Roof Rafter

What it's for

Framing a gable, hip or shed roof with common rafters. You give it the building size, the pitch and the rafter size, and it gives you the rafter count and length, the plumb cut angle, the ridge, ties, sheathing, a rafter drawing with the birdsmouth enlarged, a second drawing of the rafter marked out on the stock, and a material list. It checks the rafter's run against the IRC 2021 rafter span table and flags a rafter that is too small.

Step by step

This example uses the numbers the screen opens with.

1 2 3 4 5 6 7

1. BUILDING LENGTH (RIDGE) — the outside length of the building along the ridge. The example is 32' 0".
2. BUILDING WIDTH (SPAN) — outside of wall to outside of wall, across the rafters. On a gable or hip roof the rafter's run is half of this. On a shed roof it is all of it. The example is 24' 0". A hip roof's ridge always runs the longer side, so if you enter a hip roof with the width longer than the length the screen works it as the same roof turned: the run is half the shorter side and the ridge is the difference.
3. ROOF STYLE — Gable, Hip or Shed.
4. PITCH — inches of rise per 12 of run. Tap it and type the rise (7.5 for a 7-1/2 in 12), or use − and + to step by a half.
5. RAFTER SIZE — the stock you plan to use. The span check tells you if it is too small.
6. SPACING is the on-center spacing, in inches: it reads 16" o.c. Type any spacing, 13-1/2" included. Turn on CEILING ATTACHED TO RAFTERS for a cathedral or vaulted ceiling, where the ceiling finish hangs from the rafters. It changes which span table is used. With it off, on a gable or hip roof, CEILING JOISTS TIE THE RAFTERS asks whether ceiling joists run with the rafters and are nailed to their heels at the wall plate. They are what stops the roof pushing the walls apart. It starts on, and the line under the span check says so. Turn it off if nothing ties the rafters at the plate.
7. FRAMING SPECIES, GRADE, DEAD LOAD and GROUND SNOW pick the span table and row the check reads. Take the species and grade from your lumber stamp. Use the 20 psf dead load for a heavy roof, such as tile or slate. A note under the row then says the code caps a roof-ceiling's dead load at 15 psf in Seismic Design Categories D0–D2 (IRC R301.2.2.2), so there a heavier roof needs an engineered design. Ground snow comes from your county's snow load map: 20 psf reads the code's roof live-load tables, and 30, 50 and 70 psf read the snow tables. Over 70 psf gives a red line, ENGINEER REQUIRED — GROUND SNOW OVER 70 PSF (IRC R301.2.3): past 70 psf the code wants the roof engineered, so the result is that line alone: no rafter count, no material order and no cut list. Each one starts at Not picked. While anything is not picked, the check reads SPF No.2 at 10 psf dead and 30 psf snow, and the span line is amber.
8. EAVE OVERHANG — how far the tails run past the wall, measured level, not along the slope. SEAT CUT (WALL BEARING) — the level cut of the birdsmouth, which is how much rafter sits on the plate. The seat is never cut deeper than a quarter of the rafter's depth, measured square to the rafter (IRC R802.7.1 and its Figure R802.7.1.1, "D/4 MAX."), so on a steep pitch it can come out shorter than you typed: 3-1/2" on a 2x8 at 18/12 is cut to about 2-3/16". The drawings show the seat as cut. If you also use Hip & Valley for the same roof, enter the same seat there. RIDGE THICKNESS — the thickness of the ridge board: 1-1/2" for a 2x, 1-3/4" or 3-1/2" for an LVL ridge. Each common is cut at the ridge's face, so half of it comes off the run. A shed roof has no ridge and a hip roof with no ridge (a pyramid) has no board, so it does not show there.

