Fitting a Roof Window Into a Pitched Roof

Setting the window on the slope, not on the elevation

The two heights a client cares about are the sill and the head, and both of them are measured up the slope, which is not how anybody describes them. A sill low enough to see the garden from a chair is a different position from a sill low enough to see it standing at the wardrobe, and the difference on a 40 degree roof is most of a course of tile. Meanwhile the head governs headroom: a tall person walking to the window wants the head high enough that the sash does not catch them, and on a shallow pitch that pushes the whole unit a long way up a slope that may not have a long way left in it.

So the rafter is your ruler. Its true sloping length from wall plate to ridge, plus the tail beyond the plate, is the dimension every other position on this job is a fraction of — where the purlin crosses, where the collar ties sit, how much slope is left above the head for the drainage collar and the head deflector to work in. Working from a plan dimension instead is the classic way to arrive at a window that fouls a purlin by 60 mm, discovered on the morning of the strip.

There is a legal position as well as a comfortable one. Where the roof window is the escape window for a loft storey, the clear opening it has to give and the height of its sill above the floor are prescribed, not preferred: in England that is Approved Document B Volume 1, and in jurisdictions on the International Residential Code it is Section R310 on emergency escape and rescue openings. Both express the requirement as a clear opening dimension measured on the sash in its fully opened position, which is a smaller number than the unit size and a much smaller number than the rough opening. Read it off the manufacturer's declared clear opening figure rather than deriving it, because the hinge geometry of a top-hung sash and a centre-pivot sash give completely different answers from the same frame.

Cutting one rafter, or none at all

Roof window widths are not arbitrary. The catalogues are built around the rafter spacings the trade actually uses, so for most common centres there is a unit that drops between two rafters with the frame brackets landing on their sides and nothing structural touched at all. That is the cheapest structural decision available on this job, and it is available only while the unit is still unordered. A client who wants the next size up is asking for a rafter to be cut, and the honest way to present that is as a cost and a delay rather than as a technicality.

Where a rafter does come out, the load it was carrying has to be handed to its neighbours through trimmers at the head and the sill, and those neighbours become trimming rafters carrying more than they were sized for. Doubling them is the conventional answer and it is not automatically the correct one: whether a doubled trimmer is adequate depends on the span, the loading and the timber, which is Eurocode 5 territory in the UK — BS EN 1995-1-1 — and NDS territory in North America. On a loft conversion the whole roof is usually being altered anyway and there is a structural engineer already on the job; a roof window opening is a cheap thing to add to a scheme that exists and an expensive thing to justify retrospectively.

Two structural situations are simply not site decisions. A trussed rafter roof has no spare member and no chord that may be cut, so a window either fits between two trusses untouched or it needs a modification detail from the truss designer with its own connectors. And engineered rafters — I-joists, laminated veneer lumber — take their cuts, notches and hangers from the manufacturer's own literature, which is more restrictive than anything a general timber rule would allow. In both cases the phrase that should stop the conversation is somebody explaining that it will be fine because the window is small.

The dimension you cut to has a wrinkle particular to this product. A roof window frame is fixed with brackets into the sides of the timber rather than through a nailing flange onto the deck, so the opening is specified as the outer frame size plus a gap the maker publishes for that unit — given on the width and on the height, and not the same figure as the shimming clearance a flanged skylight expects. Read it off the installation instructions in the box rather than off a remembered rule of thumb, because it differs between makers and between ranges within one maker. Cut to the published figure and the frame goes in square first time; cut it by eye and the brackets land where a packer has to make up the difference, which is a fixing that holds for the site inspection and not for the first gale.

Cutting the membrane so what it catches can still leave

The covering on a pitched roof is a screen, not a seal — wind pushes rain past the side laps, capillary action drags it along the head laps, and snow melts underneath. All of that arrives on the underlay and runs down it to the eaves. Cut a rectangular hole in that plane for a window and you have interrupted the drainage path of everything above it, which is why a roof window that leaks almost never leaks through the window.

The membrane is therefore cut and folded, not cut and trimmed. The usual method is a diagonal cross within the opening so the four triangular flaps can be turned up and stapled to the sides of the trimmers and the trimming rafters, giving a shallow tray rather than a raw edge. Above the head, the flow coming down the slope has to be collected and led around the window and back onto the undisturbed underlay below it. Some kits supply a preformed collar or drainage gutter for exactly this, and where they do, it is a tested component and it goes in as drawn. Where they do not, it is formed from an offcut of the same membrane, dressed over the head of the frame and lapped shingle-fashion onto the sheet below on both sides.

