What fall should a flat roof have?

The minimum recommended fall for a flat roof is 1:40 (25mm per metre). This is the standard specified in BS 6229. A 1:60 fall is sometimes used, and 1:80 is the absolute minimum. Steeper falls drain more effectively.

What are firring strips?

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.

Can I use tapered insulation instead of firring strips?

Yes. Tapered insulation boards (typically PIR) are manufactured with a built-in fall and eliminate the need for firring strips. They’re more expensive but provide insulation and fall in one layer.

How do I calculate the fall for a 4m flat roof?

At 1:40 fall, a 4m roof needs 4000mm ÷ 40 = 100mm total fall. The firring strip at the high end would be 100mm taller than at the low end. Add your minimum insulation thickness to both.

Which direction should the fall go?

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.

TradeCalculator

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.

Roof Length

Length of the flat roof

Roof Width

Width of the flat roof

Fall Ratio

1:40 is recommended minimum per BS 6229

Fall Direction

Direction water should flow

Price Per Firring Strip (optional)

How We Calculate This

This calculator computes the firring strip heights needed to create the correct fall across your flat roof.

The formula

Total fall (mm) = Fall length (mm) ÷ Fall ratio

Fall per metre = 1000 ÷ Ratio (e.g. 1000 ÷ 40 = 25mm/m)

Max strip height = Min insulation thickness + Total fall

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.