I have made this article to help the average Do-It-Yourselfer when they are framing up a roof on a storage shed, barn, garage, or even a house.
I hope you find this information useful along with the accompanying videos.
You will need to adjust the length for the following situations.
- Subtract for the ridge-board (3/4 inch if 1 1/2 inch ridge-board)
- Add for any overhang beyond the bird’s mouth cut (list provided at the end of the article)
- Subtract from the rafter tail for any facia board.
(I like to leave them long and trim after installation so the facia is in a straight line)
Length of Rafter per Foot of Run
3/12 run 12.37 inches or 12 3/8 inches
Building Width with Length of Common Rafter
Below is a list of rafter lengths for building widths from the outer wall to the center peak.
(must add for overhang)
Building width ———– rafter length
4 FEET ——————- 2′- 3/4″
5 FEET ——————- 2′- 7″
6 FEET——————- 3′- 1 1/8″
7 FEET ——————- 3′- 7 1/4″
8 FEET ——————- 4′- 1 1/2″
9 FEET——————- 4′- 7 5/8″
10 FEET ——————- 5′- 1 7/8
11 FEET——————- 5′- 8″
12 FEET——————- 6′- 2 1/4″
13 FEET——————- 6′- 8 1/2″
14 FEET——————- 7′- 2 5/8″
15 FEET——————- 7′- 8 3/4″
16 FEET——————- 8′- 3″
17 FEET——————- 8′- 9 1/8″
18 FEET ——————- 9′- 3 3/8″
19 FEET——————- 9′- 9 5/8″
20 FEET——————- 10′- 3 3/4″
21 FEET——————- 10′- 10″
22 FEET ——————- 11′- 4 1/8″
23 FEET——————- 11′- 10 3/8″
24 FEET——————- 12′- 4 1/2″
25 FEET——————- 12′- 10 5/8″
26 FEET——————- 13′- 4 7/8″
27 FEET——————- 13′- 11″
28 FEET——————- 14′- 5 1/4″
29 FEET——————- 14′- 11 1/2″
30 FEET ——————- 15′- 5 1/2″
31 FEET——————- 15′- 11 5/8″
32 FEET ——————- 16′- 5 7/8″
33 FEET——————- 17′ – 0″
34 FEET——————- 17′- 6 1/4″
35 FEET——————- 18′- 1/2″
36 FEET——————- 18′- 6 5/8″
Subtract for the ridge-board (3/4 inch if 1 1/2 inch ridge-board) if required
Overhang
Add for overhang then snap a line and trim off for the facia board after installation to get a better finished product
Overhang Add
3 inch 3 1/8 “
4 inch 4 1/8″
6 inch 6 1/8″
8 inch 8 1/4″
10 inch 10 1/4″
12 inch 12 3/8″
Easy Bird’s Mouth Marking
- Cut the peak of the rafter
- Measure the length of the rafter (before adding the overhang) and mark the rise 3/12 in the same direction as the peak.
- Place a scrap board (the same width as the wall) against the length mark.
- With the edge of the scrap board corner at the edge of the rafter mark the bird’s mouth
- Cut the little wedge of the bird’s mouth out
- You now have a perfect bird’s mouth cut for the wall
Bonus
Height to the Peak from the Top of the Wall
The inches to the peak of a roof change when you have a bird’s mouth cut.
You need to add the number of inches on top of the bird’s mouth cut to the total number of inches of rise.
(example 3 inch rise, 5 feet to the center of the building 3 x 5 = 15″ + birdsmouth height = peak)
To the Peak
3 1/2″ rafter and 3 1/2″ wall
(2 3/4″ added for bird’s mouth height)
Building width and peak height for 2×4 rafter and 2×4 wall at 3/12 rise from the top of the wall.
Building width ———— top of peak–
4 FEET ——————- 8 3/4″ –
5 FEET——————- 10 1/4″
6 FEET——————- 11 3/4″
7 FEET ——————- 13 1/4″
8 FEET ——————- 14 3/4″
9 FEET ——————- 16 1/4″
10 FEET——————- 17 3/4
11 FEET ——————- 19 1/4″
12 FEET——————- 20 3/4″
13 FEET——————- 22 1/4″
14 FEET——————- 23 3/4″
15 FEET ——————- 25 1/4″
16 FEET——————- 26 3/4″
17 FEET ——————- 28 1/4″
18 FEET——————- 29 3/4″
19 FEET ——————- 31 1/4″
20 FEET——————- 32 3/4″
21 FEET——————- 34 1/4″
22 FEET——————- 35 3/4″
23 FEET——————- 37 1/4″
24 FEET——————- 38 3/4″
25 FEET ——————- 40 1/4″
26 FEET——————- 41 3/4″
27 FEET——————- 43 1/4″
28 FEET——————- 44 3/4″
29 FEET——————- 46 1/4″
30 FEET——————- 47 3/4″
31 FEET——————- 49 1/4″
32 FEET——————- 50 3/4″
33 FEET——————- 52 1/4″
34 FEET——————- 53 3/4″
35 FEET——————- 55 1/4″
36 FEET——————- 56 3/4″
3 1/2″ rafter and 5 1/2″ wall
(2 1/2″ added for bird’s mouth height)
Building width and peak height for 2×4 rafter and 2×6 wall at 3/12 rise from the top of the wall.
