What the opening actually costs in members
The arithmetic that decides everything is one line: add a member thickness to the opening width, divide by the spacing, round up. That gives the number of bays the opening spans, and one less than that is the number of members falling inside it. A 22.5 inch opening at 24 inch centres spans one bay and cuts nothing. A 46.5 inch opening at 16 inch centres spans three bays and cuts two.
The difference between those two cases is the whole job. Cutting nothing means the opening drops into an existing bay, the two members either side are untouched, and the headers exist to close the ends of the bay and carry the unit. Cutting two means both of those members were carrying load from eave to ridge, that load now has to arrive at the trimmers through the headers, and the trimmers have to be able to take it. That is why the unit size is worth choosing around your spacing rather than picking the unit first and dealing with the framing afterwards.
The pieces, and where the lengths come from
Four kinds of piece come out of an opening. Trimmers are the members either side, usually doubled, and they run the full rafter length because they are not interrupted by anything. Headers close the top and bottom of the opening and span its width, so their length is the opening width and there are two of them, each with however many plies you are using. Below the opening, each cut member leaves a tail running from the low header down to the eave. Above it, each leaves a cripple running from the high header up to the ridge.
With the defaults — a 46.5 by 48 inch opening in a 16 foot rafter, starting 60 inches up from the eave — that is four trimmer pieces at 16 feet, four header pieces at 46.5 inches, two tails at 60 inches and two cripples at 84 inches. The cripple length is the piece people get wrong, because it is the rafter length minus the position minus the opening length, and all three have to be measured from the same starting point along the slope.
The line this page will not cross
It counts pieces. It does not size a header, it does not say whether a member may be cut, and it does not check anything against a span table. Those decisions depend on the span, the dead and live loads, the snow load where you are, the size and grade of the existing framing and what sits above the opening, and they are made by a span table your jurisdiction accepts or by an engineer, with the building department having authority over the work. The loads side of that — tributary width, pounds per foot, end reactions — is set out on the header and beam load calculator, which also stops short of sizing a member, and the snow input is on the roof snow load calculator.
The one case where the answer is not a judgment call is a trussed roof. A truss is an engineered assembly where the top chord, the bottom chord and every web are sized as one thing, and removing part of it is not a field modification at any width. Openings in trussed roofs are designed by the truss engineer, and it is worth knowing which you have before planning anything: from inside the attic, diagonal webs inside the triangle mean trusses, and a plain rafter with a ceiling joist and maybe a collar tie means stick framing.
What surrounds the take-off
An opening in a roof brings three other jobs with it. The shaft down to the ceiling, if there is an attic, is geometry of its own and is on the skylight shaft and daylight calculator. The flashing that keeps water out of the hole is on the roof penetration flashing take-off, and for a manufactured unit it is the kit for that unit and nothing else. And the sheathing that comes off has to be replaced around the new framing, which the roof framing lumber calculator covers for the roof as a whole.
Finally, the practical warnings, stated plainly and without procedure. Roof work is where falls happen, and falls from roofs are among the leading causes of death in construction. An opening cut in a deck is a hole a person can fall through, from the roof side and from the attic side, and it stays a hazard until it is closed. Attics are hot enough to be dangerous in summer, are lit by whatever you bring, and are floored only where someone happened to lay boards. Insulation and other materials installed before about 1980 can contain things that need testing before anything disturbs them.
Questions people ask
How do I tell whether I have rafters or trusses?
Look into the attic. A trussed roof has triangulated webs running diagonally between the top and bottom chords, repeating identically every bay, and the members are usually thinner than you expect — often 2x4. Stick framing has rafters running from the ridge to the plate with nothing diagonal between them, a ceiling joist across the bottom, and sometimes a collar tie partway up. The distinction is not cosmetic: it decides whether cutting anything is a span question or is simply off the table.
Can I make the opening narrower so nothing gets cut?
Often, and it is usually the cheapest change available. At 24 inch centres with 1.5 inch members the clear bay is 22.5 inches, and plenty of skylights and roof windows are made to that width for exactly this reason. At 16 inch centres the clear bay is only 14.5 inches, which is too narrow for most units, so the choice there is between cutting one member or using something long and narrow. Run the width field down until the members-cut line reads zero and see what size that leaves you.
Why are the headers doubled by default?
Because that is the common approach where the header is picking up cut members, and because a single ply is rarely what ends up being specified. But the default is a default and nothing more — this page multiplies your ply count by two headers and gives you pieces. How many plies the header actually needs, and of what size and grade, comes from a span table for the loads on that roof or from an engineer. If your answer is three plies or a steel member, put the number in and the count follows.
Does the take-off include the sheathing to patch around the new framing?
It gives the area of the opening itself, which is the hole you are leaving, but not the extra sheathing that comes off to expose the members for cutting and nailing, and not the patching around the new headers. That extra is job specific and depends on where the existing sheet joints fall relative to the opening. In practice it is usually one sheet more than the opening area suggests, and on an old roof the sheathing that comes off is rarely worth putting back.
The cripple length came out as zero. What does that mean?
One of two things. Either the opening finishes exactly at the top of the rafter length you entered, so there is nothing above the high header, or the position and the opening length together run past the end of the rafter, which is flagged as an error rather than a result. The second case means one of the three figures is wrong, and since the piece counts all depend on the same slope measurement, it is worth resolving before ordering anything. Measure the position and the rafter length from the same point.