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Basement block wall/slab floor interf...

Jody_Huneycutt's picture

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I'm about to pour the concrete slab floor for my basement. The basement walls will be concrete block, inside the cells in the block is rebar that attaches to the footing. I will also pour the cells full of concrete. There are several ways, of which I am aware, for the interface between the slab floor and the block wall to be done. One is to sit the edge of the slab on the footing, with a 2x4 form around the edges, then after pouring the slab, remove the forms and lay block directly on the top of the slab; second is to lay the first course of block on the footing, and use it as the form around the outside edge; and, third, is to use a course of block (8"x8"x16") which has, when looking at the 8"x8" face of the block, a 4"x4" square left out of what would be the top inside corner of the block, when laid. This would allow the slab to sit on the shelf portion of the 4"x4" cutout, with the vertical wall of the remaining top quarter of the block acting as the form around the outside edge of the slab. The next course of block would sit on top of this block/slab combination.

Which one provides the best overall seal and the most strength to resist the inward pressure of the backfill and groundwater outside the basement? (assuming that the footing drain and waterproofing are handled correctly in either configuration.)


(post #170408, reply #2 of 9)

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Jody,
I always enjoy reading about how other people in different areas do things. I have been in this business a long time and have built hundreds of block basements and have never heard of anyone doing it the way you describe in method #1or #3. We do all of our own blockwork and concrete work and we use the second method you describe. We lay the 1st row of block and then pour the floor. I like this method for a number of reasons: It is MUCH easier to pour the floor... It provides a nice flat , clean place to store the remaining blocks...It lets you set scaffolding on a hard,dry ,flat surface. We roll the blocks around on heavy duty carts to where the masons are working. We scatter sand at the bottom of the walls on the inside and outside to catch any mortar that might fall and then sweep it up when we're done to keep the footings and slab clean. Most builders simply lay up the whole wall and then the concrete crew comes in and pours the floor. Make sure you strike the joints on the outside of the block. It will be much easier to get a good waterproofing job. Don't get too excited about the vapor barrier. It will work for awhile and then it completely disintegrates. I have dug up a lot of concrete with 6 mil poly under it, and there is ususally nothing left of it. I don't know how long this takes, but it does happen. If you have heavy soil, you must really pay attention to a good exterior drainage system. Hope this helps you........

(post #170408, reply #3 of 9)

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In our area (Arkansas) the special shape CBU you describe in method #3 is called an "L-Block." It is only used above grade to form for a slab floor. Method is footings, L-Blocks on footings, under-slab plumbing, fill to level, insulate/vapor barior to specs, and pour slab. In some cases the cells are grouted and in other cases not. A solid L-Block is available if you do not want to grout the cells. If you used this technique in a basement you would lose at least 4" of ceiling height to fill.

Method #1is what most people do for a full basement. However, Method #2 has advantages if you can get your block layer to come in for one course. You might be able to work out a deal where he (or she - I am PC) will lay blocks for a full day and then come back after the floor is poured. By the way, some of the cement finishers I know pour the basement floors after the joists are in and decked. This way they can check depth by measuring down. Method #2 would require stakes and screeds. Check with your finisher before you decide.

Here are a couple of ideas to consider:

The ceilings in most basements are about 7' 9" after the slab is poured and dry wall is hung (plate adds 1 1/2"). If you want a true 8' ceiling run one course of 4" tall block. The cost to lay is about the same as a full CBU but the result is a wall that can be studded out with regular 2" x 4" x 8' stock and finished wih 4' wide dry wall.

If you are going to grout your cells consider hirizontal rebar (if it is not already required by your local code. You can get a CBU that has knock-outs sawed lengthwise. On the job these special cells are typically laid every third course. A #3 or #4 rebar is laid horizontally in the channel formed by the knock outs. At the corners the block layer must make a couple of simple cuts tith a masonry saw to make the turn. Really ties your wall together. Block wall develop zig zag cracks when the footing settles. If you are going to grout, the additional cost for the steel will be minimal and will result in a much stronger and much more crack resistant wall. Concrete is strong in compression but it does not have much tensel strength. Without steel you are not getting full value from the concrete you pour down your cells. In basements, you need vertical and horizontal steel.

