Tapered Insulation

Tapered insulation is rigid insulation manufactured in boards of varying thickness so that, when assembled to a layout drawing, the top surface slopes toward drains, gutters or scuppers. It lets designers create falls on a level structural deck without sloping the structure or adding heavy screeds.

Materials

Tapered boards are made from polyiso, EPS, XPS, mineral wool, cellular glass and other insulants. Typical systems use a sequence of lettered boards, each increasing in thickness, with flat "fill" boards underneath where more height is needed.

Common slopes

• North America: 1/8 in per ft (about 1%) and 1/4 in per ft (about 2%) are standard tapers; the IBC requires 1/4:12 for most membranes on new roofs, and NRCA notes that a truly flat deck may drain adequately with a 1/8:12 taper.
• UK and Europe: schemes are designed to achieve at least a 1:80 finished fall, using design falls from 1:60 to 1:40 under BS 6229:2025.

Design elements

• Main falls run toward lines of drains or gutters.
• Crickets and saddles divert water between drains and around curbs, often at twice the main slope.
• Sumps of thinner insulation around outlets bring water into the drain.
• Hip and valley layouts create four-way slopes to point drains.

Thermal considerations

Because thickness varies, the thermal value varies too. North American energy codes allow an average R-value with minimum thickness limits; UK guidance requires that no point, including the thinnest areas at outlets and gutters, exceeds a U-value of 0.35 W/m2K on heated buildings.

Advantages

Tapered insulation is lightweight, quick to install, adjustable to existing decks in refurbishment, and works with single-ply, bituminous and liquid systems.