# Tapered Insulation

> Roof insulation boards cut with a built-in slope and laid in a planned sequence to create falls toward drains on a level deck.

- URL: https://roof.wiki/tapered-insulation/
- Topic: Low-Slope & Flat Roof Systems
- Also known as: tapered roof insulation; tapered scheme; tapered system
- Regions: Global
- First published: 2026-11-02 · Last reviewed: 2026-11-02
- Cite as: "Tapered Insulation." roof.wiki. https://roof.wiki/tapered-insulation/

## Key facts

- **Common tapers (N. America):** 1/8 and 1/4 in per ft (about 1% and 2%)
- **UK target:** At least 1:80 finished fall
- **Materials:** Polyiso, EPS, XPS, mineral wool, cellular glass
- **UK minimum U-value at any point:** 0.35 W/m2K

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.

## Sources

1. Falls and drainage — Bauder. https://www.bauder.co.uk/roof-systems/refurbishment-projects/solving-the-challenge/improving-drainage-falls
2. Tech Today: roof slope and positive drainage (M. Graham, March 2005) — Professional Roofing / NRCA. https://professionalroofing.net/articles/article/615
3. BS 6229:2025 - The key changes and implications for flat roof design — Bauder. https://www.bauder.co.uk/technical-centre/standards-and-knowledge/bs-6229-2025-key-changes
4. How to avoid ponding on flat roofs — Soprema UK via SpecifiedBy. https://www.specifiedby.com/soprema-uk/news/how-to-avoid-ponding-on-flat-roofs
5. Rules for long-lasting BUR systems (H. H. Pierce, Interface, Oct 2006) — IIBEC. https://iibec.org/wp-content/uploads/2006-10-Pierce.pdf