Quick answer: measure the horizontal roof run in the direction the water needs to travel, choose the required design fall, then divide the run by the fall denominator.
Required height difference (mm) = horizontal roof run (mm) ÷ fall denominator.
For example, a 3m run designed at 1:60 needs a theoretical height difference of 50mm: 3000 ÷ 60 = 50.
The important part is not to confuse the minimum finished fall with the fall you should automatically build into the firrings. Construction tolerances and roof deflection can reduce the finished fall, so the design needs enough allowance to still drain correctly when the roof is complete.
What does a 1:80, 1:60 or 1:40 roof fall mean?
A fall written as 1:80 means the roof drops 1 unit vertically for every 80 units horizontally. In practical terms, that means:
- 1:80 = 12.5mm drop per metre
- 1:60 = about 16.7mm drop per metre
- 1:40 = 25mm drop per metre
The smaller the second number, the steeper the roof. So 1:40 is steeper than 1:60, and 1:60 is steeper than 1:80.
Finished fall and design fall are not the same thing
This matters because the current flat-roof guidance changed with BS 6229:2025.
The National Federation of Roofing Contractors' summary of BS 6229:2025 states a minimum 1:80 finished fall for general roof areas and internal gutters.
The old blanket approach of simply designing every roof at 1:40 to achieve that 1:80 finished fall has been removed. Bauder's current guidance on BS 6229:2025 explains that the design fall should instead allow for the actual construction tolerances and structural deflection. For roofs below 50m², its guidance uses 1:60 as the minimum design fall; 1:60 is also a common starting point where the roof construction allows it.
That means 1:80 is useful for understanding the minimum finished fall, but it should not automatically be used as the firring slope for a new roof. The roofing-system manufacturer, project design and relevant building-control requirements still take priority.
Flat roof fall table
The table below shows the theoretical height difference across common roof runs. It does not include any extra allowance for structural deflection or construction tolerances.
| Horizontal roof run | 1:80 | 1:60 | 1:40 |
|---|---|---|---|
| 2.0m | 25mm | 33.3mm | 50mm |
| 2.4m | 30mm | 40mm | 60mm |
| 3.0m | 37.5mm | 50mm | 75mm |
| 3.6m | 45mm | 60mm | 90mm |
| 4.0m | 50mm | 66.7mm | 100mm |
| 4.8m | 60mm | 80mm | 120mm |
| 5.0m | 62.5mm | 83.3mm | 125mm |
Worked example: a 3m flat roof
Imagine the roof is 3m from the high side to the gutter.
At a 1:60 design fall:
3000mm ÷ 60 = 50mm
So the roof needs a theoretical 50mm difference in height over that 3m horizontal run before any additional allowance required by the roof design is considered.
At 1:80, the same 3m run would only produce 37.5mm of drop. At 1:40 it would produce 75mm.
Measure the run in the direction the water actually travels
Do not automatically use the longest dimension of the roof. The run is the horizontal distance from the high point to the drainage point.
A roof measuring 5m x 3m might only have a 3m fall if the gutter is along the 5m edge. The calculation therefore needs the 3m drainage run, not the 5m overall roof length.
How do you turn the calculation into a firring size?
Timber firrings are tapered strips fixed over the roof structure to create the fall. Once you know the required height difference, you can compare that with the actual taper of the firring over the distance you need.
If you know the actual taper and length of a firring, you can also calculate the fall it creates:
Fall denominator = firring length ÷ taper height.
For example, a firring that drops 60mm over 3600mm creates a 1:60 slope:
3600 ÷ 60 = 60
That calculation is why the actual dimensions matter. Product names such as 3-to-0 and 4-to-0 are useful ways of identifying the taper, but they should not be treated as an exact roof specification on their own. Check the actual taper and usable length against the fall your roof requires.
3-to-0 or 4-to-0 firrings?
A 4-to-0 firring has a deeper taper than a 3-to-0 firring, so over the same length it creates a steeper fall. That does not make the 4-to-0 automatically better.
The right choice depends on:
- the horizontal roof run;
- the design fall required;
- the actual taper and length of the firring;
- structural deflection and construction tolerances;
- where the roof needs to drain; and
- the waterproofing manufacturer's requirements.
We stock 3-to-0 timber firrings and 4-to-0 timber firrings in 3.6m and 4.8m lengths. Calculate the fall first, then select the firring that matches the roof rather than choosing by name alone.
Common flat-roof fall mistakes
- Using 1:80 as the automatic design fall: 1:80 is the minimum finished fall for general roof areas; the design needs to account for tolerances and deflection.
- Measuring the wrong roof dimension: calculate the run from the high point to the drainage point.
- Choosing 3-to-0 or 4-to-0 before doing the calculation: the product needs to suit the required fall, not the other way around.
- Treating the product name as an exact millimetre dimension: work from the actual taper and usable length.
- Ignoring deflection: the roof can flatten under its own construction and loading, reducing the completed fall.
- Creating a backfall: the overall roof and local details still need to direct water towards the intended gutter or outlet.
Planning the rest of the flat roof
The fall is only one part of the roof. For the full sequence from joists through to the EPDM covering, read How to Build a Timber Flat Roof.
If you are buying the complete job, our Build a Flat Roof collection brings together the structural timber, firrings, roof deck, PIR insulation, EPDM membrane and detailing products in the order they are used.
