Expert: Dan Griffin - 7/3/2010

Question
Hi Dan! First let me say, thank you for devoting your time and efforts to this wonderful website. Now, I am someone that is "mathematically challenged", so I am hoping you can help me. My husband and I have just bought our first house, ( a mobile home) but we can't have it set yet, because we can't agree on something... My husband wants our house set on 10 ft. pillars and then have the whole house (28x64) blocked underneath. Now I don't know anything about building pillars, so how many block does it take for each "layer?" of the pillar and also from what I have typically seen, most houses seem to have at least 12 pillars under them, does that sound right to you? How many block would you estimate it would take for us to do it his way?
Our house is 28X64, so I have assumed that is the same size our basement will be, is that wrong?
A standard size cinderblock is "8x8x16", again, assuming this is the type block we would be using for each project...if that is a wrong assumption, please correct me. :)
Knowing this how many cinderblocks will we need to cover a 28x64 area with a "ceiling" height of 8feet? 10 feet?
Any insights you can provide will be greatly appreciated and I thank you again for your time and efforts."

Answer
Angie,  I think you are planning a full basement under a mobile home.  You will want to build the basement before you move the mobile home in/on.  The basement can be concrete or block.

The perimeter of the mobile home is 184 feet (2x64 + 2x28) x 12" = 2208" divided by 16" (block) = 138 block per layer.  If you are planning on 10' walls, you will require 15 layers, so about 2070 block.

The block walls need a concrete footing with 2 #5 rebars.  You need a continuous trough block every 4'. I would do bottom, 4', 8', and top with 2 #5 bars in each.  You need to have vertical steel every 4'.  All rebar cells need a full concrete grout fill.  Plan appropriate anchors in the top trough block to provide hold down.  You need ladder ties in the block walls every 2'.

The exterior of the walls should be water/damp proofed with a perimeter french drain going to sump or daylight.

You could plan to have pilasters in the block walls.  This may be what your husband is calling pillars.  these would be coursed in with the walls.  I am not a mason, but I think 8x12x16 block would be adequate, again rebar and concrete fill.  Check with the mobile home people as to how many, but I would plan every 12' or less.  The full block wall may not need pilasters.  A good local mason should be able to advise.  I've never worked with block basement walls (always poured in place concrete), but I think most of them are 12" block walls.

I hope this has given you some good answers.  There are some handy calculators for block, sand, mortar, and steel quantities at this site:
http://www.cemexusa.com/bp/bp_cb_vc.html