membrane
The outermost layer, which takes rain, sunlight, occasional maintenance traffic and the full temperature range the roof experiences. Watertightness is a property of the completed, correctly detailed and correctly installed assembly, not of this layer alone.
- Layer 6 of 8. Order is meaningful.AttachmentRestraint against wind action
The adhesive, mechanical fixings or applied weight holding the insulation to the deck and the membrane to the insulation. It works as a chain: the outermost layer is only as secure as the layer it is fastened to, and as that layer's own hold on the deck.
- Layer 7 of 8. Order is meaningful.Edge and terminationPerimeter upstands and terminations
Every place the horizontal build-up stops and turns: parapets, verges, door thresholds and gutter edges. These are where the vapour layer, the insulation and the membrane each have to be brought to a defined end, and brought there together.
- Layer 8 of 8. Order is meaningful.Jointing and sealingLaps, seams and penetration collars
The joints between membrane sheets and the collars formed around pipes, supports and outlets. They are the parts of the weathering layer made by hand on site, and they are the point at which the field of the roof stops being a field.
Interaction
How the parts work together
The reason this is a system rather than a list of parts: what depends on what, and what stops working when one part is changed.
The vapour control layer earns its name only in relation to what sits above it. Placed beneath the insulation, it is on the warm side of the temperature drop, so internal moisture is held back before it can reach a surface cold enough for it to condense on. Lift the same material above the insulation and it becomes a cold surface for that air to arrive at instead.
Gaps between insulation boards are not local losses of thermal performance. They open a route for air to move on the underside of the membrane, which puts the coldest surface in the build-up in contact with air that came from inside the building. Continuity of the thermal layer and continuity of the vapour layer are one condition, not two separate checks.
A fixing that holds the insulation down passes through the vapour control layer to reach the deck, so the layer that is meant to be continuous is deliberately perforated by the thing keeping the roof on. That trade is why deck type, restraint method and vapour control are settled together, and why swapping one of them on site is not a small substitution.
Falls, outlet positions and upstand heights behave as a set. Water that cannot reach an outlet stands somewhere, and where it stands is decided by the taper formed in the build-up rather than by the drainage components. Move an outlet after the tapered layout is drawn and the thermal layer has been redesigned at the same moment.
Materials
Material families commonly met in each role
Commonly encountered, not recommended. Whether a material suits a given project depends on the whole assembly, the exposure, the manufacturer's documentation and qualified professional review.
Primary support
These deck families are commonly encountered beneath warm deck build-ups. Which of them can receive a given vapour layer, insulation or membrane, and by what means, is a matter for the manufacturer's documentation and the designer rather than for a general reference.
Control layer
Sheet vapour control products, and the tapes used to close their laps and returns, are commonly encountered in this position. Whether a particular combination belongs in a given assembly is read from the product documentation and settled by the designer.
Thermal layer
Rigid and semi-rigid boards from these families are commonly encountered above the deck, including in tapered form. Compatibility with the membrane above and the vapour layer below is a documentation question rather than a property of the family.
Finish surface
These weathering families are commonly encountered as the exposed layer of a warm deck. Each carries its own requirements for the substrate beneath it and for how it is terminated, and those come from the manufacturer and the roof designer.
Edge and termination
Trims, drip edges and flashings at the perimeter are commonly formed in these metals. Which metal can sit against which membrane and which fixings, without one attacking the other, belongs with the product documentation and the designer.
Junctions
Where this system meets others
Interfaces are where most assemblies actually fail, so they are set out explicitly rather than left inside the prose. What resolves a junction is a detail designed for the specific building — not a rule of thumb.
Parapet and upstand termination
The horizontal build-up has to turn up the inner face of the parapet and stop somewhere defined. The height that turn reaches, and whether the insulation and vapour layer turn with the membrane or stop short of it, decides whether the perimeter is part of the envelope or a gap in it.
Read about Parapet System →Outlets and rainwater removal
An outlet is a hole made deliberately through every layer of the roof at the point water is being directed towards. It has to be connected to the membrane, passed through the insulation and the vapour layer, and held clear of debris, which is why outlet positions are agreed with the falls rather than after them.
Read about Roof Rainwater Drainage →Pipes, ducts and support feet
Anything standing on or passing through the roof interrupts the membrane, the insulation and the vapour layer at once. Grouping penetrations, and keeping them away from upstands and low points, is easier to detail than sealing each one where it happens to land.
Read about Penetration Sealing →Rooflight kerbs
An opening in the deck turns the field of the roof into a perimeter around a hole. The kerb has to receive the membrane, carry the insulation up its outside face and let the vapour layer be closed against it, all before the rooflight itself is considered.
Read about Rooflight Kerb →Surfacing laid over the membrane
Where the roof is also walked on, the wear surface and its supports sit above the exposed membrane and conceal it. The layer order below is unchanged, but the membrane stops being visible and the outlets stop being reachable without lifting something.
