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-   -   rip floor joists down 3/4" (http://www.diychatroom.com/f19/rip-floor-joists-down-3-4-a-152368/)

DIY1 08-02-2012 03:23 PM

rip floor joists down 3/4"
 
My concrete guy poured my foundation walls about 3/4" too high :furious: for my home addition project. Perhaps it was my fault for giving him the EXACT wall height that I wanted, and not knocking off a good half inch to give myself some play. But as it is if I built my addition per spec's, there would be a good 3/4" step up between my old 1st floor living room and my new living room in my addition.

My best option at this point is to rip the floor joists down just enough required to get me back on track. The architect had me down for 2x8" S.P.F. @ 16" o.c. joists (for the 10' 9" span), but I figure if I rip the 2x8" down from 7 1/4" high to 6 1/2" high and go with 14" or 12" o.c. spacing I'll be just fine. Trouble is the span tables don't (understandably) list spans for every conceivable joist height. I think it's safe to say that 6 1/2" joists at 12" o.c. would be fine since I've seen 2x6" joists listed out to a 10' 8" span in the tables. Am I nuts?

loftezy 08-02-2012 03:35 PM

Yes, if you rip each floor joist by 3/4", you are nuts. You should look into just notching the bottom of each joist by 3/4" where the ends rest on the foundation.

DIY1 08-02-2012 03:37 PM

That is starting to make more sense. That's why this forum exists--to keep the nuts from being nuts.

DIY1 08-02-2012 03:52 PM

So the nice person above has recommended that I notch my joists at the ends instead of ripping them all of the way down.

To review, my 1st floor joists (that rest on the treated sill plate) will be notched at the ends so that the height from the foundation wall to the top of the joist is reduced by 3/4". This will keep the joists from contacting the foundation itself, so I shouldn't have to worry about using treated joists.

The only other thing that would make me feel good about keeping these notched joists at 16" o.c. is if there were some sort of joist hanger that I could attach to the sill plate to give the very bottom of the joist a little bit extra support. Does something like this exist? Or should I just wedge a 3/4" piece of treated under there? That way it would seem to me that the joist will be at full strength.

loftezy 08-02-2012 03:52 PM

This thread has a good explanation of shear versus bending moments on a beam, which explains why its OK to notch the end of a joist (up to a certain point, of course). Please note that the photo of the notch in the thread isn't how yours will look, but the discussion is still relevant.

There's also a link in the thread that will show you where and how big the notches can be.

http://www.diychatroom.com/f19/notch...emoval-111626/

mae-ling 08-02-2012 04:03 PM

Hmmmm, I was taught to not notch the bottom as this creates a place for it to crack.

DIY1 08-02-2012 04:11 PM

I feel a bit better about things. I'm probably good with the notch the way I intend to do it (which is a 4" long notch 3/4" high at the end of the joist... so that it will sit nicely on my 2x6 sill plate), but I think I'll feel even better if I wedge in a little 3/4" treated piece of wood between the foundation and under the joist (between the sill plate and the interior of the house). Then it seems to me that the joist will be at (very close to) full strength.

loftezy 08-02-2012 04:18 PM

Quote:

Originally Posted by mae-ling (Post 979949)
Hmmmm, I was taught to not notch the bottom as this creates a place for it to crack.

In conventional light frame construction, it is perfectly acceptable to notch the end of a floor joist up to 1/4 the depth of the joist. At least that's what my 2007 California Building Code says.

If you read the link that I gave above, it will explain why its OK to notch the end. In short, most joists are sized to resist bending moment--of which the maximum is the center of the span. So, a 2x8 joist is only a 2x8 because it was "designed" (we actually use span tables) to resist the maximum bending moment. If it were designed on to resist shear, a 2x8 would be over designed.

tony.g 08-02-2012 04:37 PM

1 Attachment(s)
Quote:

Originally Posted by mae-ling (Post 979949)
Hmmmm, I was taught to not notch the bottom as this creates a place for it to crack.

It's true that a sudden change in cross-section of a beam (such as at a notch on the underside) can locally increase the stress at the internal corner, with the possibility of cracking at that point.
This is generally only relevant when beams are carrying a high load, and hence high shear stress at the bearing. This does not usually apply with domestic loadings, which are relatively low, though some codes still recognise this issue. Some authorities advise chamfering the notch at 45 deg.; this reduces the local shear stress at that point.

GBrackins 08-02-2012 05:02 PM

could you put in a threshold to transition from one floor level to the other? would keep you from having to notch your joists.

just a thought ....

but if you're set in notching the joists do what tony g. recommends

loftezy 08-02-2012 05:47 PM

Quote:

Originally Posted by DIY1 (Post 979959)
but I think I'll feel even better if I wedge in a little 3/4" treated peace of wood between the foundation and under the joist (between the sill plate and the interior of the house). Then it seems to me that the joist will be at (very close to) full strength.

If it gives you peace of mind, I say go for it. :) But in reality, you won't be doing much to improve the "strength" of the joist. If you can get a piece of wood wedged snugly under the joist, you may decrease its effective span by one or two inches, thus reducing the maximum bending moment slightly. But again, the only tangible benefit would be peace of mind for yourself.

Instead of cutting a small block to put under each joist, why not just get a sill plate that covers the entire top of the stem wall? Again, I don't think that doing that is necessary, but it would be easier than cutting little blocks for each joist.

Maintenance 6 08-03-2012 07:14 AM

If you are really concerned about supporting the bottom of the joist beyond the notch, then cut a 1x (3/4") and lay it next to your 2x (1-1/2") bottom plate. No small blocks, no great cost and the joist will be supported across the width of the foundation wall which is more than what's required.


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