Falls and firrings
Check the fall of an existing flat roof
- where
- fall ratio, 1 in G
- horizontal distance between the points (m)
- measured drop between the points (mm)
- gradient (%)
Explain this equation
Checks the fall on an existing flat roof from a level reading. Use it when surveying a roof that ponds, or to confirm a new deck before the covering goes on.
h is the drop you measured between two points and L the level distance between them. Converting L to millimetres with the 1000 and dividing by h gives the 1 in G ratio. The gradient is the same thing as a percentage: drop over distance, times 100.
A fall is just rise over run; this is the rf-fall-depth calculation turned round.
Sense check: 50 mm over 4 m is 1 in 80, or 1.25%, which is right on the BS 6229 minimum. 100 mm over 4 m is 1 in 40.
Watch out: check at several points, especially mid-span and around outlets, because a deck can sag between joists and hold water even when the end-to-end fall looks fine.
Turns a measured drop over a distance (from a level, laser or straight-edge) into a fall ratio, to compare with the 1:80 minimum.
Enter your values to see the result.
Good to know
Measure along the line water will actually travel, including into gutters and outlets. Take several readings: mid-span joists often sag, so a fall that is 1:60 at the ends may be close to level in the middle of a bay. BS 6229 recommends a design fall of 1:40 to keep 1:80 or better in service.
Common mistake
Measuring only edge to edge. Mid-span deflection and old deck sag are where the ponds form.
Limits
Single straight line. Diagonal falls at valleys and hips are shallower than the main falls either side.
Sources
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow the roof covering manufacturer’s fixing specification, BS 5534, BS 8000-6, BS 6229, the Lead Sheet Training Academy guidance and Building Regulations. Wind uplift and structural design need a competent designer.