Expert: Stephen Major (Owner--Major Design Group) - 5/22/2007

Question
Stephen,

The city inspector is requiring I install 10 footings in an unlevel dirt crawlspace to install floor jacks to support the 2 story home.  I plan to purchase the 5000psi higher quality concrete as was Menards suggestion.  Question, do I need to level the dirt area in the crawlspace as their is a pretty significant grade being much lower in the middle and increasing in height out to the foundation of the home?  Also, do I need to use rebar with 5000 psi concrete? Menards didn't think so.  Do I need the more expensive 5000 psi concrete or is 3000 psi sufficient?  Also, they had many different size jackposts, I think the one I got says it is 16000 psi and its about 18.00 a piece.

Answer
Chad,

First off, you definitely need to level the grade where the footing pads will be poured.  You want all loading to transfer directly through the pad to the bearing soil, without the tendency of the pad to wander downhill, as could conceivably happen on a sloping soil surface.

Second, the choice to use 5000 psi concrete should be based on anticipated loads and the footing dimensions.  In most cases footings provide adequate support at 3500 to 4000 psi mix.  Also the final strength of the concrete is affected by the amount of water added to the mix -- too much water (usually added for ease of placement) and your ultimate compressive strength goes down.  Try to keep the mix stiff.

Also, REGARDLESS of the mix strength, you must install a rebar grid to solidly hold the pads together in the event of eventual cracking.  Install 3 pieces of #5 rebar each way (total of six in each pad, to make a grid or "fabric" of rebar.  Place the rebar grid 1/3 of the way up from the BOTTOM of each pad.  The rebar grids can be cut and tied in advance and then lowered into the wet concrete after pouring.

I don't know how long your jackposts are, but FHA-type monoposts are normally far superior to the cross-pinned cheapies.  Be sure to do your actual jacking with house jacks.  Do not use the posts for jacking.  Their screw is primarily for adjustability.  Be sure to attach the post plates to the footings with Tapcons/washers, and to the beams with lag screws.  When completed, be sure the post plates are connected to the posts, not loose in a socket hole.

That's my take, and it is how we've done many of these jobs.  It may seem like overkill, but these supports are pretty important over the life of the structure.

Please let me know if you have additional questions.

--Steve Major