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Old 03-09-2010, 04:01 PM   #1
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Replace 2x4 rafters?


I have to replace the roof on a one car garage. The house was recently redone in dimensional shingles and the owner would like them on the garage, too.

The garage is currently full of stuff so it's hard to see everything perfectly, but the roof seems to be 2x4 rafters, 24"OC with some 1x4 and 1x6 planks spaced about an inch apart. There is no plywood up there, just tar paper and pretty thin asphalt 3-tab shingles. The pitch is about 5/12 or 6/12.

I intend to tear off the shingles and paper. Should I add 1/2" sheathing over the existing planks, or tear them off too?

There is slight visible sagging now but I am concerned about the added weight of 1/2" sheathing and 40 year dimensional shingles.

Do I replace the 2x4s with 2x6s? Is 24" OC sufficeint or should I go to 16"OC? I was not going to use trusses.

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Old 03-09-2010, 04:23 PM   #2
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Replace 2x4 rafters?


I wouldn't add that weight to it. I would call an architect/structural engineer to sign off on it before recommending anything if you're unsure.

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Old 03-09-2010, 04:39 PM   #3
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Replace 2x4 rafters?


What size garage ?
Span for rafters ?
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Old 03-12-2010, 05:29 PM   #4
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Replace 2x4 rafters?


The garage is about 12 x 20 with a center ridge beam so the horizontal span of the 2x4 rafter is about 6 feet.
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Old 03-12-2010, 05:35 PM   #5
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Replace 2x4 rafters?


Well, using this span calc a 2x4 24" OC will span over 6'...depending upon wood species

http://www.awc.org/calculators/span/...rcalcstyle.asp

But, if there is already sag that will get worse with more weight
So I would probably sister in 2x6's if possible
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Old 03-12-2010, 09:41 PM   #6
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Replace 2x4 rafters?


1. How much do the new shingles weigh per square foot?

2. OSB or plywood?

3.Do you know the species of wood of existing rafters?

4. You cannot "sister-in 2x6's" without replacing the ridge board, may as well rebuild the roof per current building code .....

Be safe, Gary
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Old 03-13-2010, 04:27 PM   #7
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Replace 2x4 rafters?


>> 1. How much do the new shingles weigh per square foot?

245 lbs./square so 2.5 lbs./sq. ft.


>> 2. OSB or plywood?

Was planning on 1/2" OSB


>> 3.Do you know the species of wood of existing rafters?

Unfortunately I can't get in there easily yet but the wall studs look like spruce/pine/fir?


>> 4. You cannot "sister-in 2x6's" without replacing the ridge board, may as well rebuild the roof per current building code .....

I was afraid of that but I will be using a load calculator to see what the existing structure can hold once I can figure out the species

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Old 03-13-2010, 06:17 PM   #8
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Replace 2x4 rafters?


Check with your local building department for local snow load, may be more or less than the 35" average: http://first-time-home-buyer-s.com/m...e-mortgage.htm

The charts vary greatly with species, from your bare minimum with S/P/Fir to maximum.
You could easily sister 2x4's for 80% doubling of loads to be safe... 24" o.c. spacing, with shingles = 5#, OSB = 3.4#, rafters = 1.7#, 1x6's = 4.5#, total of -15# dead load, plus live load snow load.

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