(post #170408, reply #4 of 9)

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Steve,
I agree with you about the reinforcing. We use heavy wire pattern reinforcing in pre-formed widths between the rows as required. I too have taken to using the 4" blocks on top. It's amazing what a difference and extra 4" can make when you are finishing off a basement. If you are using a method as Jody described in #1, why don't you just pour the slab and footing as one piece like you would for a slab-on-grade at ground level instead of messing around with another form board operation to outline the slab? Just curious...

(post #170408, reply #5 of 9)

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Went back and looked at the original discription of Method #1. Agree that this is not a good way to go. One consideration with basement walls is desigining to resist inward pressure. Pouring the floor inside the foundation wall provides mechanical resistance to the forces that want to push the bottom of a foundation inward. Floor joists provide resistance at the top. That is why some contractors do not backfill until the basement floor is poured and the joists are in. In Method #1 all of the resisrtance at the bottom would be sheer. This would be true with a mono pour or a two step pour.

(post #170408, reply #6 of 9)

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I agree wholeheartedly!! Nice talking with you............

(post #170408, reply #8 of 9)

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Mike,
I have friends in L.A. and I hear you talkin'......We do use re-rod into the footings. We don't have quite the worry you do so we pour the cores every 4'. We mark the core locations on the forms before we pour and insert short lengths of rod. We add the long lengths when we pour the cores and put the anchor bolts in the same core. We are mainly concerned about the wall not bowing in , in the middle since it is solidly locked in at the top and bottom. They poured walls around here for years without any reinforcing at all. Some of them are in perfect order and those that were on unstable soil have some pretty good cracks in them. The footings in that FHB article appear to be very thick. I don't know if that is their code or custom, but a thinner footing with proper re-rod would be much stronger and cheaper. That picture on the cover dropping concrete 8 or 9 feet out of a chute is a tutorial for creating walls with a natural stone texture.......

(post #170408, reply #9 of 9)

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I'm about to pour the concrete slab floor for my basement. The basement walls will be concrete block, inside the cells in the block is rebar that attaches to the footing. I will also pour the cells full of concrete. There are several ways, of which I am aware, for the interface between the slab floor and the block wall to be done. One is to sit the edge of the slab on the footing, with a 2x4 form around the edges, then after pouring the slab, remove the forms and lay block directly on the top of the slab; second is to lay the first course of block on the footing, and use it as the form around the outside edge; and, third, is to use a course of block (8"x8"x16") which has, when looking at the 8"x8" face of the block, a 4"x4" square left out of what would be the top inside corner of the block, when laid. This would allow the slab to sit on the shelf portion of the 4"x4" cutout, with the vertical wall of the remaining top quarter of the block acting as the form around the outside edge of the slab. The next course of block would sit on top of this block/slab combination.

Which one provides the best overall seal and the most strength to resist the inward pressure of the backfill and groundwater outside the basement? (assuming that the footing drain and waterproofing are handled correctly in either configuration.)


(post #170408, reply #1 of 9)

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Option # 1 allows you to put a continuous vapor barrier under the slab, footing and even turned up the outside of the walls. You can then lap your waterproofing over this. This will be the cleanest installation, without any joints. The wall and footing are going to resist any forces without the slab so strength is not an issue. This method does seperate two trades completely, the concrete happens then masonry. The other methods involve, concrete then masonry, then concrete again, then masonry again.

(post #170408, reply #7 of 9)

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What you say may be true, where non reinforced concrete and masonry construction is permitted. In California, we would never, never have a footing to wall joint without vertical steel tied into the footing. It amazed me to see the foundation construction in the last 2 issues of FHb with no steel from the footing to the wall, and no other apparent reinforcing. Any construction detail that is structurally required, should also be be required by the governing authority. If method #1 doesn't work, plan checker or inspector should know, that is not to say that they do no. There should be standard details available that are approved in your area.