The house we just moved into has some slight sagging issues and I was hoping someone w/ more experience in crunching the numbers has some advice.
The house is a little over 30′ wide with a full basement so making adjustments is fairly easy. Originally, they ran 2×8’s 16″ O.C. for about 12′ before the supporting beams. Essentially, the ran two support beams (evenly spaced) in the basement to support the floor joists. The difficulty arrises because the two bearing walls from above are not directly above these supports. I believe that the builders assumed the cantilevered support by the 2×8’s (the joist lapping extends underneath the walls) would do the trick.
Over the last 80 or so years it hasn’t.
The difference in support placement is only about a foot to 16″, so I will be intending on extending the span to about 13′ as opposed to 12′. There have already been about 4 or 5 joist failures from knots breaking loose on the underside which will be supported and sistered on to. And the current floor does not feel spongy.
So here’s the question, Would reinforcing the span by sistering new 2×8’s w/ the new 13′ span be enough reinforcement to stem possible deflection issues of the increased span? Or, do I need to increase the size to 2×10?
I know that 2×12’s would be ideal, however, there is only about 7’4″ of space as there is and I would like to keep as much head room as possible.
Replies
How about resizing your photos so the dial people can see them this year and the rest of us can get a better perspective of what we're looking at.
Is this any better?
Here is a link to an online span calculator:http://www.awc.org/calculators/span/calc/timbercalcstyle.aspA quick run of the span says a 2x8 on 16" centers is good to 12'3". But that is for a clear floor load only. There could be a lot of weight on it from that wall up above. I would try and get a beam directly under it. If that's not possible, Then you need to calculate the total load for the upstairs wall and everything that it's holding up as well, and calculate for the point load on the span that the wall is creating. There is software (BeamCalc is one) that will do that. Figuring out what the load of the wall really might be is the difficult part.Steve
Thanks that's great. I'll definitely bookmark that one.According to the calculator 2x10's would do the job on their own once I increase the span, but do you happen to know a way of establishing if 2-2x8's would be equivalent?Thanks
Ott
that #3 photo with the knots is a classic, I saved that to my photo file.
If you do any steel work, one option would be to bolt a 1/8 thick 1-1/2 wide steel strip along the bottom of the sagging joists. Drill and place the strap, jack up the middle with the strap in place, then bolt. Let off the jack and the joist remains straight.
There is a fair amount of detail on bolt spacing for that approach (more bolts at the ends than the middle). You will likely find it easier to use the sistered joists method if you do not have a drill press and bolt driver on a 1/2" drill.
If you want to do the steel strap approach (or an angle across the edge) e-mail me the specific spans, etc. and will do a bolt spacing schedule for you as I can find time.
Thanks, I'll check into that as a possibility. Depends on costs.Ott
rule of thumb for a load that sets on a cantilevered position like that is to move over no more than the depth of the joists.So for 2x8 joists at 7-1/4", you should have the new beam support position no further than 7-1/4" from the wall position.any more than that and I want an engineeer to help me study things
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I'll be installing the new support beams directly under the walls to fix the fact that they didn't to begin with. They installed the original support about a foot from the wall. Sure, evenly supporting the floor joists across the width of the house, but not supporting the load above it. The funny thing is that there are two supports close to the bearing walls, but not underneath them, and all of the weight of the walls falls between the middle of the house and the supports. There probably wouldn't have been a problem if they had put 2x12's in that central section. And to top it all off the electrician cut a 2 1/2" hole chaseway through the joists when he did the re-wire. (between one of the support beams and the cantilevered wall) Go figure.Ott
Ott,Spans have been downrated in recent decades to accomodate lower quality framing lumber. When I built my Colorado house in 1983, #2 and better doug fir 2x8's at 16" oc were rated to span up to 13'-1". Knowing this, it seems that sistered 2x8's at 16" oc will work just fine. If the originals are okay, then the sistering puts you at the equivalent of an 8" layout. What does the table say for 12" oc? If it works at 12", it ought to work better at 8".Bill
Thanks Bill,Pure logic, that's what I needed. Your reasoning makes perfect sense I simply wasn't looking at it that way.The span calculator with #2 S/P/F 12" O.C. says 13'6" is an acceptable span. Doubled 2x8's is the equivalent to 8" O.C. like you said, so I think I'm more than covered. I thought that it should be, but I couldn't figure out how to confirm it.Thanks a bunch, you guys are great!Ott
" If the originals are okay, "IFdid you see his photos? I wouldn't use half of those for joists because they have so many knots. Somebody bought the cheapest grade they could get originally
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Piffin,I just looked at the pics. You are right, the wood is very poor grade stuff. The failed joists will need two sisters applied to get to the equivalent of 8" oc. The ones that have not failed, if not overly deflected, will add stiffness and some strength to the assembly.Bill
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