Skylight Opening Calculator

How the skylight opening calculator works

The rough opening is the hole framed in the roof for the skylight to sit in. It is the unit's outside frame plus an allowance on each side for the width, and at the top and bottom for the height:

Opening width = frame width + 2 × side allowance. Opening height = frame height + 2 × top and bottom allowance.

The arithmetic is simple. The allowance is the part people get wrong, because it depends on the kind of unit. A roof window sits down inside the opening and needs a little clearance around it. A deck-mounted skylight sits on top of the roof deck, so its frame overlaps the hole and the opening is smaller than the frame. A curb-mounted skylight sits on a curb built round the opening, so the opening is simply the inside of the curb. Enter a negative allowance for a deck-mounted unit.

Start from the outside frame, not the size code

Size codes are nominal. VELUX's data sheet for its top-hung GPL roof window lists the MK06 as 780 mm wide and 1178 mm high on the outside frame, although the code is read as 78 × 118 cm. Across the range the frame widths are 550, 660, 780, 942, 1140 and 1340 mm, and the heights 978, 1178, 1398 and 1600 mm. Take the frame size from the maker's chart for your exact model. The same sheet gives the pitch range for that window: 15° to 55°.

Roof windows in metric

The fitting guide from Roofing Superstore, a VELUX stockist, puts the opening 40 to 60 mm wider than the window, which is 20 to 30 mm each side, and sets the upper installation batten at the window's height plus 45 mm, so 22.5 mm top and bottom. The metric defaults use 25 mm at the sides and 22.5 mm top and bottom. For an MK06 that gives 780 + 2 × 25 = 830 mm by 1178 + 2 × 22.5 = 1223 mm. A small CK04, 550 × 978 mm, needs 600 × 1023 mm.

US skylights and roof windows

The VELUX USA size guide lists the rough opening next to the outside frame for every unit, and within each type the difference is almost always the same (a couple of sizes in the table are a sixteenth out). Roof windows open ½ in larger than the frame, ¼ in each side: an MK06 frame of 30¾ × 46⅜ in needs 31¼ × 46⅞ in. Deck-mounted skylights open ½ in smaller: a C06 frame of 21½ × 46¼ in sits over a 21 × 45¾ in opening. Curb-mounted skylights are sized to the curb. A 2246 unit fits a curb 22½ × 46½ in inside and 25½ × 49½ in outside, so the roof opening is framed to 22½ × 46½ in, the inside of a curb built from 1½ in lumber. The US defaults are the MK06 roof window with ¼ in each way.

Rafters, trimmers and the header

Unless the opening fits between two rafters, at least one rafter is cut. The cut ends are carried by a header running across the top and bottom of the opening, and the rafters either side become trimmers. In the US, section R802.9 of the International Residential Code says openings in roof framing must be framed with header and trimmer members. Up to a 4 ft (1,219 mm) header span, the header may be a single member the size of the rafters, and single trimmers are allowed only where the header sits within 3 ft (914 mm) of the trimmer's bearing. Over 4 ft, the trimmers and header must be doubled and sized for the rafters they carry, and over 6 ft (1,829 mm) the header-to-trimmer joints need approved hangers. The result shows which band your opening falls in. A trussed roof is different: a truss is never cut without the truss designer's say. Other countries have their own rules, and an engineer sizes the timbers either way.

Check the rafter spacing before choosing a size. Rafters 1½ in thick at 24 in centres leave 22½ in clear, so a C06 deck-mounted opening of 21 in drops between them without cutting anything, while a 30 1/16 in M-size opening does not. In metric, 47 mm rafters at 600 mm centres leave 553 mm clear, which is narrower than even the 600 mm opening for a CK roof window, so plan on trimming.

Worked through for a loft

Two MK06 roof windows side by side, each in its own opening with a rafter left between them. Each opening is 830 × 1223 mm. At 830 mm wide the header span is under 1,219 mm, so under the IRC rule a single header the size of the rafters would do. Fit the same pair as a factory combination instead, 100 mm apart, and VELUX supplies an EKY support rafter for the gap, with its own opening sizes. Swap to a single UK08 at 1340 mm wide and the opening becomes 1390 mm, over the 4 ft line, so doubled trimmers and a doubled header.

Flashing and weathering

The rough opening does not include the flashing. The flashing kit is made for the unit and for the roof covering, whether tiles, slate, shingles or metal, and its coverage sits over the roof around the opening. The right kit is the one listed for both.

What this tool does not do

It does not size headers or trimmers, check the roof for the extra load, design the light well below, or pick a flashing kit. It also does not replace the installation instructions that come with the unit, which win over any general figure here. Roof work is work at height; use a scaffold or roof ladder and follow the working-at-height rules where you are.

Using this calculator with other BuildMetricLab tools

The roof pitch calculator confirms the unit suits your roof, and the rafter set-out calculator helps plan the framing around the opening. All BuildMetricLab tools run in your browser with no sign-up, and the formula is on the page so the maths can be checked.