Expert: Dan Griffin - 1/2/2008

Question
I have read some info here, but want to run this by someone, like yourself. I want to add some blocks with rebar to an existing slab. On top of the block I will add a sill and then a wall which will extend about 7' (not typical construction) to the underside of a deck. (In other words, I'm enclosing under a deck-which is water tight, though not air tight). This room will be my new kiln space. It will be protected from the elements, but open to some air transfer.

I had visions of drilling holes in my slab and putting in rebar; then making a thin (about 1" - 3") pour of concrete to level the surface since the slab slants away from the house. Once a level surface is achieved placing block over the rebar or threaded pipe. Running the rebar/pipe through a sill plate ontop the block and screwing it down to inessence squeeze the block between the old slab and the sill plate. Does this sound do able and do you know what I should use to accomplish this plan? How do I keep the thinnly poured concrete in place without holes where they will be seen in the concrete? Because it is thin, would using heavy wood, like 4 x 6 work? Would the concrete seep under the wood? What about the latex I read about? Could I paint the slab and wood so the new thinner concrete would stick? Is the planned concrete too thin? I would love it if it were not  really visible. I can supply drawings if it helps.

Answer
I think you are telling me that you have a patio under a waterproof deck and you want to build cement block walls up to the deck to enclose this patio.  I would guess you are planning on going up on 3 sides.

If this is going to be inspected, a footing will be required.  This footing will have to go down to your local frost line which is subject to where you live and average annual freeze depths.  You could perhaps dig under the existing patio edge to achieve, but you would really need to discuss this with your local inspector.  He won't like it and I don't either as there is no way to get a water tight joint at the base of the wall and heavy rain and snow will have your floor sopping wet.  Anything sitting on the slab with the edge of the slab showing will create the same problem.  I don't know that it violates any code, but it sure does violate good construction practice.

It would be better to cut away the edge of the existing patio, dig and pour the footing with the necessary rebar, and bring the block work up on its own separate from the floor.  This will solve your thin concrete problem. provide good weather seal at the base of the wall, provide frost depth, etc.  What this will not solve is the sloping floor.  You already indicated this would be fine, but you must be very aware of the issues, especially if you ever want more than a kiln room.  I also noticed you intend to hose this room out, and you will hold water all along the low wall.

If you still intend to build on the patio, it would be best to establish grade at the lowest corner, set a block with a reasonable mortar bed, and cut all other bottom blocks to grade to maintain a level top and a reasonable bed joint, but again this is not the correct approach, I'm just trying to tell you a better method than your original approach.

You've never given dimensions of this "room", but be aware that CMU walls should be reinforced with 2 #5 rebar and filled with grout in every vertical corner, each side of any door and window, and horizontally filled every 4 feet and the top course.  In addition, windows and doors require headers.

Is there any particular reason for block walls?  You still need footings, frost depth, and water proofing, but there are other wall materials that are much lighter.  Wood or metal framing with window units, fixed glazing, siding or brick to match the house, solarium/greenhouse type lean-to come to mind.