Roof windows transform dark lofts and landings, and on Bristol’s terraces they’re often the only way to get real daylight into a converted loft. They’re also a hole cut in a weatherproof surface — so the difference between a good installation and a future leak is all in the detail. Here’s what to think about before cutting.
The flashing kit is not optional
Every reputable roof window (Velux being the household name) has manufacturer flashing kits matched to covering types — slate, plain tile, deep-profile tile. The right kit, correctly fitted, is why good installations stay dry for decades. Leaks almost always trace to improvised flashing, the wrong kit for the covering, or reused old flashing around a new window. Ask specifically which kit is being used on yours.
Your covering changes the job
Slate installations need more skill — slates must be cut and re-fixed cleanly around the opening, and on old slate roofs the surrounding fixings are often fragile. Concrete and clay tile are more forgiving. On very tired coverings, cutting a new opening can be the disturbance that tips a slope over the edge — a condition check first tells you whether the roof will take the surgery kindly.
Structure: trimming the opening
Most roof windows are sized to fit between rafters or to replace one rafter’s width, with the opening properly “trimmed” — reinforced with timber so loads transfer around the hole. Wider windows need more structural work. If an installer’s plan involves cutting rafters with no mention of trimming, stop the conversation.
Condensation and the room below
Roof windows concentrate condensation risk: warm moist air hits a cold pane and streams. Modern units manage this far better than 1990s ones, but the installation details — insulated reveals, vapour-sealed internal finishing, trickle ventilation — decide whether the window drips onto the plaster below in January. This matters double in bathrooms and kitchens.
Planning and regulations
Most roof windows are permitted development, but conservation areas can restrict street-facing installations — much of Redland, Clifton and Bishopston qualifies, so check first (our planning guide covers it). Building Regulations apply to the structural opening and, in lofts, to escape-window sizing.
Replacement is a job too
Old leaking roof windows are often best replaced like-for-like with modern units — same opening, new flashing kit, dramatically better glass. It’s quicker and cheaper than people expect. Get in touch for a quote either way — new opening or swap.