The short answer
Tapered insulation is rigid insulation board made in wedge-shaped pieces, laid on a flat roof so the surface slopes gently towards the drain. It gives the roof its fall and its thermal performance in one layer. Under BS 6229:2025, the finished roof keeps a minimum 1:80 fall, and the boards are designed to a steeper 1:40.
Water will not leave a roof that is dead level. Tapered boards build the gentle slope it needs into the insulation layer itself.
How tapered boards make a slope
Most flat roof decks are laid level or with only a slight tilt. Tapered insulation adds the fall on top of the deck. Each board is thicker at one edge than the other, and boards of different thickness are laid together in a pattern, following a plan drawn for the roof, so that the finished surface runs downhill towards an outlet or a gutter.
Why not just tilt the joists?
You can, and for new builds designers often do. On a replacement, the joists are already in place, and tilting them means altering the structure and perhaps the walls and ceilings below. Tapered boards avoid that. The deck stays level, the insulation does two jobs, and the plan can be drawn to push water into a valley between boards, towards a single outlet or to a gutter at the edge.
The boards come cut and labelled, so the crew lays them in sequence, rather like a jigsaw. A manufacturer supplies the layout for the exact roof shape, which is why measurements taken at survey have to be accurate. A wrong dimension can leave a low spot, and a low spot holds water.
The 1:80 finished fall
A fall is written as a ratio. BS 6229:2025 is the British Standard for flat roofs whose flexible waterproof covering is fully supported, meaning roofs pitched at no more than 10 degrees. The 2018 edition was withdrawn on 16 December 2025.
How much slope does the roof need?
The standard retains 1:80 as the finished minimum across general roof areas and internal gutters. In plain figures, 1:80 means the surface drops 12.5 mm over each metre of run. Manufacturer guidance, such as IKO's, is that the roof should be designed for 1:40, which is 25 mm per metre, because deflection of the deck, tolerances in the build and the thickness of the covering all eat into the slope. Design to 1:40 and the finished roof still reaches 1:80 where it matters.
Tapered boards are the most reliable way to reach those numbers over a level deck, since the slope is built into the layer and can be set out and checked. A roof laid with no fall at all will pond, however good the covering. Our page on flat roof minimum fall goes through why ponding matters and how it is measured.
BS 6229 covers flexible membranes, and does not apply to lead.
Insulation, Part L and the build-up
Tapered boards usually go in a warm roof, where the insulation lies above the deck and the waterproof covering is laid over it. The boards must also meet the thermal target, which is why the thin end matters as much as the thick one.
Does tapered insulation meet the insulation rules?
The thickness at its thinnest point still has to do its share. Approved Document L classes renewed flat roof waterproofing as renovation, and for renovation Table 4.3 sets a 0.35 threshold and a 0.16 improved target, with the 15-year payback relaxation at paragraph 4.13. A designer works out the average thermal performance across the whole slope, and building control confirms how it is read, so the plan has to show those numbers.
The thick end of the wedge raises the finished surface in one corner of the roof. That affects upstand heights, door thresholds and the height of window cills, and with a large roof the rise can be considerable. It is worth checking at the survey, and with the planning team if the outline changes. See insulating a flat roof when replacing it for the figures and the 2027 change in edition.
What to ask before agreeing to it
Tapered insulation is a specified system, not a loose heap of offcuts. A quote that includes it should be able to say how the falls were worked out.
Is tapered insulation worth the extra?
On a roof that is level now and ponds, it is often the only way to cure the problem without altering the structure. It also takes out the guesswork of laying screed or cutting wedges by hand. On a roof that already drains well, a flat board system may be enough, so it is not automatically the answer.
Ask four things. What fall does the layout give at the lowest point? Where does the water go: gutter, outlet or valley? How thick is the board at the thinnest point, and does it meet the insulation target? And how will the board be fixed and protected from foot traffic before the covering goes on? A clear reply to each is a sign the work has been planned.
The roof is measured, the deck is photographed for you and the layout is set out before any price is given, and the written price covers the finished slope. The main flat roofing page lists the systems we lay, or you can read about warm and cold roofs.
More on tapered boards
Can a roof with ponding water be fixed with tapered boards?
Often, once the cause is known. Ponding comes from a deck that is level, sagging or poorly drained. If the deck is sound, new tapered boards can raise the surface and send water to the outlet. If the deck has sagged badly, repairing the structure comes first.
Do tapered boards work under any waterproof covering?
They are designed for use under approved single-ply and built-up systems, but compatibility depends on the manufacturer. Some adhesives attack some foams, and some coverings need a specific facing, so the covering and the board are chosen together from the data sheets.
The full picture: Flat roofing in Taunton ยท More short answers