I have a 14-foot by 6-foot almost flat porch roof, and the porch roof is held up by one column one each of the two front corners. Right now, across the front there are two 2"x6" beams sistered together.
Can anyone tell me if that is considered sufficient under current building codes, or where I can find some kind of info or load span table saying what is needed for this situation?
I am in New Jersey.
Here a flat roof is constructed much the same way a floor is, in order to carry snow load, while New Jersey is a little south/east of this area, I understand they still can get some large snowfalls. Most joist and beam tables would call for four 2x10s to support this roof, since it is only 6 feet deep you may be able to get away with three 2x10s. Check your local regulations, don't take my word for it, but two 2x6s will start to do some serious sagging over time and could fail if there is some heavy snow.
I just ran a quick calc with some large assumptions. I figured Hem- Fir, No. 1 grade equivalent (older houses were generally built with better wood), 70 PSF which is definitely higher than you'd experience (figured 30 PSF live load as it's almost a floor, 10 PSF dead load, and 30 PSF snow load for a 2 month duration), and it passed in all respects. The deflection was a bit high - almost an inch - but since it's a porch it's unlikely that that would present problems ( it's still standing, right?). If the porch roof is used as a deck, the live load would be higher, but it's unlikely that you're congregating on the porch roof while there's two feet of snow or more on it.
For your own edification and reference purposes, the Canadian Wood Council web site has a nifty online span calculator. It's under the Design Tools heading, IIRC.
R
Whoops! Scratch that! My blood sugar must be running low - the beam fails and has more than a _three_ inch deflection with the loads I listed above. I also just checked the Canadian Wood Council's site, and they've changed things around since I was there last. You'd have to download one of of their demo packages to calculate beams - probably not worth it.
Here's an online calculator for a simply loaded beam:
What are you trying to do? If you're looking to stiffen the beam you could sister on a 2x10 on either side and bolt them together, or you could bolt on some steel plate or channel.
Sorry for the hit-send-before-thinking post.
R
Ricod,
70 psf seems a bit high. The 2006 IBC provides load combinations for ASD in section 1605.3. If we ignore fluid, earth, temperature, rain, seismic, and wind, the load combinations become:D + L D + (L_r or S) D + 0.75L + 0.75 (L_r or S).
L_r is the roof live load, which is capped at 20 psf. L is the live load everywhere else, and of course D is dead load and S is snow live load. Notice that you never need to apply L_r and S simulaneously to the roof. If S is 30 psf, then the controlling load for the roof is D
The OP said it was a flat roof, not a deck, but if you want to treat it as a deck, then L should be at least 40 psf. The last load combination then controls with a load of 62.5 psf.
Yours, Wayne
Stop trying to make me feel better about the initial brain fart post! ;)
I'm more curious why the OP is asking the question. Is there substantial sagging now, or are there plans to make that roof a deck?
RThanks. That definitely helps. I think I have some work to do (see my other posts).
Thanks for doing those calculations etc.
You wrote, "What are you trying to do?" Here's the story:
I have a contractor doing vinyl siding on the front of a house I own. The house has an open 14'x6' overhang over a similar size concrete area which creates a front porch. The porch had 3 wrought iron support posts across the front -- one center, and one on each corner. The plan was, and is, to replace the 3 wrought iron support posts with one steel column on each corner.
After taking off the old vinyl that covered the porch overhang, it turned out that what was there was 3/4" plywood on top, double (sistered) 6-foot
2x4's (16" on center) running perpendicular to the house out to the main cross beam, and a main cross beam of I think two 2x4's sistered together or 1 2x6 (I forget which). The contractor changed the front beam and side end beams to 2 2x6's sistered together and said he did that because there was now a 14-foot span in the front. The result was a span which looked to me like it may have a very slight sag in the middle, but I wasn't sure.The building inspector came out to do the framing inspection before it was covered up by the new vinyl. The first thing he said is there is a sag in the middle that he could see right away just driving down the street while approaching the house. Then he said he thought it may be "under span" and measured that the span is 14 feet. He said he would check whatever reference chart he had and said that using 2 2x6's is insufficient for a
14-foot span.I had no reason to doubt the building inspector, and he was very reasonable about everything and explained how snow on the roof etc. could create problems. The reason I posted the question is that the contractor seemed so confident that he was doing everything up to and exceeding code. I am waiting for a call back from the contractor, and there is no question that he will fix it so the inspector can approve the job. In the meantime, I wanted to see if I could find out more information about what should be there to be correct.
