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February 2016


We need to replace the roof of a “lean-to” type extension with two Velux windows at the rear of the house. The flat-tiled roof has a pitch of only 9 degrees, which meant we have had leak (and blow-back) problems over its 15-year life. The roof size 30ft wide x 10ft deep. We now know the pitch is too low and that the Velux fitted windows were not suitable for the roof type. We would appreciate any advice on any replacement options available to us. We need two roof windows for light.


A roof with a pitch of 9 degrees is too low for either natural slates, manmade fibre slates or interlocking concrete tiles. You cannot replace the roof with a pitched roof covering without creating a flat roof of marine-grade ply and bitumen felt under the pitched covering to prevent water ingress.

The minimum pitch for a Velux window is 15 degrees so you cannot have a pitched roof with roof lights. If the need to maintain roof lights is paramount, the only option available to you is to replace the pitched roof with a flat roof with either horizontal flat roof windows or, if you require the Velux windows to direct light into the room, then you can install the Velux windows on vertical upstands with the Velux window tilted to the required 15 degrees. Velux has technical details/sections about the required construction of these upstands, which you could give to any builder you employ. Velux is one of many suppliers of standard horizontal flat-roof lights. If you proceed with a flat roof, choose a reputable material supplier such as Moy Materials and ensure that its own technical representative attends site to sign off on the work so that the 20-year warranty can be issued.

Kevin Hollingsworth is a chartered building surveyor and a member of the Society of Chartered Surveyors of Ireland,