Start With the Structure, Not the Saw

A skylight in a sloped ceiling can transform a dark loft conversion or a gloomy upstairs landing, but the moment you cut through a roof, you are altering a structure that keeps the weather out and holds the building up. Before you buy anything, spend an hour understanding what you are working with. A roof is a system: rafters, sarking or boarding, felt or membrane, battens, tiles or slates, and sometimes insulation between and above the rafters. Every one of those layers has to be dealt with properly, or you will be chasing leaks for years.

Check whether the roof is a simple cut roof or a trussed rafter roof. Trussed roofs use thin timbers braced together as a single engineered unit, and cutting any member can weaken the whole structure. If you have trusses, a skylight usually needs the opening framed between trusses rather than through them, or it needs structural design from an engineer. In older cut roofs, rafters are more forgiving but still load-bearing. As a rule, you can remove one rafter and trim around the opening with doubled rafters and headers, but only if the span is modest and the roof is otherwise sound.

Check for Cables, Pipes and Rafters First

Nobody enjoys discovering a shower pipe with a reciprocating saw. Before any cutting, lift the plasterboard or lath on the room side and look into the void. Use a torch, a mirror and a stud finder, and where you are unsure, make an exploratory hole with a small drill or a jab saw rather than a full cut.

• Rafters: measure centres — typically 400mm or 600mm in UK homes — and mark them on the ceiling so you can see exactly where the timbers run.
• Cables: look for lighting circuits, shower feeds, alarm wiring and, in older houses, lead-sheathed or rubber-insulated cable that must not be disturbed.
• Pipes: central heating, cold water tanks, soil stacks and waste pipes often run through roof voids. Copper and plastic both drill and cut easily, and a hidden leak will show up as a stain weeks later.
• Ducting and vents: extractor fans and boiler flues may pass through the same space. A flue must never be enclosed or obstructed.

If you find anything, reroute it properly with the power or water isolated, and allow for the required clearance around the skylight frame so nothing is trapped against hot glass or sharp metal edges.

Choosing a Skylight and the Correct Flashing Kit

Roof windows and rooflights are not the same thing, and the flashing kit matters as much as the unit. For a pitched roof, buy a window supplied with a flashing kit matched to your covering — plain tile, interlocking tile, slate or profiled metal — and to your pitch. UK roof pitches commonly sit between 15 and 45 degrees, and most manufacturers publish a pitch range for each kit. Fit a kit outside its range and water will track sideways under the tiles.

Also check the Building Regulations. Replacing a roof covering or creating new openings can trigger requirements for thermal performance and, in some cases, structural calculation. If your home is listed, in a conservation area, or in a flat, you may need consent before work starts. A skylight that projects above the roof plane usually needs planning permission; a flush-fitting one often does not, but always check with your local authority.

Cutting the Opening Accurately

Set out the opening from inside, centred on the window's structural frame, not on the outer trim. Allow the manufacturer's stated rough opening size, which is usually a few millimetres larger than the frame. Mark it with a pencil and check it is square by measuring the diagonals — they should match within a few millimetres.

Cut the plasterboard or lath first, then remove insulation carefully. Cut the rafters with a hand saw or a circular saw set to the correct depth, working from inside where possible so you can see what you are cutting. Keep the cut ends clean and straight. Then fix the trimming timbers: doubled rafters either side of the opening and horizontal headers top and bottom, all screwed and bolted together with proper structural fixings. This frame carries the load that the removed rafter used to take and gives the window something solid to sit in.

Fitting, Flashing and Sealing

Work from outside only when the weather is settled, using a roof ladder hooked over the ridge, a harness anchored to a suitable point, and a second person on the ground. Never rely on a leaning ladder for roof work.

Dry-fit the window before sealing anything. Slide the flashing kit into place, tucking the apron under the covering above and dressing the side flashings over the tiles below, following the manufacturer's sequence exactly. Most leaks come from flashings fitted in the wrong order or from tiles not lapped correctly at the sides.

Fix the frame to the trimming timbers using the brackets supplied. Insulate the gap between frame and timber with a compressible foam or mineral wool, then seal internally with a suitable airtight tape or sealant. Outside, keep a small drainage path beneath the flashing so any water that gets behind the covering can escape.

Finishing Inside and Testing

Line the reveal with plasterboard, taped and filled or skimmed, and insulate the head and sides of the opening to reduce cold bridging. Fit the internal trim and handles, and check the window opens and closes freely before you decorate. Finally, test with a hose on a dry day, aiming water up the roof, not directly at the glass, and watch from inside for any dampness around the frame and flashing. If it stays dry, you have done the job properly — and the extra daylight will be worth every careful measurement.