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Flat Roofs - Through Ventilation / Cross Ventilation questions I'm always asked
Through ventilation in a cold flat roof is essential. Without through ventilation, the air in a vented space will not move at the far end. Through ventilation means a vented space with vents on both ends of the channel, and the vented space has to be clear for the air to move though it. You cannot have restrictions. We like to use cross ventilation if possible; basically cross ventilation is through ventilation with a matrix of timbers over the top, to form a venting system that vents left and right as well as front to back. If you have eaves vents, soffit vents or even mushroom vents over a cross vented roof, you will have very good cross ventilation and any way the wind blows you will get movement of air in the vented cavity of your flat roof. Unvented flat roofs have all sorts of problems with moisture buildup; not only do you have to have a good Air Vapour Control Layer (AVCL) at ceiling level, but you also have to have good through and cross ventilation. The size of the vents you use in a flat roof really depends on the size of the roof you need to vent. Generally the more venting you can place on a roof, the better. However, you must make sure the holes to the vents are around 4mm in size to stop birds and small insects from entering the vented cavity. Adding through ventilation to a cold flat roof when you have a skylight is extremely difficult; this is when we like to use cross ventilation on a vented cold roof. The fact that it's crossed means that you will not get any spaces between the joists that have no ventilation.
When doing a loft conversion and a vented cold flat roof to the dormer, and you have a ridge with tiles, today we can fit a dry ridge roof system to vent under the tiles; this enables ventilation not only from the vented cold roof to the vented ridge, but also to vent the pitched roof at the front of the building. You must be careful not to put the vents of the cold roof too low, as water can be blown back into the vented area. Often we see the wrong vented ridges being fitted to the ridge; this means that the venting to the cold flat roof is not through venting, and then you may have problems.

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Steven Dickinson
London flat roofing
07802300099
[email protected]

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