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How to measure your dormer « Back

Positioning your dormer.

The dormer windows finished cill height is recommended to fall between 800mm and 1100mm off of your proposed finished floor. This used to be a building regulations requirement but this has recently been relaxed. So with this in mind mark your required cill height on an existing rafter. Add the height of your window frame and 225mm for window lintol and rafter depth.

Cill height+window frame+225mm = Now subtract the final figure off of the measurement from the finished floor to existing property ridge height. The size that you are left with is half the maximum width of your dormer. Once you have worked out your overall width, a pair of double rafters would need to be positioned to take the structure of the dormer kit.


Once you have positioned your double rafters to position the dormer kit measure up the double rafters either from the existing roof plate (at eaves level) or down from the existing ridge beam.

To set your cill

Once you have a parallel measurement on both double rafters strike a plumb line down the double rafters (note that the finished window cill height will be 150mm above these marks) pull a straight edge or string line between the corresponding marks plumb down the marks on all rafters between the double afters. Support and cut to your plumb line. Remove a section of these rafters above the plumb line to allow the cill timber to slide into position. Incline with the lines on the double rafters, fix trimmer to the double rafters and existing cut rafters to the face edge of the positioned cill trimmer.

Take the triangular cheek sections and inter lock into the back of the cill trimmer. Fix both sections together with coach screws provided and fix the cheeks down to the double rafters below.

Slot into position the window lintel section and fix with coach screws provided.

At this stage make sure both cheeks are plumb and level and fix a temporary diagonal brace across the front to hold in position.

On top of the dormer cheek pull a level line through to the back of the double rafters the cheek is fitted to. Where the line meets the back of the rafter is the position of your supplied ceiling trimmer, fix with coach screws provided.

You have now assembled the box section of your dormer. It is now time to install your chosen roof.

Install the rafters labelled No.1 to their pre marked position on the plate. Once all of the No.1’s have been fixed at plate level, slide the pre cut ridge (& hips if required) into position. Once these have been fixed, carry on fixing numbered rafters and lay boards. Once in place install pre cut fascia, soffit and barge boards to finish your dormer.