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Flat Roof Rainwater Outlets

In the UK when designing flat roofs it is normal to base the design on a rainfall rate of 75mm per hour. This is provided any overflow will not cause damage to the building or contents the roof is protecting. The likelihood of this rainfall rate occurring for in excess of two minutes is surprisingly highest in the drier parts of the UK.

Any rainwater will run off a flat roof system. The depth of this layer will be dependent upon the length of the run to the outlet, the type of waterproofing system used and the fall. If a fall of 1:80 is used, with the correct outlets provided any rainwater will run off a flat roof in a thin layer.

Rainwater discharges into the outlets at a rate depending on the head of water at the outlet. It will collect in the gutter or on the roof until the head of water at the outlet has built up sufficiently to discharge the rainwater as fast as it falls on the roof. Just a small increase in the head of water will cause the rate of flow to increase dramatically. This does not change regardless of how the head is created.

The flow of the water into flat roof outlets can be of two types:

1. Orifice Flow

Orifice flow is caused by greater depths of water. This prevents weir flow. Orifice flow causes a swirl of vortex to form and the efficient of the outlet is lessened.

2. Weir Flow

Weir flow is the free flow of water over an edge with an unrestricted drop. The flow of water into outlets will be be weir flow when the water is relatively shallow.

Flat roof outlets which are tapered are more effective than those which has consistent diameter. If the vertical dimension of the taper is at least equal to the top opening and if the diameter at the top opening is not more than 1.5 times the diameter of the downpipe size, the  calculation of the the flow rate can be based upon the top diameter. This is often referred to as the effective diameter. If the taper of the outlet is greater than the dimensions given above, flow rate calculations should be based on a maximum design outlet diameter of 1.5 times the diameter of the downpipe.

Gravity Systems For Rainwater Disposal

Traditionally rainwater is disposed through the use of gravity. Rainwater will hit the roof, flow along the falls, through the downpipes. This is all achieved through the influence of gravity. The size and position of downpipes is selected so water drains fast enough as to not cause excessive water at the head of the downpipe. The downpipes are not designed to be full. By design a downpipe is supposed to be a maximum of 1/3 water and 2/3 air. The reason for this is to allow the water to trickle down the pipe. There should be as few bends in downpipes as possible. And any horizontal runs should be limited. If a horizontal run is absolutely necessary it should be installed to falls.

Any internal downpipes should be installed with rodding eyes at floor level. This should be positioned, so any blockage between the downpipe and the surface water drainage system can be cleared by rodding. The junction between the internal downpipe and the outlet should be sealed as a precaution agains backing up. However, over time these seals can breakdown.

Surface water drains must be of a sufficient size to remove all water drained from the roof immediately. If this is not the case there is a danger of water backing up. Many flat roof systems which make use of gravity to dispose rainwater, have a large number of downpipes. There should be a corresponding number of surface water drains to match the number of downpipes.

Gravel Guards

On flat roof outlets where the downpipe size is less than 150mm in diameter, gravel guards 1 are normally required. Outlets with downpipes which are 150mm or more in diameter and discharge water as a straight drop are normally regarded as self-cleansing systems. Although gravel guards are not compulsory on such flat roof outlets, they are recommended. On any roofing system which makes use of gravel guards inspection and cleaning is advised to be carried out on a regular basis.

Area Of Roof Drained By Outlets

Most flat roofs are drained to rooftop outlets only. A crucial aspect of flat roof design is the depth of water at the outlet. During a storm a flat roof to falls will have water collect over the area close to the outlet. This will form a natural sump and a head of water will form. On a flat roof with a fall of 1:80, a head of 50mm will be provided by a natural sump. This will extend 4 metres in radius from the centre of the outlet. This will only happen for a few minutes during a storm which rains at a rate of 75mm per hour.

Siphonic Systems For Rainwater Disposal

Large buildings often benefit from the use of a siphonic rainwater disposal system. Siphonic systems normally make use of short rainwater outlets to exposed horizontal pipes beneath the roof deck. These pipes dispose the rainwater into a downpipe to ground level. Rainwater is collected through specially designed outlets which are only open at the periphery. These outlets have a central baffle, and often vertical baffles to prevent the rainwater forming into a vortex (this is often referred to as a pipe diameter filter). These specially designed flat roof outlets are designed to prevent air entering through the centre of the outlet and to make the water enter by weir flow. Under heavy rainfall, these outlets can be covered with a large head. Depending on the design of the outlet the head needs to be between 50mm and 80mm to initiate fully efficient siphonic flow. It is common to see siphonic outlets positioned in gutters or sumps to accommodate the required head.

When there is not heavy rainfall, air will enter into the downpipes, as the outlets will not be covered with water. In this scenarios, the siphonic system will act the same as traditional gravity system with "trickle down" drainage.

In contrast, during heavy rain the amount of air contained in the pipes will reduce. This will cause symphonic flow to develop when the proportion of air is down to about 40%. The air will then move along with the water. When maximum storm conditions occur, the pipes will be almost full of water with very little air content. Most of the time, the entire symphonic system will feed into one or two downpipes. The individual outlets feed into horizontal pipes which are located directly under the roof.

Do You Have A Question About Flat Roof Rainwater Outlets?

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