Expert: Dan Griffin - 1/4/2008

Question
I have a 1 story ranch home that the floor is sagging. The joists are 2x8 @ 16" on center and 12' span. They are sagging about 1" when stretching a string from end to end. There is two walls parallel to the joists with an open attic so they are not supporting the ceiling. The house was built in 1952 and there is bridging at the mid point. I plan on putting a support beam at the 1/2 way point and jacking them back up straight. This is over a crawl space with only 24" height, so I want to keep the new beam as narrow as possible to still be able to crawl under it. If I use 2x6 for the beam, how many laminated together and how many supports would I need? The length will be 30'

Answer
I think I would use a double 2x6, but I do think a 4x4 would work.  The loads are quite minimal and the 4x4 would only be carrying about 2 joists between block stacks.    Carry the girder either with some 2x2 down off the joists or one of the Simpson ties.  I would go ahead and fasten these girder carriers to each end and center of the girder pieces.  I think I would loose lay 4" solid 8x16 blocks in the dirt, level the spot up a bit.  Stack as many 4" block as possible, buy some 1 1/4 and 2" also.  Have a bucket of shim stuff cut up ahead of time.  I think I would say with the 8x16 stuff.   Some 3/4 ply, some 1/2, some 1/4, maybe even some 1/8 masonite.  Whatever scrap you have - most construction sites will let you scrounge the scrap pile.  I would start with the piers about 6 foot on center, maybe even further apart.  The nice thing about this system is that you can always add more block stacks, move them around, and reshim as required.  I prefer this approach to forming or pouring concrete bearing pads.  I would use a bottle jack (hydraulic) next to the block stack to raise the joists about a 1/2" too high as I think the block stacks/ground/load will settle that much.  If the joists and flooring complain too loudly, you can make a partial move, wait a week, and move up again.