Why Walls Bow
Before measuring, it helps to understand what you're looking at.
Lateral Soil Pressure
Gary's wall was being pushed inward by saturated soil. We'd had an unusually wet spring. The soil against his wall was like a waterlogged sponge, pressing harder and harder as it absorbed more moisture.
The wall acts like a beam trying to resist this pressure. The top is held by the floor framing above. The bottom is held by the footing. The middle has less restraint, so that's where it bows in.
Hydrostatic Pressure
Water-saturated soil creates hydrostatic pressure. The water weight pushes horizontally. Gary's drainage wasn't great, so water accumulated against his foundation instead of draining away. The pressure built up over weeks of rain.
Frost Issues
In cold climates, frozen soil can push against walls too. The expansion from freezing adds to the lateral pressure. Gary's wall had probably been experiencing frost pressure for years, weakening it gradually. The wet spring was the final stress.
Wall Design
Gary's house was built in the early 1980s. The foundation walls are 8 inches thick, poured concrete. They're strong, but they were designed for normal soil conditions, not prolonged saturation. When conditions exceeded what the wall was designed for, it started to fail.
How We Measured Gary's Wall
You don't need sophisticated equipment to measure wall bow.
The Straight Board Method
We used an 8-foot 2x4 from Gary's garage. Held it vertically against the wall with one end on the floor. Looked for gaps between the board and the wall.
The maximum gap was at about mid-height, maybe 3-4 feet up from the floor. That's where the wall had pushed in the most. We measured the gap with a tape measure. About an inch and a half.
The String Line Method
For a longer wall, you can stretch a string line horizontally. Anchor it at each end against the wall, pull it tight, and measure the gap in the middle.
Gary's wall was about 30 feet long. The bow was most severe in a section maybe 12-15 feet wide. The ends of the wall were still pretty straight.
The Level Method
A 4-foot level held vertically against the wall shows if it's out of plumb. Gary's wall wasn't just bowing. It was also leaning slightly inward at the top. The combination made it look worse than just the bow measurement suggested.
Multiple Measurements
We measured at several spots along the wall and at different heights. The bow varied. Worst in the middle section, less severe toward the corners. The contractor did the same thing but with more precision.
What the Numbers Mean
The contractor explained how they think about bow severity.
Under 1 Inch
Minor bow. Might be within original construction tolerances or early-stage movement. Usually warrants monitoring but not immediate repair if stable.
The contractor said walls with less than an inch of bow that have been stable for years sometimes get left alone. It depends on whether it's getting worse.
1 to 2 Inches
Gary's wall was in this range. Noticeable bow that indicates significant movement has occurred. Professional evaluation recommended. If active, intervention is probably needed.
The contractor said this is the range where they do the most work. Enough movement to be concerning, but not so severe that repair is extremely complicated.
Over 2 Inches
Significant structural movement. Repair is almost certainly needed. The wall has moved well beyond acceptable limits. Risk of further failure increases.
Gary was close to this threshold. If he'd waited another year or two, the contractor said he might have needed more extensive repairs.
Activity Matters More
A 2-inch bow that's been stable for 20 years is different from a 1-inch bow that developed last month and is still progressing. The contractor asked Gary when he first noticed the crack. Gary's wife said maybe a year ago. The contractor's eyebrows went up. That's fast progression.
What Else to Look For
Bow measurement isn't the whole picture.
The Horizontal Crack
Gary's wall had a horizontal crack running along the zone of maximum bow. That's typical. The wall bends, the concrete can't stretch, so it cracks. The crack and the bow are related symptoms of the same problem.
Water Signs
Gary's wall was seeping water through the crack during rain. The pressure was literally squeezing water through the concrete. That's a sign the pressure is significant.
Gaps at Top or Bottom
We checked where the wall met the floor and the floor framing above. No gaps yet. If the wall had started separating from the structure, that would have been more urgent.
The Drain Situation
The contractor looked at Gary's exterior drainage before even going inside. Bad grading, downspouts dumping too close to the foundation. He said fixing the pressure source was just as important as fixing the wall.
Gary's Repair Options
The contractor presented options based on the severity.
Carbon Fiber Straps
For walls with less than 2 inches of bow, carbon fiber straps can prevent further movement. They're epoxied vertically to the wall and resist additional inward pressure.
Less invasive than wall anchors. Less expensive too, maybe $5,000-6,000 for Gary's wall. But they don't pull the wall back. They just stop it where it is.
Wall Anchors
This is what Gary got. Steel rods through the wall to anchor plates buried in the yard. They stabilize the wall and can be tightened periodically to slowly straighten it over time.
More invasive. Requires digging in the yard. But they actively work to restore the wall toward plumb. $8,400 for Gary's six anchors.
Steel Beams
Vertical steel I-beams inside the basement provide substantial reinforcement. Anchored at floor and ceiling. Good for situations where yard access is limited for wall anchors.
More visible inside the basement than carbon fiber. The contractor said it would have been about $7,000 for Gary's wall.
Wall Replacement
For severely damaged walls, sometimes you have to excavate and rebuild. Gary wasn't close to needing this, thankfully. It would have been $30,000+.
What I Check at My House
After Gary's experience, I added wall bow checking to my annual inspection.
The Straightedge Test
Once a year, I hold my 4-foot level vertically against each wall in several spots. Looking for any gap. Any curve. Any indication my walls are bowing.
So far, nothing. My walls are straight. But I check because Gary's wall wasn't always bowed either.
Visual Sighting
I also sight down each wall from the corner. You can see a bow visually if it's significant. My walls look straight. Gary's didn't, once you knew to look.
Looking for Horizontal Cracks
A horizontal crack often accompanies bowing. If I ever see a horizontal crack, I'll be measuring for bow immediately.