Expert: Daniel Humphrey - 1/9/2008

Question
QUESTION: We bought property in southern WV that has a concrete block foudation (62x30) already built.  But, it was built 8 years ago and has been exposed.  The gravel was poured, but not concrete was poured. We'd like to put a roof on it, and close it in.  The foundation looks very sturdy, no cracks.  How can we determine if it will support a roof?

ANSWER: Dear Carole,

If the blocks are still solid,  without cracks in the joints, it is probably fine. You can check to see if it has heaved or subsided by checking the wall for plumb and level, but usually the mortar cracking is the most reliable sign of this.

You didn't say how tall the foundation is, but you ask about it supporting a roof.

A block wall needs a reinforced concrete cap all around the top to tie the walls together before it can be used. A roof, potentially bringing some forces pushing out or in, especially needs such a cap. To get even more strength, the hollow blocks are sometimes filled with concrete and rebar, forming solid pillars where needed.

Hope this gets you started.

Daniel

---------- FOLLOW-UP ----------

QUESTION: Thanks so much! What's interesting is that there is a row of solid concrete blocks (3-4 inches thick) around the top of the walls, followed by anotherlayer of regular blocks. No mortar is cracking, or blocks cracking even around window openings. Maybe they needed the walls to be taller after they poured the gravel. How does the builder check to see how much the wall will withstand? Or, do the reinforcements prevent any problems?

Answer
Carole,

It does sound as if the mason made a height adjustment or perhaps made the layer as an architectural detail. From what you're observing, the foundation is good to build on, provided you finish off the top of the wall with the reinforced concrete cap I mentioned. You will also be placing your tie-down bolts in this cap. In an earthquake zone, you would extend some long tiedown bolts even into a pillar of new concrete you would pour into the hollows periodically.

As long as a block wall is plumb and has sufficient rigidity, it can withstand tremendous weight--far more than a single story weighs, especially if there are no point loads and the weight is distributed along the entire sill. If you do plan for point loads, pour the hollow parts of the blocks there to make a solid reinforced concrete column, and add your earthquake tiedowns at those points.

Rigidity is supplied by the reinforced concrete cap, and any interior walls acting as shear walls.


--Daniel