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spooksmcgee 05-04-2008 01:06 PM

Strengthening joists
I am replacing a subfloor in my house. The joists are 24" on center with a 16' span, and a little bigger than 2x8 (their dimensions are exactly 2"x8"). I'm putting in slate and a clawfoot tub, so since the joists are so far apart, I'd like to add more support. I cannot maneuver a 17' long sister in there unless I cut it in half or something. If I do that, do I have to provide support piers under the section I cut in half? I also can't use joist hangers since the joists are too thick for normal size hangers (unless I use a double hanger and fill in the sides with scrap wood?).

My only other thought is to use something like this:

and use a 6' long 4x4 beam in 2 or 3 placed under the joists. I kind of like this option. Is this the best way to provide support? And if I do it this way, is it okay to just use 6 ton bottle jacks and leave the jacks there when done? Or should I use lally columns or more 4x4s for the support posts? And do I need to pour some kind of concrete footer for the jacks/beams, or can they just sit on the ground? What about concrete dek blocks, can I use those? Thanks for any help and any suggestions anyone might have. Below are pictures of what I'm working with:

glennjanie 05-04-2008 02:58 PM

Welcome Spooks:
Since the earth under your floor will maintain a fairly constant temperature and will not be rained on, you can use the concrete block to rest the weight on. Just dig down to undistrubed earth to set the block.
Yes, a center beam added will cut the joist span in half and you will be amazed at the additional support. I would not leave the bottle jacks to support the floor because they can eventually blow an 'O' ring and loose their pressure. I would cut a piece of treated 4 X 4 cut to size to support the floor after you get it level. The 4 X 4 posts will hold as long as I will live.

guyod 05-04-2008 09:20 PM

You can use screw jacks if you have the clearance. they are permanate.

spooksmcgee 05-24-2008 03:39 PM

Thanks for the advice. I added two 4x4 beams under the joists for extra support, and added two sisters, although there was no way to fit a 16' span under there, so I had to split the sisters in two. My original joists are also a weird size (literally 2"x8") so I couldn't perfectly match them. They're also apparently not completely straight, as you can see by my sisters that don't line up perfectly.

I have 4 jacks under the beams, and I'm also raising up the left hand jacks a half turn every few days to try to raise that wall an inch. Should I add anything else before putting in the subfloor, like cross bracing or blocking? I'll be putting in a very heavy clawfoot tub, as well as 1.25" of subfloor, plus hardibacker, plus slate tile. Also, what is the difference between blocking and cross bracing? Thanks again!

Daryl in Nanoose 05-25-2008 08:14 AM

There should be screw jacks under the 4x4 where it meets the floor joists as well as a concrete slab under each jack.

spooksmcgee 05-25-2008 01:45 PM


Originally Posted by Daryl in Nanoose (Post 19438)
There should be screw jacks under the 4x4 where it meets the floor joists as well as a concrete slab under each jack.

So I need to add 4 more screw jacks under the joists as well? I didn't pour footings for the jacks but they are sitting on concrete blocks. They have some dirt on them so you can't see them unless you look closely.

Daryl in Nanoose 05-25-2008 01:49 PM

Sorry I meant to say 4x4's unless you want to use screw jacks.

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