Flat Roof Fall Calculator: Firring Sizes for a 1:40 Fall

A flat roof is never flat: it is built with a slight slope, the fall, so rainwater runs to the outlet instead of sitting on the covering. The fall is normally created with tapered timber firrings over the joists, and getting their depths right is the difference between a roof that drains and one that ponds. Enter your run below for the total fall in millimetres, the firring sizes and the timber to order.

How much fall does a flat roof need?

British Standard 6229, the code of practice for flat roofs, sets a minimum finished fall of 1:80 and recommends designing to 1:40 to achieve it. The gap between the two figures is deliberate. Joists deflect once they carry the deck, the covering and a person walking on them, and site tolerances eat into the slope as well, so a roof laid at exactly 1:80 ends up with sections that are level or falling the wrong way. Designing at 1:40 gives you double the fall you need, which is why almost every specification you will see for a domestic extension or garage roof says 1:40.

In plain numbers, 1:40 means 25 mm of drop for every metre of run. A 4 m deck falling to a gutter along one edge therefore drops 100 mm from the high side to the low side.

Turning the fall into firring sizes

Firrings are the tapered lengths of timber laid on top of the joists to create the slope. Their thin end is normally 25 mm, partly so there is something to fix into and partly because a knife edge splits when it is nailed. The deep end is simply the thin end plus the total fall, so the 4 m roof above needs firrings running from 25 mm to 125 mm. At 400 mm centres across a 3 m width that is nine firrings of 4 m each, 36 m of timber before waste, cut from stock at least 125 mm deep.

Where the joists run across the fall instead, you cannot taper each joist. The usual answer is cross firrings: lengths of constant depth laid over the joists at 400 or 600 mm centres, each one deeper than the last. At 400 mm centres and a 1:40 fall, each row steps up by 10 mm. The calculator gives you the shallowest, the deepest and the step so you can rip them all from one board.

Which direction should the fall run?

Towards the drainage, and away from the house. Falling a roof back towards a wall or an abutment pushes water at the flashing, which is the weakest point of any flat roof. If the run is long enough that the deep end gets silly, fall the roof two ways to a central outlet instead, which halves the run and halves the timber depth. Standard sawn depths stop at 225 mm, so a single-direction fall much over 8 m at 1:40 stops being practical in timber.

The other check is height. Adding 125 mm of firring plus the deck and covering raises the finished roof level, and roofers normally want around 150 mm of upstand at any abutment, plus clearance below a door threshold or a damp-proof course. Work this out before you order, not after the joists are up.

Firrings or tapered insulation?

On a warm deck build-up, where the insulation sits above the deck, the fall can be built into tapered insulation boards rather than the timber. That is the normal route on larger commercial roofs and on recover jobs where the existing deck is level and cannot easily be lifted. It costs more per square metre than firrings but avoids rebuilding the structure, and it is often the cure when an old roof ponds because it was never given enough fall. On a small domestic roof being built from scratch, firrings are cheaper.

Whichever way the fall is formed, the covering still has to suit it. Both EPDM rubber and GRP fibreglass are laid on a falling deck, and you can compare what each costs on the flat roof cost comparison calculator.

Common questions

What is the minimum fall for a flat roof?

1:80 as a finished fall, which is 12.5 mm per metre. Because joists deflect and site work is never perfect, roofs are designed at 1:40 (25 mm per metre) so the finished result still clears the minimum.

Can a flat roof be laid completely level?

It should not be. Water that sits on the surface for days accelerates the breakdown of the covering and finds any weak joint, and most manufacturer guarantees expect a designed fall. A level deck is the single most common reason a flat roof ponds.

Do I still need firrings if I use tapered insulation?

No. On a warm deck the taper is in the insulation boards, so the joists and deck stay level and the fall is formed above them. You cannot double up: pick one method.

How do I add fall to an existing flat roof?

Either strip the covering and deck and fit firrings over the existing joists, or leave the deck alone and lay tapered insulation on top as part of a warm deck recover. The first is cheaper in materials, the second is far less disruptive and adds insulation at the same time. Both are re-covering jobs rather than repairs, so price them alongside a full flat roof replacement.