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Old 08-26-2015, 12:35 PM   #1
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existing rafter sizing for new roofing


The rafters are true 2x6 and about 14' from ridge to birdsmouth and no collar ties or supports. The ceiling joists are also the rafter ties. The ridge board is 2x8 but is not a structural ridge beam. The lumber is rough sawn from the mid-late 30s. They are about 7:12 pitch but only 24" o.c. It has clean plywood deck, probably in the 70s. Rafters are straight.

Can I put architectural roofing shingles on this roof? I can't tell if there are one or two layers (existing is 3 tab shingles), but it will be stripped to the deck. The shingles I chose is GAF timberline architectural.

As always, thank you in advance.

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Old 08-26-2015, 02:59 PM   #2
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Rafter sizing is usually driven by snow load. You need to tell us your location and which code you are under to determine if the rafters are adequate.

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Old 08-26-2015, 03:01 PM   #3
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Your have plenty of slope and a single layer of anything (shingles) can't possibly weight more than two layers.

BTW, my roofer tells me that architectural roofing shingles are better than 3 tabs for a roof where the slope is marginal. I assume that is because there are less places for water creep.
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Old 08-26-2015, 04:16 PM   #4
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Quote:
Originally Posted by carpdad View Post
The rafters are true 2x6 and about 14' from ridge to birdsmouth and no collar ties or supports. The ceiling joists are also the rafter ties. The ridge board is 2x8 but is not a structural ridge beam. The lumber is rough sawn from the mid-late 30s. They are about 7:12 pitch but only 24" o.c. It has clean plywood deck, probably in the 70s. Rafters are straight.

Can I put architectural roofing shingles on this roof? I can't tell if there are one or two layers (existing is 3 tab shingles), but it will be stripped to the deck. The shingles I chose is GAF timberline architectural.

As always, thank you in advance.
Hey carpdad,

"One or two layers" is essential to the question, as is snow load criteria. It may be a trade off if there are two, but let's compare actual numbers. There is a wide range of dimensional shingle. Does it give a weight per square or bundle? This may not be an issue unless your snow load is already taxing the roof.

Last edited by keymaster; 08-26-2015 at 04:21 PM.
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Old 08-26-2015, 06:08 PM   #5
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I'm in north NJ. I've only worked with 16" o.c. rafters and I'm not sure about this house. I will go look at the rafter size table. Sorry, just got little lazy. I would appreciate a lesson on how to use the rafter table. I know I have to find the regional snow load, although I forgot about it until you reminded me, thanks.
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Old 08-26-2015, 06:39 PM   #6
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I take being sawed in the 1930s means they were put up as rafters then. Being they are still straight I'd strip however many layers are there, shingle it and not even bother looking a a table.
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Old 08-26-2015, 06:58 PM   #7
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"I'm in north NJ"-------------- http://www.cmdgroup.com/building-codes/new-jersey/

Check for local amendments as listed. IRC; http://publicecodes.cyberregs.com/ic...9_8_par027.htm

IBC; http://publicecodes.cyberregs.com/ic..._15_sec010.htm

http://publicecodes.cyberregs.com/ic...?bu2=undefined

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Old 08-26-2015, 07:21 PM   #8
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Found some things, some questions.
Also if anybody else was wondering, this is how I got some numbers.

Ground snow load (necessary for roof snow load) by zipcode - mine is 30 psf
7:12 roof pitch is 30.26 deg
used 1.0 for exposure and temp factors
above info gave me 23.1 psf roof snow load (roof snow load calculator)

MSR lumber council table - I used the table specifying:
20 psf live (snow load)
10 psf dead (light roofing)
//180 for deflection (no finished ceiling material)
load duration (Cd) of 1.15
2x6 24" o.c.

I used 2100f (1.8E) because it was the middle, which gave me max span of 14'1" for the rafter. This is just what I have (formula for hypotenuse of a triangle).

Would you explain what those 2100 and 1.8 mean?

Thank you Seniorsitizen for a good laugh. If not for the internet, that is what i'd done.

