Roof Windows and Rooflights
PremiumSpecify, order, and install roof windows correctly for your extension. Covers Part L U-values (max 1.4 W/m²K), Part F ventilation, membrane grade, lead flashing, and the pre-scaffold inspection that prevents expensive re-work.
The roof goes on. The tiles look right. Scaffold comes down. Then it rains.
Leaks at the window junction are one of the most common post-construction defects on extensions with pitched roof windows. They're also one of the most avoidable. The failure almost never comes from a faulty window. It comes from what surrounds the window: the wrong grade of breathable membrane, a roofer who doesn't do lead work, or a flashing kit specified for the wrong tile profile. By the time you discover the problem from inside the extension, scaffold has been gone for months.
What follows covers how to specify, order, and install roof windows correctly. And the one inspection you must do before scaffold strikes to catch problems while they're still accessible.
The roof structure must be complete with correctly sized trimmer rafters forming the window opening before your windows can be delivered and installed. Confirm the structural opening dimensions from the manufacturer's actual unit spec (not your design drawings, which may have been drawn before the window was finalised) and make sure your structural engineer has signed off the trimmer arrangement before the carpenter starts cutting.
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