rafter layout

[Terry Barr]

hi guys the problem lies with the layout of the birds mouth in the rafter i seem to be coming up about 3/4 inch short on this cut. I've measured the run and the rise with no differences in the measurments what am i doing wrong

[Matt Ocel]

hi guys the problem lies with the layout of the birds mouth in the rafter i seem to be coming up about 3/4 inch short on this cut. I've measured the run and the rise with no differences in the measurments what am i doing wrong

Terry - There are a few different ways to measure the length of a common rafter. If you are using a framing square table, the measurement I take is from the bottom of the plump cut at the ridge, to the inside edge of the top plate.

[Rob Wright]

Terry,

Did you use a rafter table? Sometimes they have been figured for a ridge board (2x3/4" - 1.5" = 2x8 ridge board). That would be my first thought...

- Rob

[Matt Ocel]

If you already have the ridge beam set, just hold up a piece of rafter stock and scribe it.

[NICK BARBOZA]

3/4" would be half of your ridge pole. deduct that and you get your birds mouth right on...

EDIT: i miss read.... nevermind

[Matt Ocel]

If he deducts another 3/4" won't he be 1 1/2" short then?

[Charles Cannon]

If you are making your measurement ftom the long point at the ridge to the seat of the birds mouth you would be short. I would cut the birds mouth then with the framing square set on the pitch bring that point to the top of the rafter, then measure the length to the ridge cut along the top of the rafter. I guess my point is that the measurement should not cross the rafter.
Cannon

[Paul Girouard]

Are you sure your building measurements at "right on"?

Your not pulling the B/M up against the wall sheathing and haven't figured that in?

The last poster mentions the "measuring line" it must be measured parallel not "across" the rafter. It doesn't matter if it heel of ridge to heel of B/M , or tip to tip ( project the B/M pencil line to the top of the rafter in other words.

Those are a few common error points / places.

I generally cut two rafter (one pair) then try them on either end of the building , before cutting the rest of the rafters. Yes you'll need to compensate for ridge board / ridge beam thickness , or use a scrape piece when you do your "test" fitting.

[Matt Ocel]

Terry -

Give us your building width(framing to framing-do not include sheathing), roof pitch, thickness of ridge board and lets see what we can come up with for a rafter length.


This should be interesting!:)

[Terry Barr]

the pitch is 5/12 the distance is 22' 11"
I've tried the square and the computer for figuring the lengths and come out with in 1/64 of an inch or overall 1/32 which this is rough framing

[Terry Barr]

With a building length of 24' and width of 22'11", a roof overhang of 2' and gable overhang of 1'4" the roof is 26'8" long and 26'11" wide
With an angle of 22.6199 °, the overall height above outer wall is 5'6~11/16" . The roof area is 777.59 ft²
Rafter spacing is 1'11~7/8" (Next larger spacing = 2'2~1/32" Next smaller = 1'10~1/32" )
107'4" lineal eave @ 2' overhang side and 1'4" overhang gable = 175.1 ft²
Gable Area (under rafters ) = 53.52 ft²
You need: 1 Ridge @ 26'8" × 1~1/2"
26 Rafters @ 14'10" × 9~1/4" × 1~1/2"

http://www.blocklayer.com/Roof/ImageGen/DrawRafterEng.ashx?rd=296&sb=123&a=67.4&rl=5696&apc=true&pw=48&ltb=4741

[Charles Cannon]

My rafter book says that the rafter for a 23' building should be 12' 5 1/2". I assume that is the total width of the building. After laying out the rafter for a 23' building subtract 1/2 inch from the rafter (perpendicular to the plumb line) to make it for a building 22'11". Then subtaact half the thickness of the ridge the same way.
Cannon

[Carl Hill]

Hey Terry, are you still working on this? There are some figures on your diagram that don't jive. The 14'-10", the 1 5/8" and 5/8" birdsmouth seem to be off. If you're at your computer, let's talk.

[William OConnell]

hi guys the problem lies with the layout of the birds mouth in the rafter i seem to be coming up about 3/4 inch short on this cut. I've measured the run and the rise with no differences in the measurments what am i doing wrong
It sounds like your not allowing for the ridge thickness

[Matt Ocel]

Terry -

Assuming 1 1/2" ridge board.

Rafter length if 2 x 6 walls 11' - 10 3/16"

If 2 x 4 walls 12' - 0 3/8"

This measurement is from short point of 5/12 plumb cut along bottom of rafter to start of birds mouth.

I would then run rafters long and cut tails after.

If you give me the material for ridge board I can calculate elevation to set ridge accordingly.

[Terry Barr]

2 x 10 (1 1/2" x 9 1/4") rafters
2 x 12 (1 1/2" x 11 1/4") ridge

[Matt Ocel]

Terry - If my math is right, try these numbers

If 2 x 4 walls the bottom of the ridge beam is to be set at 4' - 6 1/4"

If 2 x 6 set bottom of beam at 4' - 5 7/16"

I would string a line from birds mouth to birds mouth and measure up from there.

Let me know how it comes out.

Good Luck!

P.S. I like to frame my roofs with wood I-Joist.

[Matt Ocel]

Terry -

If your intertested, this is a great book. Old school stuff.