What Is Tapered Insulation?

Why a flat roof needs a fall

A truly flat roof does not exist in practice. Water must be able to leave, or it will sit in pools, load the structure, speed the ageing of the membrane and collect dirt, moss and algae. Many commercial buildings have a level concrete or steel deck, and the slope has to be created on top of it. There are three ways to do this: build the fall into the structure, lay a sloping screed, or lay tapered insulation. The third is common because it combines two jobs in one layer and keeps weight down.

The roof must drain with some margin, because deflection and small errors at the deck, and later movement, all reduce the slope. A design that has no spare fall will pond in places. If you have standing water on an existing roof, our page on repair, overlay or replace explains how that affects the choice of work.

How the boards work

Each board is a wedge. Boards are made in a few slopes and in a range of thicknesses, and several boards laid in sequence build up a taper across the roof. A flat filler board, in a standard thickness, is used to make up the depth where the taper is not needed. The supplier works from a plan of the roof, with the outlets marked, and produces a layout drawing showing where each numbered board goes. The installer follows that drawing, board by board.

On a roof with a single direction of fall, the layout is simple. On a roof with outlets in the middle, the boards slope down towards each outlet from all sides, creating shallow valleys. Between two outlets, the roof forms a ridge, and behind a parapet or upstand, a cricket or saddle is used to push water around the obstacle instead of letting it pool behind it. The corners and valleys are the trickiest parts, because the boards must be cut to fit.

What goes under and over the boards

Beneath the insulation lies the vapour control layer, which prevents moist air from reaching the boards. The boards are bonded or mechanically fixed to the deck, following the maker's pattern for wind uplift. Over them goes the waterproof covering: single-ply, built-up felt, liquid-applied or asphalt. The covering needs to be compatible with the board facing, and the maker will confirm this. Our guide to comparing commercial flat roof systems describes each covering.

Thickness is set by the insulation target. A roof is a thermal element, and when a roof covering is renewed, the insulation has to be considered under Approved Document L. The tapered boards are part of that calculation, so the thinnest point must still meet the target. Ask the building control body how it wants the U-value shown.

Outlets, crickets and overflows

The drainage layout sets the whole design. Each outlet drains a catchment area, and the designer checks that the outlets can pass the rainfall expected on that area. Where a flat roof is enclosed by parapets, an overflow is added so that if an outlet blocks, water escapes before it builds up. The fall toward each outlet should be clear, with no flat shelf round the outlet where silt can settle. Good design puts outlets away from the corners, where the boards are hardest to shape.

Tapered boards or screed

Screed is a sand and cement or lightweight mix laid to a slope. It is rigid and can be formed to any shape, but it is heavy, it takes time to dry and the moisture can be trapped under the membrane. Tapered boards are lighter, faster to lay and drier. Screed may still suit an unusual shape or a very thin build-up. Sloping timber firrings suit timber decks, but not usually large commercial roofs.

Tapered insulation does have limits. It adds thickness, which affects upstand heights and threshold levels, and a wrong layout, or a roof measured badly, means boards that do not fit. A site survey should come before the drawing is ordered. The boards must also stay dry, so deliveries are stored off the ground and covered until the day they are laid. Watch for the effect on rooflight kerbs and parapets, where the added depth can leave an upstand too short.

We survey the roof, check the levels and the outlets, and photograph the deck so you can see why a fall is needed. Then we give a fixed written quote that shows the insulation, the fall and the covering together. Related business work is listed on the commercial roofing in Basingstoke page.