Framing a Skylight Opening

The rough opening is the manufacturer's number, not yours

A unit skylight arrives described by four or five dimensions that all sound like the size of the skylight. Outside frame, rough opening, installation clearance, daylight opening, and — on curb-mounted products — a curb size that is different again. Only one of them is a cutting dimension. Reading the wrong line off the sheet is the single most common way a skylight ends up with a 15 mm gap on one side and nothing on the other.

There are two conventions in circulation and they are not interchangeable. Some makers print the rough opening outright, in which case that figure is the whole answer and no arithmetic applies: cut to it, and any clearance the unit needs is already inside it. Others print the unit's outside frame dimension and a required installation clearance per side, and expect the installer to add the clearance twice in each direction. The arithmetic is trivial; knowing which of the two sheets you are holding is the part that matters, and if the sheet is ambiguous the answer is a phone call to the supplier, not a judgement call on a roof.

Take a unit measuring 780 mm by 1180 mm across the outside of the frame, with 13 mm of clearance called for on each side. The framed opening becomes 806 mm by 1206 mm, and both trimmers and both headers have to land on those lines with their faces, not their centrelines. It is worth writing the two numbers on the trimmer stock in marker before the cut, because at that point in the day you will be holding three dimensions in your head and only one of them is the one you saw to.

The clearance exists so the frame can be shimmed square, level and plumb without binding against timber that has moved since it was milled. It is not slop, and it is not an allowance to be padded. An opening cut generously so the unit will definitely fit leaves the fixing brackets reaching for framing that is no longer under them, and it leaves a gap too wide for the perimeter insulation and air seal to bridge honestly. Cut it tight to the number and shim it; that is the sequence the fixings were designed around.

Curb or no curb

Deck-mounted units sit down onto the sheathing and carry their own upstand inside the frame, so the framed opening is the finished thing and the flashing kit lands straight on the deck. Curb-mounted units want a site-built timber kerb standing on the deck, and the unit lands on that. Which one you have was decided in the showroom along with everything else, and the two are not swappable at the point of installation — the flashing kits are different parts.

Where a curb is built, its height above the finished roof surface is the number to get right, and the maker states a minimum. Building it to that minimum on a roof that gets driven rain, or on one where snow lies, is a decision to be at the bottom of the tested range for the life of the roof; the extra material to build it taller costs almost nothing and buys the flashing a longer upstand. Build it square, build it with the deck cut back to the framing beneath so nothing is cantilevered, and check the diagonals again before it is fixed, because the unit will not correct a curb that is out of true.

The perimeter of that curb is the flashing takeoff. Apron at the foot, step or soaker pieces up each side interleaved with the covering course by course, and a head flashing at the top lapped by the covering above it — the same downhill logic as everything else on a steep roof, applied to a rectangle standing in the middle of it. Under all of it, a self-adhered membrane meeting ASTM D1970 Standard Specification for Self-Adhering Polymer Modified Bituminous Sheet Materials Used as Steep Roofing Underlayment for Ice Dam Protection, run up the curb faces and lapped shingle-fashion onto the deck, is what keeps a flashing detail from becoming a leak the first time a fixing works loose.

The saddle that keeps water off the upslope side

Everything the roof collects above the skylight arrives at the head of the curb and stops. On a narrow unit it parts and runs around, and the head flashing handles it. On a wide one it does not part cleanly: the flow spreads across the upstand, slows, drops its grit, and in the first winter the resulting shelf holds leaves, then ice, then a standing pond above a joint that was designed to shed. A saddle — a cricket — is a small wedge of framing that gives that water a shape to leave by.

There is a code floor under the decision and a manufacturer's figure above it, and they are not the same thing. The floor is the familiar 30 in (762 mm) threshold: the roof flashing provisions in Chapter 9 of the International Residential Code call for a cricket or saddle on the ridge side of any chimney or penetration more than that wide, measured perpendicular to the slope. The wording is worth reading carefully, because the clause is usually quoted as a chimney rule and it is not — a curbed skylight is a penetration, so a wide one is caught by it directly rather than by analogy. Above that floor sits the maker's own trigger, which varies with product and with pitch because a shallow roof spreads the flow and a steep one throws it, and which can call for a saddle sooner. Where it does, it is the tested condition and it is the one to build to.

The geometry of a symmetric saddle is one line of arithmetic and one comparison. Its ridge runs up the slope from the centre of the curb head; its two faces fall from that ridge to the deck at the curb's outside corners. If those faces are built at the main roof pitch, the ridge height is half the width facing upslope multiplied by the pitch ratio. Run it, then hold the answer against the height of your curb above the deck.

That comparison is the whole decision, and it catches people out. An 850 mm curb on a 6:12 roof wants a ridge 213 mm high, while the curb itself may stand only 150 mm above the deck — the saddle would tower over the head flashing it is supposed to sit under. The resolutions are to build the saddle faces shallower than the main roof, to build the curb taller, or to accept the maker's own head deflector if the unit is inside the width it covers. Shallower faces have their own floor: they must still exceed the minimum slope the roof covering is permitted at, which for asphalt shingles and for slates and tiles is set by Chapter 9 of the code and by the covering manufacturer, and below that the saddle stops being a shingled surface and becomes a metal or membrane one.

Build it as framing, not as packing. A ridge board or a beveled ridge, two sets of short jack members falling to the deck, and sheathing over them, all bearing on the deck and the trimmers rather than floating on top of the covering. Line the two valleys that the saddle's faces make with the main roof before anything else goes on: those are the most concentrated few inches of water on the whole slope, and they are exactly where a saddle that was tapered out of insulation board or offcuts starts to fail.

The shaft under the hole

Below the roof plane the opening becomes a light well, and it is the part of the job the client sees every day. Straight-sided wells are quick and give a narrow, hard shaft of light. Splayed wells — flared at the ceiling, often more on the downslope face — spread the light and are the reason one skylight can feel like two. The splay is decided at framing, not at boarding, because the shaft studs have to lean.

Whatever its shape, the shaft is an exterior surface on both of its faces. It gets the same insulation continuity as the roof around it, an air seal at the ceiling plane, and vapour control on the warm side, all of which are easier to build before the boards go on and effectively impossible afterwards. Skimping here is not a thermal problem so much as a moisture one: the glazing is the coldest surface in the room, moist air runs up the shaft to meet it, and the resulting drip gets reported as a leaking skylight in a dry week. BS 5250 Management of Moisture in Buildings, and the psychrometric material in the ASHRAE Handbook — Fundamentals, are the documents behind that behaviour; the fenestration U-factor and SHGC requirements in the International Energy Conservation Code, as adopted, are what the unit itself has to meet.

There is a finishing consequence too. Well faces are seen in raking light from the glazing above, which is the least forgiving light a plasterer ever works under, and every fastener line and joint shows. Board them with that in mind, and paint them white or near-white unless the client has a specific reason not to — the shaft is a reflector, and the difference between a white well and a mid-tone one is visible in the room below.