Q: Why is there poured concrete between my floor joists?
During one of the many restoration projects on my 100-year-old home, I ripped up the original bathroom floor on the second floor and found what looks like 2-inch-thick concrete between all the floor joists. Do you have any idea why it's there or what purpose it might have served?
—Joe Leksich, Fort Wayne, Ind.
Norm Abram replies: You never know what you are going to find when you open up an old house. The concrete in your photo sits below the top of the joists and isn't very thick, so maybe it was for fireproofing or to control drafts. Or perhaps it was an unfinished attempt at a "mud job" for a tile floor.
I've uncovered many mud jobs in the bathrooms of old houses. These thick beds of mortar, resting on boards placed between the joists, provide a solid, flat, and very durable base for tile. In every case I've seen, however, the mortar bed covers the top of the joists, just enough to bring the tile even with the surface of the finished floor in an adjacent hall or room.
Mud jobs are still done today, except that now we frame the joists lower and cover them with plywood and tar paper before laying the bed of mortar. It's quicker and less likely to crack than the old method.