Read about Trafficked Roof Deck →Rooftop plant and equipment supports
Equipment mounted on a warm deck applies point loads onto a compressible thermal layer and needs access around it for servicing. Whether load is spread, how the support meets the membrane and how maintenance routes are kept clear of laps are questions for the designer.
Considerations
Topics worth discussing
Which topics genuinely apply to this system and what to raise about them. Measured values, classes and ratings come from a qualified professional's design for the specific building, not from a reference page.
- Moisture
- Every layer of this build-up affects where moisture can arrive and whether it can leave. Whether interstitial condensation is a risk in a given roof depends on the whole build-up, the internal conditions and the climate, and is assessed by a qualified professional rather than assumed from the layer order.
- Thermal
- The thermal line runs above the deck and has to continue somewhere at the perimeter, at upstands and around penetrations. Where it does not continue, the assembly has a bridge; what that means for a particular roof is a matter for the designer and not for a general statement.
- Movement
- The exposed membrane sees the widest temperature range in the assembly while the deck beneath it, being inside the envelope, sees very little. The layers therefore move differently, and how that difference is absorbed at laps, upstands and changes of direction is part of the design.
- Buildability
- The build-up is completed from below upwards, so each layer is covered by the next and cannot be revisited. Weather during the works, and how the roof is kept dry before the membrane closes it, therefore affect what ends up trapped inside the assembly.
- Maintenance and access
- The membrane is exposed, which is an advantage for inspection and a liability for damage. Access routes, protection where people walk, and keeping outlets clear are ongoing duties rather than one-off provisions, and belong in a maintenance discussion at design stage.
- Documentation
- The layer order, the restraint arrangement and the terminations are only recoverable later from drawings and product information. Recording what was actually built, including where fixings pass through the vapour layer, matters when the roof is altered or a penetration is added.
Design
Questions the design has to answer
The decisions this assembly turns on. They are questions rather than answers, because the answer depends on the building.
- Is the fall formed in the deck, formed in the thermal layer, or shared between them, and who is setting it out?
- Where does the vapour control layer terminate at each upstand, threshold and penetration, and what closes it there?
- How is the insulation held down, and does that method pass fixings through the vapour control layer?
- Are the outlet positions fixed before the tapered layout is drawn, or is the taper being drawn first and outlets placed afterwards?
- How does the thermal layer continue past the parapet, the wall head and any upstand without leaving a break?
- Will anything stand on this roof, and if so where do its supports meet the membrane?
- Does the roof need to be walked on in normal use, which would move part of this design into the trafficked deck entry?
Boundaries
Commonly misunderstood points
Distinctions that are easy to blur, and the places where a familiar term means something narrower than it sounds.
- The name refers to the position of the insulation relative to the deck, not to a covering type or to how warm the roof feels.
- A vapour control layer is not a second waterproofing layer, and a roof that leaks through the membrane is not repaired by relying on it.
- Tapered insulation is not a drainage product added to a roof; it is the thermal layer doing the additional job of giving water a direction.
- A membrane that looks sound from above says nothing about whether the insulation beneath it is dry or still fully restrained.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- What condensation assessment has been carried out for this build-up, and what internal conditions was it based on?
- How has the wind restraint of the insulation and the membrane been established for this particular deck?
- What is the intended termination detail where the roof meets the parapet, and how does it deal with the insulation?
- How will water reach the outlets from every part of this roof, including the corners furthest from them?
- Which layers are penetrated by the fixings holding the build-up down, and how is that accounted for?
- What provision is being made for future penetrations so that they do not have to be improvised into a finished roof?
- Who is responsible for setting out the tapered layout, and how is it coordinated with the drainage design?
What this page does not do
- This entry describes how the assembly is organised. It is not a specification, a build-up for a particular building, or a substitute for design by a qualified professional.
- No thermal, fire or watertightness performance is stated here; those are properties of complete, tested and correctly installed assemblies in a specific context.
- Whether this layer order is appropriate for a given roof depends on the deck, the internal use, the climate and the jurisdiction, and is a design determination.
- Altering a completed warm deck, including adding a penetration or a mounted item, affects layers that cannot be seen and should be taken to the roof designer.
Related systems
How this system relates to others
Every link states what the relationship actually is, rather than leaving a bare list of related pages to be read as a suggestion.
Alternatives
Different systems answering the same need. Listing them together is not a comparison and does not suggest one is better — which, if either, suits a project is a design decision.
Meets these systems
Systems this one physically meets. The junction is usually where the design problem lives, so these are worth reading together.
Commonly built alongside
Systems routinely present in the same building without necessarily touching this one.
Go deeper
Related Build Design Hub guides
Planning guidance behind the decisions this assembly involves.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this system comes up.
Inspiration
Related Ideas Library pages
Design directions where this system commonly appears.
Roof Assembly Systems
Roofs answering two questions at once: where the insulation sits relative to the deck, and how the water-shedding layer achieves continuity across the whole surface.
Browse all roof assemblies entries →