The Self Build Self Help Site Keeping Water Out

External Beading

If you are building in an area of particularly driving rain you should consider and discuss the use of window external beading with your window suppliers - more on this in the Hints & Tips/Design/Windows page.

Dormer Roofs - running outlets

Not a critical aspect but its about attention to detail and minimising risks as much as possible (but balance the cost of reducing the risk with the likelihood of the risk manifesting itself). The rain coming off a dormer window roof runs down the main roof and into the main gutter - but the rain has to be kept away from the cheeks of the dormer.

The cost of the countermeasure is very low.

The cheeks on dormer windows are more vulnerable than the rest of the building structure - they are often constructed of marine ply and covered in a render on expanded metal mesh. If you finish off with sheet lead or slates then this provides a very waterproof finish (although slates can break away) but there is still the weak spot of the flashing to form a water tight joint between the roof and dormer cheeks.

We have seen, in very old buildings where the rain comes out of the gutter at the back, that the water has run down and contacted the wooden fascia underneath and caused it to rot.

Anyway, to the matter in hand. Simple - just incline the running outlets to send rainwater away from the dormer checks. That's it.

Diagram of structure of soffit

The cheeks, made of marine ply, have been covered in roof membrane with the soakers in place, prior to the flashings and final waterproof layer. The running outlet can just be seen pointing away from the cheek

Photo os soffit

Dormer cheeks finished off - and water tight!

(Click on photos for a larger view.)

Openings in roof

If putting in a roof light or a slate/tile vent then make sure you follow the instructions. The roof membrane has to be cut by making diagonal cuts, rather than removing a piece of the membrane. But the instructions will be specific on which way the cut pieces are turned, either into the roof space or to the outside, so that any water getting past the apron will not be allowed to run inside the roof.
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