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Building Homes or Extensions/Best practices in rebar in block foundation


Hi Dan,

I have a house with a poured foundation on clay soil. Frost depth here is 5 ft. I am planning on building an extension using a block foundation. What are the best practices in regards to vertical and horizontal rebar as well as filling the cores of block?

I know we must put vertical rebar and fill the cores on all corners. We must also use galvanized mech very other course. And also fill the last row and put in j bolts. Being that the soil is clay, would it make sense to fill all the blocks and put vertical rebar very 24 inch, 48 inch? Only fill one core in the middle of a span or no fill at all? Any help is quite appreciated.


Good question.  You need to verify your plan with your local building and inspection department.

You need a concrete spread footing at the bottom of the wall.  I would use #5 rebar at the corners and every 4'.  The grouted cells should be done with concrete, not mortar.  Use 2 #5 bars in the trough block with anchor bolts for the base plate - again grouted with concrete.  It would be best to damp proof the outsides of the wall and if there were any way to drain water away from the wall, do so.

here is some basic information:

here is another:

There are many more if you enter "cmu foundation wall design" in google.

If the information is too difficult to understand, do not hesitate to ask back with questions.

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Dan Griffin


I can answer almost all questions related to the total construction process. My expertise is in commercial construction, though I can field most any residential question. I have hands on experience in concrete, heavy equipment, masonry, all phases of carpentry, interior finishes, and I am fairly strong in mechanical and electrical.


I have over 20 years experience as a commercial carpenter and commercial construction superintendent. I have another 20 years experience in facility management for a major school district.

My favorite hobby for he past 12 years has been singing bass in a The OkChorale men's barbershop chorus and the Mature Moments quartet.

I hold a Bachelor's degree in English and Math. I have completed many continuing education hours in the building trades. I hold a Master Carpenter card from the AGC, Associated General Contractors.

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