- Insulation between timber joists without proper vapour control, causing condensation in the roof
- Too little fall, so water ponds on the roof
- Extra weight from ballast or a roof terrace that the structure was not designed for
- Insulation that does not connect at the roof edge and upstands
Building detail · Roof
Insulating a flat roof: warm roof or inverted roof
A flat roof is usually insulated in one of two ways: as a warm roof, with the insulation under the roofing, or as an inverted roof, with compression-resistant insulation on top of the roofing. Which build-up fits depends on the existing roof and its use.
- Two build-ups compared side by side
- The order of the layers decides how moisture behaves
- Fall and load-bearing capacity are part of the choice
Principle sketch
Build-up of the detail
Two build-ups of an insulated flat roof side by side. The numbers in the sketch refer to these parts:
- Roofing
- Insulation
- Vapour control layer
- Roof deck
- Joists
- Ballast (gravel or tiles)
- Compression-resistant insulation on top of the roofing
- Roofing under the insulation
- Structural deck
Common mistakes
Where it often goes wrong
Leaks, condensation and cracks usually start at the junctions, not in the middle of a wall or roof.
In the drawing
What we look out for
We work out the detail for your home, based on a site survey and the chosen materials.
- The existing roof build-up and the condition of the roofing
- The position of the vapour control layer
- Fall and rainwater drainage
- Load-bearing capacity for ballast, a roof terrace or solar panels
- Connections at roof edges, upstands and roof lights
A renovation often replaces the existing roofing: the right moment to improve the insulation as well. Sustainable renovation
Detail FAQ
Frequently asked questions about this detail
What is the difference between a warm roof and an inverted roof?
In a warm roof the insulation lies under the roofing. In an inverted roof, compression-resistant insulation lies on top of the roofing, covered with ballast.
Can I insulate a flat roof from the inside?
It is possible, but it needs careful vapour control to prevent condensation in the construction. Insulating on top is usually simpler in terms of building physics.
Is an inverted roof suitable for a timber roof?
That depends on the load-bearing capacity, because ballast adds weight. A structural check is part of that choice.
Can I use this principle sketch for my own home?
The principle sketch shows the build-up, but it is not a working drawing. Dimensions, materials and fixings depend on your home. We work out the detail for your situation.
Detail library