I put in an under-garage space in my new house project using pre-stressed concrete plank. The garage is 24' x 28'. The 8" thick planks are 2' wide and span the 24' direction. The prestressed company told me they could have used 6" planks for that span for regular residential vehicles, but the 8" thick doesn't really cost that much more and was what the architect had called out on the drawings and what the foundation was already set up for.

One small hitch in this plan is that they wanted the foundation in place so they could come and make as-built measurements before they cut the planks. This added a 3 week delay between pouring the foundation walls while we waited for them to fabricate the planks and schedule the installation.

Their price of $3800 included delivery and installation using their own crane and crew. They also drilled and pinned the ends of the planks into the tops of foundation walls to resist any side loads on the walls. Finally, they groated the seams between the planks.

I had my concrete contractor put on a 3" topping over a rubber waterproofing membrane (Bituthane). Since that work was lumped in with a bunch of other work on the same bid, I am not exactly sure what it cost, but I know that the mebrane was a couple hundred dollars of cost, but easy to install.

They also offered to add pre-stressed planks under the front porch area (5' by 14') to create a safe room or wine cellar for an extra $500 since they could fit them on the same truck and be installed at the same time.

- Rick