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Flat Roof Drainage

The design of falls and drainage patterns will have a considerable influence on the depth of the total roof construction. The depth of the total roof construction is sometimes referred to as the roof zone. This should be a fundamental consideration when planning the construction of a new building. A designer can only decide on aspects of construction above the level of the flat roof after assessing the depth of the roof zone.

It is common for the depth of a roof zone 1 to be underestimated. This can result in problems on site such as: upstands being too low under windows and falls being inadequate. The reason for this is designers are unwilling to sacrifice window size or increase the height of construction in order to  ensure good design principles.

Flat roofs may be drained in two basic ways:

1) Towards the outer edges of the flat roof and into external gutters.

2) Towards outlets within the main roof area into internal gutters.

Straight falls to external gutters are simple to form by sloping the roof deck, by screeding or by using tapered insulation boards 2. Internal drainage will be achieve by straight falls to gutters or a pattern of falls and cross falls to outlets.

When the roof falls are created by a screed it should always be possible to drain the whole roof efficiently. This can be done with falls and cross falls to outlets, without the need for gutters. If the falls are formed in the structure, a pattern of falls and cross falls will be difficult to achieve. In this instance, straight falls to a gutter or to outlets will normally be used. Falls between outlets can be provided by adding tapered firrings to the purlins between outlets. Additionally a fall can be introduced into the purlins themselves.

When internal gutters are used they should be laid to falls. This can result in a very deep gutter at the low point. In most cases, it is not advised to use perfectly level gutters as this can result in a high amount of standing water. Rather than have a perfectly level gutter it is better to have sections of flat roof between rainwater outlets. This is one of the advantages of flat roof as you can have wall to wall waterproofing without the need of a gutter. In most situations, a well designed flat roof will not have a gutter, but have a number of outlets.

Tapered Insulation Systems

Tapered insulation systems provide both insulation and falls. This is particularly useful for re-roofing existing flat roofs. The reason for this is many do not have sufficient falls or insulation. Tapered insulation can provide falls in one direction to a gutter or level valley. And can also, provide falls in two directions to form falls and cross falls. If this is done it is important to keep the intersection at 45 degrees to avoid over-complicating the installation.

The skirting/upstand height must be 150mm. This often causes problems with tapered insulation systems as it needs to be ensured the skirting does not raise above damp course level. If it is not possible to keep the skirting below damp course level, the waterproofing should be abandoned, or the whole wall above the skirting completely waterproofed. In the case of ow parapet walls the system can be taken to the top of the parapet and tucked in under the damp course beneath the coping. For higher walls metal cladding is advised.

Drainage Crickets

Drainage crickets 3 can be used to improve the drainage of a flat roof which is installed to straight falls to an otherwise level valley. Crickets displace any standing water and provide a moderate fall between outlets. It must be noted, crickets do not provide fully efficient falls and cross falls. They cannot be expected to completely eliminate standing water. The objective of drainage crickets is to provide a new valley and improve falls. The main falls of drainage crickets are formed with tapered insulation to a falls of 1:60. The crickets are wedge shaped to a steep slow of approximately 1:40. These are laid over the main insulation in a diamond shape. The size of the roof will constrain the width of the drainage cricket overlaid, this means the smaller the roof the less effective fall can be achieved.

Crickets can be used on existing roofs with ponding issues. Before doing this it is important a roof survey is conducted to know the levels involved, so the crickets can be designed correctly.

Drainage Layouts

Internal rainwater pipes are usually positioned against the main columns. Outlet should be positioned to divide the roof into convenient drainage areas. The positioning of outlets will normally be governed by the roof itself. If the level at the outlets is taken as zero, then the pattern of the drainage can be drawn and the level at the high points calculated. There are many different approaches to  the design of drainage patterns. With four drainage layouts been used most frequently.

Do You Have A Question About Flat Roof Drainage?

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

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