My livingroom is 12′ by 15′. The floor joists are 2x8s running the long way, spaced 24 inches on center. They sit on top of the stone foundation walls at both ends. Needless to say, the floor is like a trampoline. I have purchased a bunch of 16 foot 2x8s and was wondering if I should sister them with the existing ones (effectively making them 4x8s 24″ oc) or set them in the middle of the existing ones (making them 2x8s 12″ oc) or if it makes any difference at all.
I was also considering “letting-in”a beam (I believe its called). Can I double up 2x8s down the center. One end of this beam would sit on top of the foundation wall and the other end would attach to a 6×8 beam with a joist hanger and possibly a lally column? if necessary. Then I would attach all the 2x8s to the new beam with joist hangers.
Some other things to consider:
My basement is only 6 feet high between the joists. I dont want to lose any height.
The basement floor is 150?? year old concrete?? about an inch thick on top of field stone. Would I need to pour a footer for the lally column?
Any other options or thoughts?
Thanks,
Rob
Replies
It probably doesn't make much difference whether you "sister" the joists or space the new ones between the old ones.
A "let in" beam would, in general, be better, but is pretty complicated to do. And you'd probably want something stiffer than the doubled 2x8.
Something to consider is X blocking between the existing joists. If done right it transfers weight to adjacent joists, reducing bounce somewhat. Probably not as much as the other options, though.
It does make a difference
Actually, it does matter if you space 12" O.C. or sister your 24" O.C. The 12" O.C. will offer less deflection because their is less tributary load on each joist member.
Anyhow, if you can break the living room joist clear span in half with a beam below, that would give you a much stiffer floor system over both above options...and use much less material doing so. If you have the space and access to do that , then I would go for that soultion. Size the beam according to an engineers calculation for the most effecient beam member. Cross blocking or metal cross bracing can do nothing but help with floor diaphragm stiffness, but be careful to avoid squeaks.
I like the idea of a "let in"
I like the idea of a "let in" beam. I would use a 4*2x8 beam since that's what my 1940's house has, and still has some deflection. Some problems my own house has are related to some splits in the lumber, which may be some of your problem, But your floor joists are made of overspaced undersized lumber.
You would have to build a couple temporary walls then cut the existing joists to receive the new beam. Then I would cut the new joists and get them in place. Then test fit the beam to see if corrections are needed. If the beam starts in out of level you will probably have to make a slight taper of the top of the beam to get clearance to get it in on the high side. If everything is good you can attach the beam to the 6x8 beam now. The rest should be easy.
A person who used to be very active on this forum had a similar problem and sistered 1/4 inch plate steel to both sides of the joists and drilled and through bolted the sandwich together. Said it worked real well.
Here's some info you might want to read. http://www.awc.org/technical/spantables/tutorial.php
Span calculator (click on online span calculator on bottom right of screen): http://www.awc.org/technical/spantables/index.php
Stiffing floor
What is the the 6 x 8 beam you mentioned now supporting and what is it's span? What makes you think that it will support the additional loading you will be adding to it?
With a let in beam you might not have to double up your floor joist. What is your floor decking/sheathing?. Is it springy between the joist also.? If so, install new joist 12" o.c.
A wood let in beam only the depth of the floor joist spanning 12 feet is ify when stiffness is an issue. At least use LVL lumber. Your local real lumber yard could get it properly sized for you free for the cost of the material.
Best way: Use a steel let- in beam on a new lally column set on a new footing.
and I have had excellent success stiffening a floor using a recommendation of the guys at the tile forum. I attached (glued and screwed) a horizontal 2x4 to the bottom of each joist which in effect creates 1/2 of an I joist. The ability of the joist to deflect is lessened in the same way the top and bottom cords of an I joist prevent the thin plywood center from deflecting. You would lose 1 1/2 inches of head room but it would be a lot easier than any of the other methods mentioned and a whole lot less expense and very quick as well. Since it is only 8 2x4's, you might give that a try first. If you are concerned it might not work and would be in the way of sistering, just forgo the glue initially, screw them in place, try it out and if you are okay with the improvement go back and glue them. You can also add solid blocking between the joists if you like.
Another approach would be to attach plywood to the bottom. It could be done with plywood spanning two adjacent joists, then skipping one joist bay then spanning another pair, in order to minimize the obstruction of wires and plumbing.
I doubt that doubling up joists would make a noticeable difference. You need more depth or less span.
A few years back I did a thread on floor vibration. It gives you an idea what's going on.
http://forums.finehomebuilding.com/breaktime/general-discussion/floor-vibration