Rafters 32" on center

huai · 04-04-2010, 09:57 AM

Inherted a seasonal cottage with a problem. It's an A-Frame (16/20 pitch) with a relatively new roof. I pulled off the sheetrock on the ceiling to add insulation and discovered that the roof rafters are 2x8s, 32" on center. Because the length of the rafter is 17', the builder used two 10 footers bolted together in the middle to span the 17 foot distance.

Need to know if this is going to be a problem as time goes on. If yes, is there any way to strengthen the roof that doesnt involve pulling off the sheathing.

One idea that a contractor (not roofer) friend suggested was to add cross members between adjacent rafters every 4 feet to give them strength, but I'm worried this will block air flow once I add insulation and roof will rot.

Any ideas welcome

Thank you in advance.

Scuba_Dave · 04-04-2010, 10:14 AM

Do you have a pic of how they are bolted together ?

I think I'd add 20' cut to size in between for 16" OC

tpolk · 04-04-2010, 10:22 AM

whats a 16/20 pitch

jlhaslip · 04-04-2010, 12:16 PM

that's metric, I think...

:D

**kwikfishron** · 04-04-2010, 02:02 PM

32" centers on A Frames is pretty common but there Usually 4x8's with car decking on top and no sheetrock.

Daniel Holzman · 04-04-2010, 02:20 PM

You did not indicate where you are, so it is impossible to tell what the maximum live load (read snow) is on your roof. If you are in snow country, typical live plus dead load could be as much as 40 pounds per square foot, in which case your rafters are overloaded. If you are not in snow country, the maximum live load is probably due to wind, and you would need to check with your local code enforcement official or read the code book to see what the wind load would be on the rafter.

As for your particular case, the strength of the rafters is limited either by the depth and spacing, or by the bolt job, whichever is weaker. Without a hands on inspection, it would be difficult to compute the strength of the bolt job. Even if the bolts are adequate, you are probably substantially undersized. Adding rafters between the existing rafters to make 16 inch OC is one way to go, another would be to strengthen the existing rafters by sistering new wood, or possibly by adding steel plates. You may want to consult a structural engineer, they would no doubt have some good ideas.

As to whether this is going to be a problem, if you never get any snow or wind, maybe not. If you do, the roof could collapse.

huai · 04-04-2010, 03:22 PM

I'm in eastern pa (poconos) where snow is definitely a factor. The cabin is registered as a "recreational cabin" with the state so it doesn't have to meet residential code, but clearly I want to make it safe.

The pitch is very steep - 8 ft rise for 10 horizontal. The plywood used for sheathing is the pressed woodchip kind, not the glued layers of wood kind, so I think 32 inches is way too much of a gap

Daniel Holzman · 04-04-2010, 08:24 PM

So you have two issues. One is that the rafters are overspanned, i.e. overloaded. Two is that the roof sheathing is overspanned. So what you do depends on whether you want to spend the time and money to fix the problems, or take the chance that a heavy snowfall will collapse the roof.

huai · 04-05-2010, 08:26 AM

Well I knew I had a problem before I posted =/

Daniel, in your opinion do I have any way to backfill the missing rafters or is my only way out is to pull the roof off?

mark942 · 04-05-2010, 09:19 AM

Scuba Dave said it. Just split the 32" to 16" add new rafters.

Good Luck to You..............................

huai · 04-05-2010, 03:07 PM

Should I use the same rafter material (2x8s?) I have heard good things about lightweight wooden ibeams. Also - how would one go about locking both ends of the rafter in.

I can lock the lower end into the top of the wall, then bring the top of the rafter up to the central beam - then what? Notch it and move the end back and forth horizontally until it fits in?

mark942 · 04-05-2010, 04:53 PM

Go with what you have already. Here is a link http://www.ehow.com/how_2346313_build-rafters.html In your case look more at 1 thru 3

There was a question about how the sister was bolted up. Two 1/2 x 4 inch bolts per 2x8 with a square piece of wood on either side, or better? 6 bolts? 2 bolts?

**kwikfishron** · 04-05-2010, 05:03 PM

Hey Haui, can you post some pictures? Worth a thousand words.

huai · 04-05-2010, 05:48 PM

I'll be back working on the cottage this weekend, will post pictures then. I don't recall how it's bolted - will make sure to note that.

huai · 04-14-2010, 05:31 PM

I bought a 20 ft 2x8 and was able to cut it down and fit it in perfectly in the middle of the 32 inch span resting one end on the ridge and the other on the top of the wall. The only question that remains is how to nail it to the sheating.

If I just let the sheating rest on top of it, the beam will prevent roof from collapsing, but wont prevent bubbling and bowing of the sheathing. However if i try to nail it from the inside, I am afraid nails will penetrate the sheating and shingles, and my roof wont be watertight.

I can get up on the roof and do the nails from that direction, but not sure how to hit the new rafters directly and i might still ahve water problems.