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Are Flat Roofs Really Flat?

No flat roofs are not really flat. All flat roofs should have a slight angle to allow for the drainage of rainwater. A flat roof should have a minimum fall of 1:80 (0.72⁰). When constructing a flat roof, all construction inaccuracies must be accounted for. If this is not done, issues such as the pooling of water on the roof can occur.

Often a fall of 1:40 (1.40⁰) or 1:60 (1.00⁰) is planned for when designing a flat roof. The reason for this is to assume a fall of at least 1:80 will be present once the roof is ready to install. However, with screeded flat roofs this can be problematic as it can lead to a higher price. The reason for this is more screed will be required than planned for. As much as double as the planned screed could be required as the highest points. Obviously this will cause problems with the roofing system being thicker than planned for as well as being more expensive.

With a screeded flat roof, an alternative approach the designer may take, after considering the accuracy and deflection of the roof is as follows: take 1:80 as a finished fall and adds an arbitrary adjustment for inaccuracies. Such figures could be 25mm for concrete substrates and 15mm for metal ones.

Deck Deflection When Planning A Flat Roof

Once a fall of 1:80 is in place, the effects of deck deflection should be considered. Deck deflection may have a positive or negative effect on drainage flow 1. Rainwater outlets which are positioned in the central area of the roof may be positioned or near the point of maximum deflection of the deck and would therefore assist drainage flow. However, in practice there is usually a requirement to place internal downpipes closes to walls or columns. The reason for this is to provide protection and support. This means the downpipes will be positioned away from the natural low point of roof deflection. Under these circumstances, the effect of mid-span deflection will be to reduce the fall to the outlet. This should be taken into account when calculating the design fall.

When allowing for these deflections, in most circumstances it should not prove necessary to allow  for deflections from imposed loads on the roof. The falls will ensure there is no significant load from standing water, and it is only necessary to take account of the dead load deflection.

Assuming the deck takes a circular shape when deflecting, a reverse fall will be avoided entirely by raising one end of the deck by four time he defection. A typical deflection for a metal deck under dead load is span/650 in which case as additional fall of 4/650 will compensate for deflection adverse to drainage. Some decks however are so stiff their deflection due to dead load can be ignored.

When the falls are provided by screeding, the deck deflection will be taken out by the application of the screed and no allowance need be made for deflection. Where deflection is favourable to drainage, it should only be necessary to include an allowance for construction inaccuracies. The design fall could be reduced in line with the anticipated deflection but this would not be wise unless the designed is confident the dead load deflection can be accurately predicted and the construction can be completed within design tolerances.

Cross Falls On A Flat Roof

A cross fall is formed at the junction where two roof surfaces with a different fall meet. The effective slope of a cross fall will be less than the slope of the main fall. Designers often favour a slope of 1:80 for the cross fall, which produces a steeper slope for the main fall. On a square flat roof this slope for the main fall would be 1:56. The problem with this approach is a higher cost. The reason for this is more screed will be needed and an increased parapet height to accommodate tis increased screed depth. The alternative approach to this situation is to accept the cross fall will be slightly less, but will not impair the overall efficiency of the flat roofs drainage.

Frequently Asked Questions About The Slope Of A Flat Roof

What Is The Minimum Slope For A Flat Roof?

The minimum slope for a flat roof should be 1:80 (0.72⁰).

Does A Flat Roof Need A Slope?

Yes, flat roofs need a slope. Although a flat roof appears to be level and is called a flat roof, it needs a slight slope of at least 1:80 (0.72⁰), to allow for the drainage of water.

How Do You Increase The Slope Of A Flat Roof?

The slope of a flat roof can be increased in a number of ways. Using firrings 2 is one way. If this is done a waterproofing membrane would have to be applied over the firrings. Another way, is to strip back your roof and to use a screed to elevate the slop and reinstall a new roofing system.

Do You Have A Question About The Slope Of A Flat Roof?

If you would like RJ Evans to provide any information related to the slope of a flat roof please contact us or call us now on 01277 375 511. One of our friendly flat roofing specialists will be delighted to help.