Building on Existing Concrete Which is Below Grade

Trying to utilize existing concrete slabs on grade in new post frame building can bring with it some unique challenges. Reader DARREN has one of these situations which most people do not face.

Please read on:

“I want to build a pole building on an existing cement pad.  The cement pad is a few inches lower than grade.  I’m planning on cutting out the cement where each pole will go.  In order to keep out water due to the lower grade can I use a non bearing 3’ cinder block wall?  If so how is it incorporated into the build? Are there any other recommendations?”

Mike the Pole Barn Guru Responds:

My concern would be in regards to the top of the slab being below grade. The Building Codes require the top of slab to be higher than the surrounding grade. Building Codes generally require the non-impervious area beyond the building perimeter be graded away from building a minimum 5% slope to drain surface water away in all directions. A 5% slope is a six inch drop in 10 feet.

Provided your Building Official will allow your circumstance, the blocks could be placed between the columns. You will need to provide us with the finished height of your wall, above grade. We will then design with a pressure preservative treated plate on top of your blocks, between the columns, to attach base trim and the bottom of the siding.

You will also need to deal with sealing your cinder blocks to both the existing concrete slab as well as the columns which will be frequently interrupting your cinderblock wall. Keep in mind, wood and cinder blocks have a different rate of expansion and contraction with temperature. The columns will also be subjected to swelling due to water, as the existing slab in effectively in a hole.

Personally, I’d be investing in grading, rather than having a perpetual fight with water trying to flow into my building.