Bld. width peak Bld. width peak
4 FEET ——————- 8 1/2″
5 FEET ——————- 10″
6 FEET ——————- 11 1/2″
7 FEET ——————- 13″
8 FEET ——————- 14 1/2″
9 FEET ——————- 16″
10 FEET ——————- 17 1/2″
11 FEET ——————- 19″
12 FEET ——————- 20 1/2″
13 FEET ——————- 21″
14 FEET ——————- 23 1/2″
15 FEET ——————- 24″
16 FEET ——————- 26 1/2
17 FEET ——————- 27″
18 FEET ——————- 29 1/2″
19 FEET ——————- 30″
20 FEET ——————- 32 1/2″
21 FEET ——————- 33″
22 FEET ——————- 35 1/2″
23 FEET ——————- 36″
24 FEET. ——————- 38 1/2″
25 FEET ——————- 39″
26 FEET——————- 41 1/2″
27 FEET ——————- 42″
28 FEET ——————- 44 1/2″
29 FEET ——————- 45″
30 FEET ——————- 47 1/2″
31 FEET ——————- 48″
32 FEET ——————- 50 1/2″
33 FEET ——————- 51″
34 FEET ——————- 53 1/2″
35 FEET ——————- 54″
36 FEET ——————- 56 1/2″
5 1/2″ rafter and 3 1/2″ wall
(4 3/4″ added for bird’s mouth height)
Building width and peak height for 2×4 rafter and 2×6 wall at 3/12 rise from the top of the wall.
Bld. width peak Bld. width peak
4 FEET ——————- 10 3/4″
5 FEET ——————- 12 1/4″
6 FEET ——————- 13 3/4″
7 FEET ——————- 15 1/4″
8 FEET ——————- 16 3/4″
9 FEET ——————- 18 1/4″
10 FEET ——————- 19 3/4
11 FEET ——————- 21 1/4″
12 FEET ——————- 22 3/4″
13 FEET ——————- 23 1/4″
14 FEET ——————- 25 3/4″
15 FEET ——————- 26 1/4″
16 FEET ——————- 28 3/4
17 FEET ——————- 29 1/4″
18 FEET ——————- 31 3/4″
19 FEET ——————- 32 1/4″
20 FEET ——————- 34 3/4″
21 FEET ——————- 35 1/4″
22 FEET ——————- 37 3/4″
23 FEET ——————- 38 1/4″
24 FEET——————- 40 3/4″
25 FEET ——————- 41 1/4″
26 FEET ——————- 43 3/4″
27 FEET ——————- 44 1/4″
28 FEET ——————- 46 3/4″
29 FEET ——————- 47 1/4″
30 FEET ——————- 49 3/4″
31 FEET ——————- 50 1/4″
32 FEET ——————- 52 3/4″
33 FEET ——————- 53 1/4″
34 FEET——————- 55 3/4″
35 FEET ——————- 56 1/4″
36 FEET ——————- 58 3/4″
5 1/2″ rafter and 5 1/2″ wall
(4 1/4″ added for bird’s mouth height)
Building width and peak height for 2×4 rafter and 2×6 wall at 3/12 rise from the top of the wall.
Bld. width peak Bld. width peak
4 FEET ——————- 10 1/4″
5 FEET ——————- 11 3/4″
6 FEET ——————- 13 1/4″
7 FEET ——————- 14 3/4″
8 FEET ——————- 16 1/4″
9 FEET ——————- 17 3/4″
10 FEET ——————- 19 1/4
11 FEET ——————- 20 3/4″
12 FEET ——————- 22 1/4″
13 FEET ——————- 23 3/4″
14 FEET ——————- 25 1/4″
15 FEET ——————- 26 3/4″
16 FEET ——————- 28 1/4
17 FEET ——————- 29 3/4″
18 FEET ——————- 31 1/4″
19 FEET ——————- 32 3/4″
20 FEET ——————- 34 1/4″
21 FEET ——————- 35 3/4″
22 FEET——————- 37 1/4″
23 FEET ——————- 38 3/4″
24 FEET. ——————- 40 1/4″
25 FEET ——————- 41 3/4″
26 FEET ——————- 43 1/4″
27 FEET ——————- 44 3/4″
28 FEET ——————- 46 1/4″
29 FEET ——————- 47 3/4″
30 FEET ——————- 49 1/4″
31 FEET ——————- 50 3/4″
32 FEET ——————- 52 1/4″
33 FEET ——————- 53 3/4″
34 FEET ——————- 55 1/4″
35 FEET ——————- 56 3/4″
36 FEET ——————- 58 1/4″
I hope you have found this article helpful
Uncle Gary 17 Sept 2023