Innovation
Tapered insulation — engineered roof falls under the membrane
The drainage fall is engineered into the insulation itself, then welded over with sheet membrane.
Instead of chasing falls with sand-cement screed, the drainage slope is built into the thermal insulation. Flat fill boards level the deck first, tapered boards then form the falls, crickets and valleys that drive water to the drains — all under a PVC / TPO sheet membrane.
The detail that decides the outcome is the base layer. The first layer is flat board, laid across the whole deck to take up substrate deviation; only the second layer is tapered, and that is what forms the fall. Without the flat levelling layer, deck deviation carries straight through into the finished surface and the designed fall is no longer achieved on site — so the set-out has to start from a level base.
Why it matters
- Where a roof needs thermal insulation anyway, one engineered layer can deliver both the R-value and the drainage fall — a single set-out instead of two separate trades.
- The falls are designed and cut in the factory to a drainage layout, so the achieved slope is documented rather than judged by eye on site.
- Tapered insulation is far lighter than sand-cement screed to falls, which matters on remedial work where the existing structure sets the load limit.
- Positive drainage to every outlet is what keeps water off the membrane long-term — ponding is a leading cause of premature roof failure.
How it's built up
- 1Vapour control layer to the prepared structural deck, sealed at upstands and penetrations.
- 2Flat fill (base) layer across the entire deck — takes up substrate deviation so the fall starts from a level plane.
- 3Tapered boards over the flat base, laid to the manufacturer’s shop drawing to form the designed falls.
- 4Crickets and valleys cut on the diagonal between outlets so every part of the roof drains to an outlet.
- 5Sheet membrane (PVC / TPO) heat-welded over the completed insulation falls.
- 1 · Structural deck (concrete slab or metal deck)
- 2 · Vapour control layer
- 3 · Flat fill insulation — levels out deck deviation
- 4 · Tapered insulation — forms the designed fall
- 5 · Sheet membrane (PVC / TPO), hot-air welded
Visuals to add
PHOTO — flat base layer laid out across a full deck (levelling pass).
PHOTO — tapered boards set out forming falls + a cricket diverting water to a drain.
PHOTO — finished PVC/TPO membrane welded over the tapered falls, water shedding to drains.
Standards & references
- • AS 4654.2 (external above-ground waterproofing) is the governing Australian standard: the surface must drain, with falls set out to the outlets in accordance with the standard and the project specification — no ponding on the membrane.
- • BS 6229 (UK flat roofs), widely used as reference practice for tapered schemes: the finished fall on a completed flat roof, internal gutters included, should be at least 1:80, and the design fall is set higher than that so deflection and site tolerance are absorbed (long the 1:40 rule of thumb; the current revision derives it from structural analysis instead).
- • Tapered layouts are supplied as a shop-drawn scheme by the insulation manufacturer, with board thicknesses, crickets and cut lines set out per roof — we install to that drawing and the membrane manufacturer’s system requirements.