Hey Fellas,
I’m building a 25×15 foot deck and I want to build a girder that cantelevers. To the best of my knowledge I can cantilever 1/4 of the length of the span. My deck will be 15′ out from the house which means I can make the cantilever a max. of 11.25′ correct? I’m using 2×10’s for the framing and my question is, can I span 11.25′ safely with 2×10’s? I would step it up to 2×12’s but I’m finishing the deck with composite lumber and I had to special order all the facia which is quite expensive and a 2-3 week wait for delivery. All the facia I ordered is 2×10. I’ll be making the girder the beams bolted to the posts. (6×6 posts and 2×10 beams). I have a “deck book” that has a chart that shows that a 2×10 spaced 16″ OC can span 16.5′. It seems like alot to me. Anyway just wanted some thoughts on “my span”. Thanks.
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Best thing to do is call those in charge in your jurisdiction. I've read that 25% rule before as well, but it doesn't fly here. No matter what, we can't cantilever anything more than 2 ft. All I'm saying is we're just going to be guessing at what rules apply to you where you're at. As to the span, yes, 2x10's ought to do fine. I think even a 2x8 comes close to that 11 ft on a table, but I've seen that span ten and it's a little spongy for my taste.
"If you pick up a starving dog and make him prosperous, he will not bite you. This is the principal difference between a dog and a man." - Mark Twain
Thanks RW. appreciate your input.
Yes it will be attached to house. Why do you reccomend against that?
You might want to look at some archived discussions about this. Just recently there were quite a few. Someone was saying in some places code won't even allow you to lag the ledger to the rim joist--must use carriage bolts through both. If I'm not mistaken, a double cantilever is better for strength because the reactions of the weight are upward in the center of the beam, which counteracts the load; the supports acting like fulcrums and the cantilever like a teeter-totter.
Danno, What do you mean by a double cantilever?
Basically what I'm concerned about is how far can I span a 2x10 joist out from the house(ledger) to the girder? I would like to just have 1 beam running under the deck if possible.
Like RW said, 2x10 should be adequate--I have a book Simplified Engineering for Architects and Builders by Harry Parker. A table in there for live load of 40 psf with little deflection (1/360 of span) for 16" spacing gives allowable length of 14'-6" to 17'-4", depending on factors like modulus of elasticity and allowable extreme fiber bending stress (yeah, I know, gobbledegook); sufice it to say, just about any wood you encounter will be okay for 2x10 joists to span 14'-6" and a cantilever will make them able to span more. Douglas fir and southern yellow pine are the strongest.
Your girder will need to be sized to support the load, I'll look in this book some more and see what it says, unless you have that figured out.
To do that, I'd need to know the distance between posts. It's been a long time since I've done these types of calculations, so I wouldn't trust them, but it looks like if you only had posts at thirds, you'd have to have 3 2x10's for the girder. With four posts, one every 6'-4", you would only need double 2x10's. BUT don't believe me--have someone knowledgeable check this out! I probably shouldn't even guess at this stuff! Building inspectors who view these messages are probably rolling their eyes!
Edited 8/26/2004 9:47 pm ET by Danno
Danno, Thanks for input. I already have the ledger on the house. Bolted every 16" top and bottom. I'm using all 2x10 doug fir for framing. My plan is to make my girder with 2x10's on 6x6 posts. The girder will be 12' from the ledger with a 3' cantilever. I'm not worried about the 3' cantilver, 2x10's over a 3' area is plenty strong. I just want to be sure a 2x10 spanned 12' will be adequate.
It will. Look at my previous reply--took me too long to edit it that your last message came before I was done!
Yeah that's what i thought Danno, just did a search online and found that anywhere from 12'-15' was an exceptable for 2x10's spaced at 16".
BTW I plan on using (4) posts to support the girder (25')
Might be the wrong terminology--but like one reply said, a beam under the joists two feet from the house (rather than a ledger) and another two feet from the far edge away from the house. So it looks like the Greek letter 'pi'. Or two capital ''T's" like so: TT. The left and right tops of the T's being the cantilevers.
Cantilever: if I am understanding you on the rule of thumb that your are referring to is so the fulcrum you are creating by extending the joist beyond the post and beam don’t lift the deck behind the fulcrum. It has nothing to do with how far you can physical extend the cantilever beyond the fulcrum. That distance has more to do with strength of the joist (depth x widith) and what the joist is made of (steel, oak, and pine) or the combination of the materials. Most building code limits the cantilever to 2 feet with out an engineer signing off on it.