How much should I worry about this large crack in the foundation slab?

Question

I recently bought a 3-story townhouse in northern Virginia. I pulled up the carpet on the bottom level (foundation is a slab on grade) and discovered a large crack running along the whole room and into the mechanical closet.

The crack is about 3/8" - 1/2" wide and runs about 16 feet, under the wall at the top of the first photo, into the mechanical room. From there it goes under the furnace, but doesn't appear to continue outside of that room. It starts beyond the bottom of the first photo, at or underneath a gas fireplace. It looks to be up to 3" deep at points, maybe more. I was able to get the ten-dollar bill about halfway into the crack.

Putting a level across the crack produces a slight rocking, maybe a couple degrees, but nothing huge. Each side of the crack appears level on its own.

The townhouse is in a row of 7 townhouses; mine is adjacent to one of the end units. I have no idea how recent the crack is, but the townhouse was built in 1999. I haven't seen any moisture or anything coming up from the crack during recent rains.

How much should I worry about this crack? I plan to get it evaluated by an engineer, but is it something I should do immediately or could I wait a little bit?

Answer 1

Sigh. Cheap builders are forever spending thousands on concrete, but unwilling to spend a few hundred on steel to make the concrete properly reenforced. Nobody sees the steel, but the lack of it does show up eventually.

Unlikely to be a major issue, might be a path for water or bugs, so probably sealing it would be best. But consulting an engineer is good for being careful. Unlikely that the house will fall down any time soon (if ever), so probably not too urgent.

Now, if you change to a hundred dollar bill and it gets pulled down into the crack, you have dwarves, and $10 wasn't enough to tempt them... ;^)

Answer 2

Concrete cracks because of movement. Either movement from shrinkage at the initial pour, or movement due to structural settlement.

Because the two slabs are not level with each other (you say there is, “a slight rocking” across the crack when a level is used), I doubt the crack occurred due to shrinkage. Rather, it’s cracked due to settlement.

As the house is “loaded” (i.e.: additional loads during the construction phase, furniture, people, etc.), loads are transferred down to the foundation and soil. If the foundation is not properly designed for such loads, the building will settle and crack.

Actually, we know all buildings settle as they are built. It’s just that we hope they settle uniformly so they don’t crack.

Hopefully the building has finished settling. In order to determine this, I’d keep a journal. I’d make marks across the crack about every 4’, number the marks, measure the width, and photograph each. I’d remeasure every 3 months for about 1 year. If little or nothing occurs, I’d change and measure every 6 months.

You’re lucky there’s no moisture coming up through the crack. If that changes, you’ll need to add drains around your house.