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wheezer11 · Jul 30, 2010, 06:22 AM

My new shed is 12 feet wide. I want to use a 6/12 pitch for the roof. How long will my rafters be and how do I cut the birdsmouth. Secondly how tall will my ridge board be.
Thanks Rick

creahands · Jul 30, 2010, 10:13 AM

Hi Rick

If u are making a gable roof, the height of the ridge will be 36'' above top plate. 6'' up for every foot of run.

If u have a framing square, it will give the length of common rafter on it long leg.

Hold 6 and 12 on square and step it of 6 times. The 6'' side is the plumb cut that goes to ridge board. Reduce this cut by half the thickness of ridge board.

The seat cut is on the 12'' leg. This cut is made the width of top plate.

Finish off end of rafter with a plumb cut for overhang.

Chuck

dannac · Jul 30, 2010, 10:27 AM

Tried uploading PDF file... not sure if will work.

I may send a pic instead

dannac · Jul 30, 2010, 10:35 AM

creahands · Jul 31, 2010, 07:55 PM

Hi Dannac,

The measurement u have as 4 1/2'' is relative to the size of lumber used. The larger the lumber the larger this distance.

I u measure from top of ridge to plumb mark at wall u will have 36'' for a 6/12 pitch.

Chuck

dannac · Aug 1, 2010, 07:01 AM

Chuck , I agree... the 4 1/2" will be relative to lumber size.

I drew and assumed .... they would use 2 x 6, which may not be the case.

I measured from top plate to top of ridge.

6/12 from top plate ,
which I assumed the OP was asking,
should be 3' plus the 4 1/2" allowed above birdmouth... or 3' - 4 1/2"

But the rafters do not come to a point at ridge,
the ridge is flat on top, thus the loss of 3/8"
Making it 3' - 4 1/8".

hkstroud · Aug 1, 2010, 07:41 AM

Top of ridge board will be 36" above top wall plate, rafter length will be 93 1/2" (assuming 2X material for ridge board and 1/2" wall sheathing), angle of cuts will be 22.5 degrees.
Measure point A to point B, divide by 2. Measure down the side of ridge board that distance, make a mark. Measure from mark on side of ridge board to outer edge of top wall plate or outer edge of sheathing if wall sheathing is installed. Measure down center of rafter that distance, make X. Draw line through X at 22.5 degrees. Draw second line 90 degrees to first. Cut bird's mouth.

wheezer11 · Aug 1, 2010, 08:56 AM

Thanks everyone for all the great info this is an awesome sight. I think I can get it done now. :D

dannac · Aug 2, 2010, 02:45 AM

I disagree with post of dividing lumber size in half to get
plumb cut / seat cut.

Will weaken rafter... no need for a 6" seat cut on a 2 x 4 wall.

hkstroud · Aug 2, 2010, 05:56 AM

Why do you assume 2x6 material?
This is high slope rafter (greater than 3 in 12)
Rafter span in 6' 8".
Allowable span of 2x4 SPF 16" on center is 7' 4".
The seat cut of a 2x4 at 22.5 degrees is approximately 3 1/2".

Not concerned about weakening the rafter. The remaining material after cutting the bird's mouth only supports the over hang. More concerned about amount of rafter material that rest on the wall.

If wheezer11 wants to use 2x6 material for the rafters and modify depth of seat cut that's OK too. That won't make a measurable difference in the location of the cut. We aren't building a Swiss watch here.

dannac · Aug 2, 2010, 06:20 AM

Originally Posted by ;

More concerned about amount of rafter material that rest on the wall.

That's precisely the point I was trying to make.

I assumed 2 x 6 because
I would build with 2 x 6 at 24" O.C. instead of 2 x 4 at 16" O.C.

Just my opinions...

hkstroud · Aug 2, 2010, 07:00 AM

I errored.

Failed to read note in code book. Span is measured as horizontal projection, not along slope of rafter between two supporting points.. That makes span the 6'. Actually 5'8", if 12' is exterior dimension of shed. That mean 2x4 meets span requirements even on 24" centers.