How to Monitor a Foundation Crack with a Crack Gauge

How do you tell if a crack is getting bigger?

Measure it at the same spot, the same way, over time, and compare readings from the same season. One measurement only tells you how wide the crack is today. A series of dated measurements tells you whether it’s moving.

Season matters because concrete and masonry move with temperature. The National Park Service notes that temperature cracks in masonry walls open and close with the season (opens in a new tab), growing wider in cold weather and narrower in hot weather. Basement cracks also move with soil moisture: ASHI’s Tom Feiza says horizontal cracks often will change seasonally (opens in a new tab).

So a crack that’s a little wider in January than it was in July may be normal. A crack that’s wider this January than last January is growing.

What is a crack gauge?

A crack gauge is any tool that measures the width of a crack or tracks how it moves. Homeowners use two kinds: a crack width card you hold against the crack to read its width, and a two-plate crack monitor that stays fixed across the crack for months.

For most cracks, pencil marks plus a width card are enough. Add a two-plate monitor when the crack is near one of the engineer lines, such as close to 1/4 inch wide, wider at the top, or on a wall you think is moving.

How do you monitor a foundation crack?

Mark it, measure it and date it, then repeat at the same spot on a schedule.

1. Mark both ends

   Draw a short pencil line across each end of the crack, right where it stops, and write the date beside it. If the crack grows longer, it will run past your line.

2. Pick one measuring spot

   Choose the widest point and draw two short ticks on either side of the crack, so you measure exactly the same place every time.

3. Measure and write it on the wall

   Read the width at your ticks with a crack width card or feeler gauges. Write the width and date in pencil next to the ticks.

4. Check for offset

   Lay a straightedge across the crack. If one side sits higher or farther out than the other, measure that step too, and note it.

5. Photograph it

   Take one close photo with the gauge or a ruler flat against the wall and one wider photo showing where the crack is. Keep them in one folder with the dates.

6. Keep a log

   Write each reading in a notebook or a spreadsheet: date, width, length, offset and whether the crack was damp. Note the weather too, since a wet week can change things.

How do you install and read a concrete crack monitor?

Fix one plate on each side of the crack, so the plates overlap across it and the crosshair starts at zero on the grid. When the crack opens or closes, the crosshair moves sideways along the grid. When one side slides past the other, it moves up or down.

1. Pick a flat, sound spot at the widest part of the crack, away from foot traffic.
2. Clean off dust and loose paint so the plates sit flat.
3. Center the monitor across the crack with its zero line running along the crack.
4. Fix each end to its own side of the crack with the screws or adhesive the maker specifies.
5. Remove the setting pins or clips that hold the plates together, so the two halves can move independently.
6. Record the crosshair position on the day you install it, then at every reading.

Most monitors come with a record sheet. Write the reading on the sheet and in your log, so you have it even if the monitor gets knocked off.

A crack width gauge card is the cheaper starting point for marking and logging width at one spot. Most people should start there.

How often should you check a crack, and for how long?

Check it once a month for the first three months, then at each change of season for at least a full year. A year captures the seasonal swing, so you can compare winter to winter and summer to summer.

There’s no official schedule for homeowners. The point is enough readings across the seasons to separate normal seasonal movement from real growth. Pillar To Post’s inspectors note that ongoing settlement is very difficult to identify from a one-time visit (opens in a new tab), and that confirming it can take repeat visits over a few years.

Check sooner after anything that could move the wall: a very wet spring, a hard freeze, a new crack nearby or water coming through. A single reading after an event doesn’t prove much, but it belongs in the log.

What does a growing crack look like?

It’s longer than your end marks, wider at your ticks than it was in the same season last year, or has developed an offset. Any of those means the crack is active. The NRMCA says to seek professional advice to evaluate and repair active cracks that are widening with time (opens in a new tab).

Signs that mean an engineer, not more monitoring:

• The crack has run past your end marks.
• It’s measurably wider than the same season a year ago.
• It has reached 1/4 inch or more at any point.
• The crack monitor’s crosshair has moved up or down, which means one side is sliding.
• A crack that was sealed has opened again wider than a hairline. Pillar To Post says a repaired settlement crack that has re-cracked (opens in a new tab) could mean ongoing movement.
• The wall itself is leaning or bowing. For a horizontal crack, Feiza says inward movement over 1/2 inch signals a potentially serious problem (opens in a new tab); the horizontal crack guide shows how to measure it.

Take your log and photos with you to the structural engineer. A year of dated readings answers the first question they’ll ask: is it moving?

Can you seal a crack while you’re monitoring it?

Wait if you can. Sealing a crack covers it up, and then you can’t see whether it’s growing. Monitor first, then seal once you know it’s stable.

A leaking crack can’t always wait a year. If it’s a thin, even crack that fits the shrinkage profile, you can seal it now and keep watching the repair and the wall on either side. A crack that reappears next to a fresh repair is a sign the wall is still moving.

Anything on the engineer list should be looked at before it’s sealed. The when to worry guide covers those signs, and the leaking crack guide covers the repair once it’s cleared.