Now, after reading the feedback here, it looks like I need to start over and have the whole overhang replaced with one that meets all of the codes. Obviously, all that existing 2x4 stuff doesn't cut it even if the main
14-foot cross beam were made of titanium. I'm guessing I'll need (and want) an architect or engineer's seal on the new plans. The good thing is that it's not that big of a job and it will be good to have it done correctly.Thanks aga>> >>
This is not obvious. A 2x4 rafter may well span 6 feet. If the roof was performing OK before you removed the central column , there's no need to change the rafters, just get the outer beam sized properly.
Cheers, Wayne
As usual in these type questions the answer is:
You have to satisfy the building inpsector. If he insist on something being way overbuilt...oh well, he won't pass anything less.
I went through the whole thing with my inspector when I "repaired" my porch roof. That is the way I worded the application and that is the way the permit was written. I even beefed it up considerably. Time I was done all that was original was a few 2x4 rafter.
His inspection wouldn't have passed it if it hadn't been grandfathered by the "repair" bit. He also said that the clerk who issued the permit would be "counseled".
Bottom line again: What the inspector says, is what happens.
Harry K
Only your local codes can say, no permit can mean you take it down, why ask here. Call and take out a permit or risk having a no sale or a suit when you sale. Do it right, get a pemit.
Is hem-fir likely in New Jersey? I thought east coast wood was mostly pine, which is substantially weaker than western woods.
I would think an engineered member (glu-lam) would be cheaper than dimensional lumber for this application. Any decent lumber yard could run the load calcs from a drawing and order the right beam.
Larry,
Ever hear of southern yellow pine? It probably the most commonly available structural lumber in the east and in equivalent size and spacing can span greater distances than douglas fir.
Thanks. That would make it easier to fix, and the existing roof was performing okay.
In my case, the building inspector is being completely reasonable. He wasn't looking to create problems. And, I think if the new crossbeam didn't already have a sag that he could easily see from the roadway, he may not have even decided to measure the span and check the code charts he has.
As usual in these type questions the answer is:
You have to satisfy the building inpsector. If he insist on something being way overbuilt...oh well, he won't pass anything less.
I went through the whole thing with my inspector when I "repaired" my porch roof. That is the way I worded the application and that is the way the permit was written. I even beefed it up considerably. Time I was done all that was original was a few 2x4 rafter.
His inspection wouldn't have passed it if it hadn't been grandfathered by the "repair" bit. He also said that the clerk who issued the permit would be "counseled".
Bottom line again: What the inspector says, is what happens.
Harry K
That's an odd occurrence. A building inspector is able to reject things whether they're on approved drawings or not. Usually they won't step on an engineer's toes, but if they see something obviously wrong they're bound to red flag it. I also don't understand how the word repair changes things. If you write repair and you materially change the structure it's not a repair and the inspector could and should deal with it accordingly.
As an aside, if it's a repair, why did you get a permit? Around here, where they want permits for everything, they don't require permits for repairs.
R
I admit that I do get a bit tired of people asking questions such as this when the answer is "see your inspector" in all cases.
Harry K
didn't
Ooops. I meant "I get a bit tired.... when people who are working with a permit..." These type questions are appropriate when no permit is in play.
Harry K
I think you missed the point on this, but that's okay.
Thanks for sharing your story of what happened between you and your building inspector in your situation.
Ooops. I meant "I get a bit tired.... when people who are working with a permit..." These type questions are appropriate when no permit is in play.
Harry K
Here in WA state, I would probably use 40psf for that roof, but I'll assume you get more snow for longer periods than us and will go with 60 psf.
I assume this roof is supported by the house on one side?
14' x 6' x 60psf = 5040 lbs totalHalf of that is supported by the house, leaving the beam to support the other 2520 pounds.
According to the charts I have, you would need three 2x12's to support that weight over 14' (using a 1/360 deflection, typical for floors).
If you can tolerate a little more deflection (shouldn't be a problem for a roof), you could get by with three 2x10's.
If you want an easier option than replacing the beam, you could add a third post in the middle of the span. This would reduce the span to 7', and the total load to 1260 pounds. Your existing 2x6 sistered beam would easily support that weight over 7'.
Anthony
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