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I am enclosing a tiled porch on concrete slab with a 3' short wall of concrete block, with windows above. Should concrete blocks be laid over a tile floor?

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The tile floor shouldn't be a structural weakness, but if it's glazed you won't get a good mortar bond. I'd install anchor bolts every 32" or so, aligned with block cores, and fill those cores with mortar.

It might be easier to simply cut the tile inside the wall line and remove it, though.

Also be aware that your slab may not be designed to carry a heavy wall. If it doesn't have proper footings underneath it may crack and settle.

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NO, NO, NO, never install concrete block on tile. You don’t say what kind of tile is installed on your concrete slab, but it doesn’t matter if it’s “ceramic tile” or “terra-cotta tile” or “porcelain tile” or “stone” or “slate”. Regardless, you’ll have future problems because: 1) differing strength in compression, 2) different expansion coefficients, 3) moisture absorption, and 4) mortar adhesion issues.

1) Concrete block is a structural material with high compressive strength compared to floor tiles. This will be a structural exterior wall and will move from Environmental events (wind, earthquake, settlement, etc.) which will cause pressure on the wall...causing the tile to crack. Cracks allow moisture into the wall and probably room.

2) Because the tile is made of a different material and thickness, it will expand and contract differently than concrete block depending on exterior temperature. When differential expansion occurs, it causes cracks.

3) Depending on where you live, moisture will adhere to the tile differently than the concrete block. If you cover the exterior edge of the tile with mortar, it will help reduce (but not eliminate ) the issue.

4) With a “finished” surface on a tile, it will not allow mortar to penetrate the surface and it will not adhere. Without adhesion, there will be a crack between the tile and mortar. This will allow moisture to penetrate the wall. Then, it could freeze, expand, and develop huge cracks.

Besides the “tile” issue, there are other issues too:

A) The concrete block wall must be tied into the roof structure or it’s a retaining wall...and you’ll need a ton of steel, depending on wind and earthquake conditions in your area.

B) This is en exterior wall and the footings must extend below the frost line. This could be anywhere from 12” in the south to 48” in the north. Otherwise the patio will “heave” and crack all those windows you are installing on top of that concrete block wall.

C) If the new room is considered a “habitable space” the Code says you need to have the wall and ceiling insulated. In addition, the perimeter of the slab will need insulation, there will be electrical issues, etc.

D) Structurally, you’ll need #4 horizontal reinforcing steel at 32” on center plus #4 vertical reinforcing steel at 32” on center at a minimum...if it’s not a retaining wall and if it’s more than 12’ long. If it gets to be 20’ long or so, it’ll need expansion control too. If you live in an extra cold climate, you may need to install more horizontal reinforcing steel.

E) Soil conditions: If you use 8” thick block it will weigh about 100 lbs. per square foot. So, with a 3’ high wall plus’s the windows, plus the roof...I’m guessing you’re loading about 1,000 lbs. per square foot on the existing slab and soil. It’s not a lot, if the existing slab is reinforced.

You asked about setting concrete block on an existing tiled concrete patio, so I won’t get into setback requirements, lot coverage ratios, etc., but you’ll need to verify that with your local Building Official.

You’re getting a Building Permit, right.

  • You seem to be assuming something about the nation/location of asker. In that your example locations for the frostline doesn't hold the same for north/south of Britain as is does for north/south of Australia. And your statements about the "Code" certainly are not true of every Code. – Lyndon White May 22 '18 at 6:03
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    @LyndonWhite The asker’s unit of measure is in feet. (BTW, I like your tree top walk.) – Lee Sam May 22 '18 at 6:55
  • A fair point, however in a lot of the world (or at least in my parts) feet are used as unit of convenience (as a short hand for about 30cms); so not conclusive. (Yeah, that tree top walk is awesome) – Lyndon White May 22 '18 at 7:01
  • Your confidence in the likelihood of failure is amusing, Lee. You raise some concerns, but most of them are edge cases and unlikely. Expansion rate variance probably isn't enough in the width of a block to be a problem. (The tile is bonded to concrete, after all, and can't really move.) Moisture would be handled as with any block wall--as needed. I mentioned sill bonding in my answer, and it isn't a terrible concern, either if handled well. If OP is in an earthquake zone (s)he probably wouldn't be using block to begin with, and there's no mention of it being a retaining wall. – isherwood May 22 '18 at 13:46
  • @isherwood You say, “Moisture would be handled as with any block wall—as needed.” The point is it will be a continuing problem due to lack of adhesion if mortar, differential thermal expansion of tile to block, movement causing cracks on tile, etc. These items aren’t “maybe issues” they will cause problems. Bad design is what we’re trying to eliminate. No tile manufacturer will recommend their tile as a structural element. Retaining walls aren’t just from soil bearing on a wall. Any “lateral load” (like wind, earthquake) on a wall that is not restrained at the top and bottom is considered a – Lee Sam May 22 '18 at 14:21

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