Ventilation and moisture control ride on the same detail. Whether the roof is a cold one that needs an air path from eaves to ridge, or a warm one relying on a vapour control layer and a breather membrane, the window is an obstruction to whatever that strategy is, and it needs the path around it kept open rather than stuffed. BS 5250 Management of Moisture in Buildings is the document behind those choices in the UK, and the practical consequence is that the counter-batten or the ventilation gap above the head of a window is not the place to cram the last of the insulation.

Most of the time you are doing this in the middle of a slope that is already stripped, and the underlay for the whole slope is on the order. That quantity is taken off the sloped area, not the footprint. It is the same trap as ordering the covering: a 40 degree slope is about 30 per cent longer than the plan it sits over, and a roll count derived from the floor plan comes up short somewhere near the last course of the day.

1. Strip the covering back well past the marked opening on all four sides, and lift the battens clear rather than cutting them in place.
2. Mark the opening on the underlay from the trimmers already fixed, so the membrane is cut to the timber and not to a chalk line.
3. Cut the membrane corner to corner within the opening, then turn the four flaps up and staple them to the inside faces of the timber.
4. Form or fit the drainage collar above the head first, lapping it onto the undisturbed sheet on both sides of the window.
5. Re-batten to the original gauge either side of the opening, with short lengths picking up on the trimming rafters so the courses still line through.
6. Fit the apron, then the side skirts working up the slope, then the head deflector last, each lapped over the piece below it.

When nothing in the catalogue matches the covering

Stone slate, heavily cambered handmade clay, a deep double-Roman well outside any published profile height, a standing-seam metal roof, a conservation slate laid to diminishing courses: these are the roofs where the merchant looks at the three measurements and says there is no kit. The answer is site-formed weathering, usually lead, and it is a different trade and a different takeoff. An apron across the foot of the frame, soakers up the two sides interleaved course by course with the covering, and a back gutter across the head with its own upstand and its own cover flashing.

That work is specified rather than improvised. The lead code, the maximum length of an individual bay, the amount of lap between bays and the fixing method all come from the Lead Sheet Training Academy's Rolled Lead Sheet — The Complete Manual, with the material itself to BS EN 12588 Lead and Lead Alloys — Rolled Lead Sheet for Building Purposes. The bay-length limits exist because lead moves a great deal with temperature, and a back gutter formed in one long piece to save a joint is a detail that splits itself within a few summers. The takeoff that goes to the leadworker is a perimeter, broken into those four runs, with the bay count falling out of the length limit for the code you are using.

Plumb below the sill, level above the head

Inside, the opening becomes a reveal, and the roofing trade's oldest instruction about it is a good one: cut the bottom reveal plumb and the top reveal level. It is not decoration. A plumb lower reveal lets warm air off a radiator or a pipe run below rise directly up the inside face of the glass, which is what keeps condensation off it in February; box that reveal in on the square and you have built a trap that holds cold air against the coldest surface in the room. A level upper reveal throws daylight back into the depth of the room instead of down onto the wall immediately below the window, and it makes one window feel like a considerably larger one.

The consequence is that the two faces are different sizes and both are larger than the opening. Take the lining area off the reveal you are actually building rather than off the rough opening, and remember it is four faces, not two — the sides splay as well, usually less dramatically, and they get boarded, taped and painted like everything else.

Every one of those faces is an external element on its other side. It carries the roof insulation around the frame with no gap, an airtightness layer on the warm side, and the same continuity of vapour control as the ceiling it joins. BS 9250 Code of Practice for Design of the Airtightness of Ceilings in Pitched Roofs is the document for that plane, and the reason to care is not the heating bill: an unsealed reveal is a chimney that pumps moist room air into the cold roof space around the frame, and it condenses on the first cold thing it touches. When a client reports a leaking roof window on a bright dry morning in January, that is almost always what has happened.

Finish it knowing the light will not be kind. Raking daylight straight down a reveal shows every fastener line, every taped joint and every ripple in the board — it is the most punishing light a plasterer will ever work under, and there is no angle from which the client will not see it. Board it flat, tape it properly, and paint it white or near-white unless somebody has a specific reason otherwise, because the reveal is a reflector and a dark one measurably costs the room daylight.