Mysterious Cracking — Causes and Remedies - 8/23/2004 - Home Foundation Structure Framing

Mysterious Cracking — Causes and Remedies

I got called to fix a large sheetrock crack in an older home. It was obvious the problem was foundation settlement. But the house is built on a solid bedrock outcrop. I didn’t know rock settled. Your thoughts? — Rick M., Golden, CO

Solid rock does not settle, particularly under the relatively light loading a residence would apply. Even fractured rock is very unlikely to settle unless an earthquake or bomb blast were involved.

I was involved in a similarly puzzling project once. It was a large custom home built on a rock outcrop at the top of a knoll. The owners complained they’d had the same drywall crack repaired several times, but it always came back.

I knew the problem had to be settlement, but on solid bedrock? Into the crawl space I went. All the footings were placed directly on exposed bedrock — except one. Unfortunately, this particular footing supported a main roof beam and a floor girder. With a little probing and digging, I discovered a five-foot deep “bowl” in the rock. At first I couldn’t tell it was a bowl because it was full of soil and leaves.The original contractor, rather than scoop this debris out, built his footing on top of it. Over time, the material decomposed, compacted and settled. The rest of the house, being built literally on rock, didn’t budge. This is called differential settlement.

The fix was to jack the settled area up, scoop out the bad soil, pour a new footing directly on bedrock and let the house back down. Had the bedrock been sloping, rotohammered steel rebar dowels would have been embedded into the rock and extended into the new footing to keep it from sliding.

Sheetrock cracking is caused by structural movement of some sort. If it’s not vertical settlement, then it almost has to be lateral (sideways) movement.

I recall a project with severe plaster cracking on all four exterior walls. It was an old schoolhouse converted into a residence. The foundation was sound, the floors level. Inspection of the roof framing told a different story, however.

The roof was hip style, fairly steep. Rafters bore on the exterior walls and extended upward, terminating at the ridge or a hip, depending on their location. There was a serious problem: there were no collar ties or ceiling joists; nor were there any ridge beams or hip beams. In short, there was nothing to take the outward thrust of the rafters, except the exterior walls. Over the years, all four exterior walls had bowed outward — nearly a foot in the middle of the long walls. It was a miracle that this place did not implode in one of our snowy, Northwest winters.

The fix was to jack the roof system up, and using come-alongs, winch the exterior walls back together as best we could. Then new hip and ridge beams were installed, their bearing extended all the way down through the structure to existing or new footings below. It was a messy, expensive job, but the insurance company paying the bills determined it was more cost-effective than a complete roof system replacement.

In summary, wherever you’ve got significant drywall cracking, you’ve got structural movement. The trick is determining the cause and properly fixing it. Simply replacing or patching the drywall is like putting a Band-Aid over a broken bone — it won’t work. To complicate matters, a small amount of cracking may be due to wood shrinkage — a common phenomenon, especially in newer homes. So, if in doubt, don’t be afraid to get a second opinion. The wrong “fix,” in the long run, will cost far more than a few hours of an expert’s time.

Tim K. Garrison P.E. of ConstructionCalc.com has authored books and short courses and lectures on topics relevant to builders. Got a technical or management issue? E-mail buildersengineer@constructioncalc.com. Tim reads every one.

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