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Flat Roof Fall Calculator — Firring Strips & Tapered Insulation
Calculate firring strip dimensions for flat roof falls at 1:40, 1:60 or 1:80 gradients. Includes strip spacing and tapered insulation.
Length of the flat roof
Width of the flat roof
Firring strip centres, set by the joist spacing (default 400mm)
Timber left at the thin (drainage) end of the firring - feather edge ~0–25mm. The high end equals this plus the total fall
Insulation laid over the deck/firrings (a separate layer). Leave 0 for a cold/uninsulated build-up. BS 6229:2025 thermal limit ≈ 0.35 W/m²K
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How We Calculate This
This calculator works out the firring depths and number of firring lines needed to create the correct fall across your flat roof, to BS 6229:2025. A firring is a tapered timber batten fixed along each joist; insulation (if any) is a separate layer laid over the deck or firrings.
The formulas
Total fall (mm) = Fall length (mm) ÷ Fall ratio
Fall per metre = 1000 ÷ Ratio (e.g. 1000 ÷ 60 ≈ 16.7mm/m, 1000 ÷ 40 = 25mm/m)
Firring depth: low end = nominal (feather) depth; high end = nominal depth + Total fall
Build-up above deck = Firring depth + Insulation thickness (firring and insulation are separate layers)
Firring lines = ceil(cross dimension ÷ spacing) + 1 (one per joist line)
BS 6229:2025 sets the minimum finished fall at 1:80; 1:60 is the recognised design fall for many roofs (and the minimum for roofs ≤ 50 m² and for tapered insulation); 1:40 is the conservative design fall where deck deflection is not proven within tolerance.
Frequently Asked Questions
BS 6229:2025 (which superseded BS 6229:2018 on 31 December 2025) sets the minimum FINISHED fall for most flat roofs at 1:80 (12.5mm per metre). It removed the old blanket “design at 1:40 to achieve 1:80” rule. A design fall of 1:60 (16.7mm per metre) is now the recognised starting point in many cases, and the minimum aim for small roofs (≤ 50 m²). A steeper design fall of 1:40 (25mm per metre) is used only where structural analysis cannot show the deck stays flat within tolerance under load. Backfalls (water running away from the outlet) are never acceptable.
Firring strips are tapered timber battens fixed on top of flat joists to create a fall. They are widest at the high point and thinnest at the low point (drainage edge). They’re typically spaced at 400mm centres.
Yes. Tapered insulation boards (typically PIR) are manufactured with a built-in fall and eliminate the need for firring strips, providing insulation and the fall in one layer. The scheme must be designed by the board manufacturer: BS 6229:2025 and NHBC Chapter 7.1 require a design fall of not less than 1:60 for tapered insulation. They are more expensive than timber firrings but avoid building the fall on site. Treat the area figure here as a starting point — the ~5% cutting allowance is typical, but confirm the exact board count against the manufacturer’s tapered scheme.
Total fall = fall length ÷ fall ratio. At a 1:60 design fall, a 4m roof needs 4000mm ÷ 60 = 67mm of fall (at 1:40 it would be 4000mm ÷ 40 = 100mm). The firring is the timber that creates this fall: it runs from a thin feather edge at the low (drainage) end up to that edge plus the total fall at the high end. Insulation is a separate layer laid over the deck or firrings — it is not part of the firring depth, so add it on top when working out the total build-up height above the deck.
The fall should direct water towards the gutter or drainage outlet. Typically this is along the shorter dimension to minimise the total fall height needed. For roofs with a parapet on three sides, fall towards the open edge. Once you have the fall sorted, use our Flat Roof Membrane Calculator to work out membrane quantities.
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Last updated: June 2026
Verified against UK standards · estimates only, confirm with your supplier.