Locating structural concrete foundation problems
Look at a concrete foundation from inside and outside to tell if it’s plumb and level. Slight variations are normal, but foundation walls should not be visibly bowed or leaning. Check all doors and windows; they should also be plumb and level, and the units should operate smoothly. Sagging rooflines are sometimes due to poor roof construction, but often they can indicate concrete foundation problems as well.
If necessary, run string lines along the interior and exterior of concrete foundation walls to see if they are bowed. Place a 6-foot level along the walls in several spots to check for plumb. A foundation that is out of square is not uncommon, and can result from sloppy workmanship during forming and pouring. But walls that are out-of-square and cracks in or near a corner indicate a larger problem that needs closer inspection. A combination of cracks and movement means repairs are required. Walls that are more than 3 inches out of plumb should be repaired immediately.
Reading the cracks in a concrete foundation
All concrete foundations settle and slight cracking in walls and slabs is very common. Typically, if these cracks are less than ⅛-inch wide and there is no movement on either side of the crack, the problem is just cosmetic.
The same may not be true for masonry block walls. Vertical cracking with no movement is common, but steeply angled or “stair-step” cracking and even small cracks within the block and not the joint are indications of a bigger problem.
Horizontal cracks in either type of foundation are the worst-case scenario and a full foundation replacement is usually warranted.
Common structural cracks, or those that need to be repaired, are those that run up at an angle from the lower corners of a wall, accompanied by vertical cracks in the center. Either soil swelling or soil desiccation causes these problems, and they require either a wall bracing system or foundation underpinning system to restrain and stabilize the wall and prevent any further movement.
Wall bracing systems vary, but the most typical incorporate either bolting steel braces along the wall or bonding a carbon-fiber mesh to the wall with an epoxy. These typically cost less than $5,000 for an average home.
Underpinning the slab typically consists of cutting through the slab and setting helical screws or concrete piers to support it from below. This is more detailed and expensive, often running between $10,000 and $20,000.
In either case, consult a structural engineer to definitively determine the cause of the concrete foundation problems and specify the appropriate solution.