I have built two cedar strip canoes and about to push the Guillemot out the door any day now, my garage floor is concrete but I think who ever poured it had already passed out the 6 pacs to the guys before they started the job. It has bumps and dips no large flat spots at all.

The strong back is alligned to build the boats, just don't move the strongback around until the boat can support itself and everything is fine. I left the strongback in place until finished building the boat.