Gary in WA, I will check those tables.

Again, thank you for all replies and would appreciate it if you will confirm or disagree with my steps. For example, I am not sure, with 23 psf roof snow load, if I can use 20 psf live snow load table.
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Old 08-26-2015, 07:24 PM   #9
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So what's the plan of a structure built in the 30s doesn't meet today's code?
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Old 08-26-2015, 07:33 PM   #10
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That ICC table says I'm under supported. Taking #2 douglas fir, the max is 11'9". The MSR table seems to assume fairly high grade lumber.
I'll see what the town inspector says.

Is there a span table for full size lumber, not the nominal size tables?
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Old 08-26-2015, 07:36 PM   #11
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I think that is why no town inspector ever stopped to ask if I had a permit to reroof. But I did only about 5-6 shingle roofs and 1 low slope roof for my parents.
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Old 08-26-2015, 07:51 PM   #12
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Quote:
Originally Posted by carpdad View Post
I'm in north NJ. I've only worked with 16" o.c. rafters and I'm not sure about this house. I will go look at the rafter size table. Sorry, just got little lazy. I would appreciate a lesson on how to use the rafter table. I know I have to find the regional snow load, although I forgot about it until you reminded me, thanks.
I pulled this ground snow load map from the 2012 IBC, it is an approximation (not down to the address). Your best source for snow load data is the building department. Ground snow load is used to determine design loads.



It looks like you're between 20-30 psf, depending on the location. I had to zoom in to make sense of NJ. Deflection limits will also be in the code.

From the IRC, rafters with no attached gypsum, and slope greater than 3:12, your limit is L/180. (Limit means the max deflection the code allows for those conditions.) It's hard to truly "know" as the species is unknown and grade is unknown. Using favorable assumptions (Doug Fir select structural and L/180) your max span for 2x6 is 14'. I used 20 psf--the actual design snow load might be less (or more). If I used 30 psf--without accounting for slope--your roof would not meet minimum deflection criteria.

I used tables. If you plan on adding more weight, talk to an engineer. I can say this without engineering: if your roof is bearing two layers of 3-tabs without problems, shingles of comparable weight shouldn't be a problem.

Last edited by keymaster; 08-26-2015 at 08:09 PM.
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Old 08-26-2015, 08:19 PM   #13
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Quote:
Originally Posted by carpdad View Post
That ICC table says I'm under supported. Taking #2 douglas fir, the max is 11'9". The MSR table seems to assume fairly high grade lumber.
I'll see what the town inspector says.

Is there a span table for full size lumber, not the nominal size tables?

Are your rough cut rafters two inches in actual dimension?
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Old 08-26-2015, 08:53 PM   #14
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The rafters are actual 2x6. Although full of dust, I also didn't see knots. At least no radial cracks over the knots.
I just found out those numbers (2100, 1.8) are grading of the lumbers. Keymaster, absolutely right, there is no way for me to even guess what grade the rafters are. If I take the lowest grading listed in the MSR council table, these rafters aren't good enough.
I'm asking because I want to use heavier architectural shingles. I assume it had cedar shake roof, ply decked in the 70s with 3 tab shingles. I also assume the cedar roof would be lighter than asphalt shingles. I know this can be answered by going up to the roof, but I also wanted some number backed answers, esp for when talking to the town inspector.

If the inspector says no, I'll just sister 2x6 every other rafters.
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Old 08-26-2015, 09:32 PM   #15
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I don't think those numbers are grades. I think the 2100 is allowable stress in psi, and the 1.8E means 1.8 million psi is the modulus of elasticity. Of course you have no idea what the actual allowable stress is for your beams, or the modulus of elasticity.

Lumber grade would be something like select spruce, pine, fir (SPF select). Typically you get the allowable stress and modulus of elasticity from the grading, since it is pretty rare to have actual test results for lumber.

There are probably no tables for full sized dimensional lumber, but you can adjust the existing tables based on the ratio of the moment of inertia of the sections.

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