Roof framing questions
#1
06-06-06, 07:30 AM
Roof framing questions
I have completed the framing of the walls for my 12'x12' shed. They are plumb and square with a double top plate. I am having trouble figuring out the measurements for the rafters and ridge height. It will be a 4/12 pitch gable roof. I purchased a 2x8(overkill?) for the ridge beam thinking it would create a stronger roof than a 2x6. I was planning on using 2x4 rafters 16" OC. I was thinking of using a 6" overhang. Is that adequate?
First step I'm assuming is putting up the ridge beam, but my question is "at what height?" It will be resting on each end on 2x4s sandwiched between to other taller 2x4s to create sort of a "U" shaped notch for the ridge to rest in. How tall should that center 2x4 be cut assuming the 7.5" tall 2x8 ridge and a 4/12 pitch or do the gable end rafters hold the ridge beam in place?
Thanks for any calculation help you can offer.
First step I'm assuming is putting up the ridge beam, but my question is "at what height?" It will be resting on each end on 2x4s sandwiched between to other taller 2x4s to create sort of a "U" shaped notch for the ridge to rest in. How tall should that center 2x4 be cut assuming the 7.5" tall 2x8 ridge and a 4/12 pitch or do the gable end rafters hold the ridge beam in place?
Thanks for any calculation help you can offer.
#2
06-06-06, 07:54 AM
Member
John - I probably would have gone with 2X8 rafters and 2X4 ridge. 
The strength of the roof comes primarily from the opposing rafters.
Assuming you are putting up a gable roof, you can calculate the height of the ridge board. 1/2 the total length (12/2=6') less the width of the bird's mouth (3 1/2") will give you the run 6' - 3 1/2" = 5.71') . The rise is the run times the pitch (4"/ft of rise or .33') of the roof (5.71' X .33' = 1.88'). The bottom of the ridge should be at 1.88' or about 1' 11".
If you haven't done a roof before, the best way to visualize the roof assembly is by laying out the gable end rafters flat on the ground, using a small piece of 2X to simulate the ridge. Mark and cut the ridge plumb cut and the bird's mouth cut on the gable rafters and use them as a template for all the common rafters. The bottom plumb and tail cuts can be done after the rafters are up or at the same time you cut the bird's mouth. I usually cut them in place because I think it makes for a straighter eave.
The strength of the roof comes primarily from the opposing rafters. Assuming you are putting up a gable roof, you can calculate the height of the ridge board. 1/2 the total length (12/2=6') less the width of the bird's mouth (3 1/2") will give you the run 6' - 3 1/2" = 5.71') . The rise is the run times the pitch (4"/ft of rise or .33') of the roof (5.71' X .33' = 1.88'). The bottom of the ridge should be at 1.88' or about 1' 11".
If you haven't done a roof before, the best way to visualize the roof assembly is by laying out the gable end rafters flat on the ground, using a small piece of 2X to simulate the ridge. Mark and cut the ridge plumb cut and the bird's mouth cut on the gable rafters and use them as a template for all the common rafters. The bottom plumb and tail cuts can be done after the rafters are up or at the same time you cut the bird's mouth. I usually cut them in place because I think it makes for a straighter eave.
#3
06-06-06, 08:56 AM
Wayne, thanks a lot! I'll return the 2x8 and use a 2x6 I already have that I used for the concrete slab form. 
Could you tell I haven't framed a roof before? 
OK. I figured the height at 1' 10 7/8", so I guess thats a start. Now you're saying that this measurement is to the "bottom" of the ridge beam? Doesn't that take into account the size of the ridge beam or should that be to the top of the ridge?
What about the rafter length? a²+b²=c² , right?
a=6' and b=1.88, right? So it would be 6.28' plus whatever the overhang will be?
Could you tell I haven't framed a roof before? 
OK. I figured the height at 1' 10 7/8", so I guess thats a start. Now you're saying that this measurement is to the "bottom" of the ridge beam? Doesn't that take into account the size of the ridge beam or should that be to the top of the ridge?
What about the rafter length? a²+b²=c² , right?
a=6' and b=1.88, right? So it would be 6.28' plus whatever the overhang will be?