7 8 9 10 11

9. On a gable roof only: RAKE OVERHANG is how far the roof runs past the gable wall, measured level, and BARGE RAFTERS adds the outermost rafters that carry it. A rake over 9" also orders 2x4 lookouts at the rafter spacing along each rake: roof sheathing may not overhang the gable wall more than 9" without gable overhang framing (IRC R803.2.3).
10. FASCIA adds the board across the rafter tails. It runs the whole eave, past the rake overhangs at each end, and on a gable a Rake fascia row covers the four rakes. With it on, SOFFIT adds the underside of the overhang, and on a gable a Rake soffit row for the underside of the rake overhang.
11. COLLAR TIES sit high, near the ridge. They do not stop the roof spreading. RAFTER TIES sit low and stop the roof pushing the walls apart. Each switch adds its count and length to the results. With the ceiling on the rafters and no rafter ties, nothing ties the rafters across. The ridge then has to carry the roof as a beam (see the red line below).
12. SHEATHING adds the sheet count. With it on, pick the SHEATHING TYPE: OSB, Plywood or ZIP System. ZIP System also counts tape rolls.
13. WASTE FACTOR — the extra you buy over the measured quantity.
14. Read the answer: 50 commons @ ≈ 14' 5-9/16".

Reading the results

1 12 13 2 3

1. The headline is the number of common rafters in the roof and the length of each. The next line gives the order with the waste factor in it: the example's Order 55 commons (incl. 10% waste). On a hip roof the jacks read the same way: the roof's count, then the order. The length is the one you cut: from the end of the tail to the face of the ridge, so it includes the eave overhang and has the ridge already taken off. ≈ means the figure has been rounded for display.
2. The lines under it. First, the length to the ridge centre — the theoretical length, longer by half the ridge carried up the slope (the example: ≈ 14' 6-7/16"). Then the run and the plumb cut angle. The roof area and the ridge length. On a hip roof, the hips with their length and the number of jacks. The hip length runs along its centre line from the tail tip to the ridge centre. Hip & Valley prints the cut, with the ridge taken off, so the two tools differ by that much on the same hip. Then the ties and the sheathing, if they are switched on.
3. The span check. It names the table and what it was read for: Within IRC 2021 R802.4.1(3) — SPF No.2 · 30 psf snow · 10 psf dead · ceiling joists at the plate. It is amber until you have picked the species, grade, dead load and ground snow, because until then the row is the app's assumption, not your job. Southern pine and hem-fir span less than SPF, and every rafter spans less under a heavier snow load. Once all four are picked, the line turns green. The cut sheet and the drawing name the same basis, and the cut sheet lists your picks. If the code's own table misprints the cell you picked (two Douglas fir-larch No.3 cells), the line says so and gives no maximum. If nothing ties the rafters at the plate, or the pitch is under 3 in 12, a red line reads RIDGE MUST BE A DESIGNED BEAM — IRC R802.3 / R802.4.4. On a hip roof under 3 in 12 it reads RIDGE AND HIPS MUST BE DESIGNED BEAMS — IRC R802.4.4. The material list then gives a ridge beam for an engineer to size, not a ridge board. A roof with no ridge never shows the ridge line: not a shed, and not a hip roof on a square plan, where the hips meet at a point. Under 3 in 12 that square hip roof reads HIPS MUST BE DESIGNED BEAMS — IRC R802.4.4. A SPAN EXCEEDS MAX line gives the longest run that size allows and names the size to go to. Change RAFTER SIZE, or tighten SPACING. If no size in the list will do, the flag says the run needs an engineered rafter or a mid-span support. If you type a spacing wider than 24", the flag says it is not in the table and names the spacings that are. A spacing between the listed ones is checked at the next wider one. A spacing tighter than 12" is counted at 12" — the table goes no tighter — and a red line says so: SPACING 8" IS BELOW THE 12" TABLE FLOOR — COUNTED AT 12". The cut sheet prints the same red line with its other checks. The last line is the seat. It is green when the seat bears at least 1-1/2" on the plate (IRC R802.6), and says when the quarter-depth limit cut it shorter. Under 1-1/2" it is red and says what to do, for example Seat 1-7/16" — under the 1-1/2" bearing IRC R802.6 requires; cut a longer seat, or "use a deeper rafter" when the quarter-depth limit is what keeps it short (a 2x4 at 12/12). Under 3 in 12 the material list orders each hip as a designed beam, not a sawn hip rafter.
4. BASIS & WHAT WAS ASSUMED — tap it. It names the span table, and the species, grade, snow load, dead load and deflection limit the table was read for. Check those against your lumber and your local snow load. A span that passes here can fail under a heavier snow load. It also says how the rafters were counted. It ends with IRC 2021 — verify local code and a sentence that states and municipalities amend the code: check the adopted edition and local amendments with your building department.
5. BASIS also says what is not checked: the loads are gravity only, so where IRC R301.2.1.1 requires wind design, or in Seismic Design Category D0 to E (R301.2.2; townhouses from C), the added rules are not applied.
6. The drawing shows the rafter from plate to ridge with the run to the ridge centre, the cut length to the ridge face and the height above the plate, and the birdsmouth enlarged in the corner with its seat and heel. Pinch to zoom, drag to pan, double-tap to reset. The expand button opens it full screen. See Reading the drawing.

