Expert: Bruce E. Johnson - 4/11/2011

Question
I am planning to build a Pier and Beam Foundation Home.  It will be a 1 story house and about 2,000.  I have looked into several methods, concrete tube, cinder blocks, wood post etc...  I have 2 questions.  1 I live in central Texas and have black clay like soil that is hard as concrete when dry and sticks like Taffy when wet, What size piers, how far apart and how deep?  Question 2:  What is the best and cheapest foundation material should I use?  I plan on doing a lot of the work myself and with help from family.  Being a single mother, I need BUDGET FRIENDLY as possible, but also want a strong Home for my Son.  Thank you

Answer
Hi Denise, pretty much any foundation type will work with your soil conditions provided you keep good drainage around the perimeter which will help keep the soil as consistant as possible.  Black mud or clay is expansive which means it swells up when wet and contracts when dry, this is the foundation killer.  It is best to build on black clay when the soil is not saturated nor when it is dry like a rock.  Your particular house pad should have a soil bearing test done on it to determine what capacities it has which will help determine the best size and spacing for your foundation pads.  I like monolithic concrete slabs or post and beam construction for these soils and if you live on a hillside where shifting is a potential hazard I like to auger down and incorporate poured concrete pilings into my foundation wall footings.  As far as spacing goes, for a typical post and beam foundation where you use 4x8 beams sitting on 4x4 posts the spacing is 6-8 feet apart supporting the beams on  2'x2'x 12" deep concrete pads. How far apart you space your beams will depend on your floor system, 5' for 4x6 joists at 32", 8' for 2x8 joists at 24" centers, 10' for 2x10 joists at 24" centers..etc.

However your soil report may require larger pads if your bearing capacity is less than 1500 pounds per square foot.  A 2'x2' pad will support 6000 pounds if the soil capacity is 1500 psf.  I would use a concrete perimeter foundation or a masonry foundation wall on top of concrete footings.  Again the soil report will help determine what the width of your foundation wall footing should be.  Generally a 24" wide by 12" deep foooting will suffice giving you 3000 pounds of bearing per lineal foot depending on your bearing capacity.  Again I stress that your bearing capabilities will depend greatly on the moisture content of the soil.  A good soil report will also give you suggestions regarding how to prep the building pad, if the pad needs to be stripped down a foot and reconstituted with more stabilized fill or compacted to certain density or other methods to ensure a solid base for your foundation.

You are going to need a building permit and probably a structural engineer to sign off on the structural portion of building plans.  It is in your best interest to get an engineer on board early to ensure that the foundation planned for your building pad is adequate for your actual soil bearing conditions. I hope this information helps, please feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com