Answers to Slab and Foundation Questions #2 Answers to Slab and Foundation Questions #2

Q. I have a 12'x25' 25-year-old slab for an old shed sitting on a slight slope. There is a crack running the length of the slab about a quarter of the way in from the left side edge. When I looked at the left side, I noticed that there is a cavity along the whole side under the slab. I was planning to dig 2' or 3' down beside the slab, put down some gravel wrapped them metal mesh, and add some sand or concrete into the cavity. Would this be sufficient? Are there any other ways to stop the soil from being washed away, sinking the slab?

A. It will be difficult to pack the material beneath the slab sufficiently well to stabilize the slab manually. Mud jacking or its equivalent in solid footing combined with lifting the slab may be worthwhile depending upon the value of the slab. Managing the water can prevent the continuation of the problem. Grade, drain, or contain the water to stop the erosion.

Q. On a 4 foot tall foundation wall for a crawl space, does the foundation have to be water proofed? If so, is it just the concrete block foundation wall or must the footer also be water proofed? Can I buy something and use a paint roller to put it on after power washing the surface?

A. If there is adequate drainage and water control, it should not be necessary to waterproof the wall. If this is new construction and the outside of the wall that will be subterranean is accessible, waterproofing it now might be a wise investment.

Q. I am going to enlarge a 10x15 utility building to 18x15. It sits on a slab foundation. Will I have problems with the two slabs moving separately that could affect the structure?

A. With good footers, there should be minimal movement. The addition is so small that it seems unlikely that tying it to the larger slab would matter. It is just a utility building.

Q. I have two issues:
1. There is a loquat tree, which is larger than any loquat I have ever seen before - about 25 feet - that runs right up to the slab addition of my house. Oddly, the slab sinks down towards the tree slab. The loquat isn't directly causing the slab crack, as the slab runs parallel to the tree. I would like to remove the loquat, but am concerned about whether the eventual degeneration of the roots will cause a void. I assume that the tree will eventually cause slab damage, but I really just want to get rid of the tree because I don't like it.

2. The original driveway was built in the 1930's, and is about 7 ft. wide until it spreads out to about 15 ft. at the apartment area. Over the years, the driveway was enhanced in a piecemeal way. I would like to remove the driveway and rebuild, but it is obviously connected to the slab of the apartment. Is there any way to sever the connection to the apartment, such as some sort of masonry circular saw blade, and (more importantly) is this an idiotic idea from a structural standpoint?


A. The removal of the tree will leave whatever roots are beneath the slab. Even if the tree abuts the house, then there is likely an insignificant number of roots of any size under the slab. All in all, a loquat is not a substantially large tree. The root system of a tree tends to mirror the canopy of branches of the tree. If the loquat has not heaved the slab, removing it should not cause any damage in and of itself.

If the driveway is part of the slab, then it can be cut away from the rest of the slab. However, cutting through a 4-inch thick slab 15 feet will be quite a task. Renting a saw for this purpose will be a good idea. I cut some concrete blocks with a masonry blade on my circular saw. The dust looked to have ruined the saw. I am glad it was old and cheap. A rental saw will have a tad more power than what you and I have lying around the shop.

Cutting the driveway away from the rest of the slab will have no structural effect on the remaining slab. Are you going to hire someone to demolish and remove the driveway? If so, it may be prudent to have the slab cut at the same time.

Q. I have a 12'x18' 4" slab that was poured nearly 60 years ago. It has brick piers set every 4 feet around the perimeter. I am planning to pour around and over this slab to make a new 16'x24' slab with 12"x16" footers. There was a shed on it and it had nothing anchoring it to the slab. The old shed has since rotted and fallen down. How deep should I pour over the existing slab? What prep should be done to the existing slab for the new slab to adhere to it?

A. All in all, it would really be better to remove the old slab and make a fresh monolithic pour.
1. You have a 60-year-old slab of unknown construction: what the composition of the concrete is, what kind of base is underneath it, no vapor barrier.
2. Piecing together is not as satisfactory as a monolithic pour.
3. If problems develop later, any correction will be costly. Most of the time, a slab would be 4 inches, so you would pour the same 4 inches over the old slab, as you would install on a new base. You would have to pour more than 4 inches for the rest to come to the same level, since the base will likely be lower than that of the old slab. So this is already more concrete than a new slab.

A thin layer of sand on top of the old slab prior to pouring is recommended. Since this slab is only going to be used for storage/shop and not vehicle parking, 1.5" of concrete over the old slab should suffice with the remainder of the slab at 3.5" thick, 4" as in lumber 2"x4" on edge and 12" x 16" footers.

Q. I have plants growing up from the molding in my recently bought home. There is a crack in the slab. The house is five years old and had some foundation work before I bought it. Has anyone heard of this problem before?

A. If you like the plant, try Miracle Gro every two weeks and keep the curtains open for light - or, get rid of it, remove the molding and find the crack. Seal it with appropriate materials depending on the size and shape. On the outside, fill and seal the crack. You have an opening for a lot of little critters and especially termites. Do some spraying or call an expert to put up the warning sign for bugs. You probably do not have a structural problem unless the crack is large and there is displacement (up and down) from one side to the other.

Q. I've been in my house for about 20 years and with the recent stretch of high heat and humidity, the concrete slab floor in garage is showing a very heavy condensation condition in various areas of the floor. As far as I know, there are no pipes under slab, and the floor has never been sealed or painted. Is this a normal occurrence?

A. The condensation is not too normal, but it does happen when you have a lot of hot weather and a very cool garage floor. Think of it as dew on the grass. There's not much you can do, but either open you door and let it get hot in there to equalize the temperaturess, or leave the door closed and put a box fan in blowing across the floor to dry it. This is a result of your weather conditions.

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