4 6 7

7. The rafter stock drawing is the same rafter lying flat, the way you mark it. Hook the tape at the tail tip and run it. It is a running tape, not stepped off. The board is drawn to the face of the ridge — the ridge is already taken off — and the overall mark is the headline length. The notes give the plumb angle, the birdsmouth, the height above the plate and the length to the ridge centre.
8. MATERIALS lists rafters with the stock length to buy and the length to cut them to — the same cut the cut list shows. A rafter longer than 24' of stock says so and still gives the real cut. On a hip roof the hip row is one size deeper than the commons. That is the depth the jack cuts need (IRC R802.4.3), not a size: the row says to brace the hip at the ridge to a bearing wall or have it designed. The span check covers the commons only. A hip roof also lists Ridge brace posts: two, one where each pair of hips meets the ridge (one at the apex of a pyramid roof), down to a bearing wall. Cut them to fit and size them for the load, or have them designed. Then the ridge, barge rafters, fascia, soffit, sheathing and ties. The ridge row names the board: IRC R802.3 wants it at least as deep as the rafters' plumb cut, so a 2x8 at 6 in 12 takes a 2x10, and past a 2x12 no sawn board is deep enough (an LVL). On a hip roof the ridge length is the theoretical length less the span; cut the board to fit at the hips. On a gable with BARGE RAFTERS on, the ridge runs past each gable wall to carry them: the building length plus, at each end, the rake overhang less the barge's 1-1/2" (the sample: 32' 0" + 2 × 10-1/2" = 33' 9"). With barge rafters off, the lookouts carry the rake and the ridge stops at the walls. When the ridge must be a beam, the ridge row is a ridge beam for an engineer to size, with a wall or column under each end. Barge rafters and rafter ties give a stock length; a tie longer than 24' is two pieces lapped over a bearing wall. The waste factor is in these quantities.
   Where ceiling joists or rafter ties hold the heels, 16d common nails — heel joints gives the nails per heel from IRC Table R802.5.2(1), read at the next lower slope and the next higher spacing and span. A ridge beam needs none (note b). Past the table (a span over 36', or a 20 psf dead load) the row says to have the heel designed. Hurricane ties gives one per rafter seat (IRC R802.11). Toe-nails per Table R602.3(1) are allowed instead only under its exceptions; the row says when the pitch, span or spacing rules out exception 2. Wind speed and exposure are not asked.
9. CUT SHEET, the list icon at its right end, CUT LIST and SEND LIST stay dim until you change at least one input. SAVE stays lit, but while every input is still a default a tap only answers Enter your job's numbers first and saves nothing. CUT SHEET makes a PDF, the list icon at its right end shares the material list only, SAVE keeps the result in History, CUT LIST puts the list on the Lock Screen and SEND LIST sends a checkable list to another phone.

Common mistakes

• Entering the overhang along the slope. EAVE OVERHANG and RAKE OVERHANG are level measurements.
• Entering the run as the width. BUILDING WIDTH (SPAN) is the whole building, outside to outside. The calculator halves it for you on a gable or hip roof.
• Leaving the ridge at 1-1/2" on an LVL ridge. Enter the ridge you are using in RIDGE THICKNESS. The headline is cut to its face; a thicker ridge makes every common shorter.
• Taking the ridge off twice. The headline and the cut list are already to the ridge face. Only a layout to the ridge centre line starts from the second figure.
• Trusting the green line without reading the basis. The table behind it is for one species, one grade and one snow load. Yours may differ.

Related

• Hip & Valley — cut-ready hips, valleys and jacks
• Roofing
• Framing
• Reading the drawing
• Cut sheets and 1:1 templates
• Cut list on the Lock Screen
• Saving and History