Choosing Your Monitors
Different types exist. Here's what I've actually used.
Two-Plate Monitors
This is what I recommend and what I use. Two overlapping plastic plates with crosshairs on one and a grid on the other. One plate goes on each side of the crack. If the crack moves, the plates shift relative to each other and you can read exactly how much movement occurred.
I bought mine online for about $12 each. Got three of them to cover my worst-looking cracks. They track both widening and shear movement, which is when one side of the crack slides up or down relative to the other.
Tell-Tale Strips
Simpler option. Basically a strip that bridges the crack and breaks if the crack moves. Very cheap, maybe a few dollars each. But they only tell you yes or no. No measurement of how much. I don't use these because I want actual numbers.
The DIY Method
Rick told me about an old-school approach. Epoxy a piece of glass or a thin strip of plaster across the crack. If it breaks, the crack moved. If it doesn't break, the crack is stable. Free, simple, works. But again, no measurement. Just yes or no.
Choosing Where to Install
Location matters more than I expected.
At the Widest Point
Install the monitor where the crack is widest. That's where movement is most pronounced and easiest to detect. My widest crack is about 1/8 inch at its worst point. That's where my monitor sits.
Where You Can See It
Sounds obvious, but I made this mistake. Installed one behind my water heater where I basically couldn't see it without moving things around. Checked it maybe twice before giving up. Put monitors where you'll actually look at them.
On Solid Concrete
Don't install over spalling, flaking paint, or deteriorating concrete. The adhesive needs good surface to bond to. If the concrete is crumbling, the monitor will pull off with it.
Multiple Points on Long Cracks
Gary's crack ran about 15 feet along his wall. We put monitors at three points: both ends and the middle. The middle showed the most movement. If we'd only monitored one end, we might have missed how bad the center was.
Surface Preparation
This is where I messed up the first time. Don't skip this.
Clean the Area
Wire brush or stiff plastic brush to remove loose material, dust, old paint flakes. The concrete should be clean and solid. I spend more time on prep than on actual installation now.
Degrease If Needed
Any oily residue needs to go. Rubbing alcohol works. Let it dry completely. My first failed attempt was partly because I didn't degrease properly.
Dry Is Critical
This was my main mistake. The concrete was slightly damp and I installed anyway. The adhesive never bonded properly. Now I check by pressing a paper towel against the surface. If it comes back damp, I wait.
Gary's wall was seeping water through his crack. We installed the monitors on either side of the wet area, not right on it. The monitors are on dry concrete even though the crack between them is damp.
Mark Position First
Hold the monitor in place and mark where it will go before applying adhesive. Make sure it's centered across the crack with equal mounting surface on each side. Once epoxy is applied, you're committed.
Installation Process
Step by step, the way I wish someone had shown me.
Mix the Epoxy
Most monitors come with two-part epoxy adhesive. Mix equal parts thoroughly. You have maybe ten minutes of working time before it starts setting, so have everything ready before mixing.
If your monitor didn't come with adhesive, buy two-part construction epoxy. The kind rated for concrete. I've used J-B Weld with good results.
Apply to First Plate
Put a thin layer on the back of the first plate. Not too thick. Thick globs mean the plate sits off the wall at an angle. You want it flush against the concrete.
Position and Press
Press the first plate firmly against the wall at your marked position on one side of the crack. Make sure it's level. Hold it for about 30 seconds. Then either keep holding or tape it in place while the epoxy sets.
I use painter's tape to hold plates while curing. Easier than standing there for ten minutes.
Install Second Plate
Same process on the other side of the crack. The key is alignment. The crosshairs on one plate should line up with the center of the grid on the other. Take your time here. Bad alignment makes readings confusing.
Let It Cure
Don't touch it for 24 hours. I made the mistake of checking too soon once, nudged the plate, compromised the bond. Now I install and walk away. Come back the next day.
Taking Your First Reading
After cure, establish your baseline.
Read the Grid
Look at where the crosshairs sit on the grid. Most monitors have numbered lines. My readings are usually something like "0, 0" for a centered crosshair. I write down this initial position with the date.
Photograph It
I take a photo of each monitor showing its initial reading. This is my baseline photo. If I ever think the reading has changed, I can compare to this photo and know for sure.
Record In Your Notebook
Date, monitor location, initial reading. Simple but important. This baseline is what all future readings compare against. Without it, your monitoring is useless.
Reading Your Monitors Over Time
Here's what I check and what the readings mean.
Horizontal Movement
If the crosshairs shift horizontally on the grid, the crack is widening or narrowing. Movement outward means widening. Movement inward means narrowing. Some seasonal variation is normal.
My monitors have never shown more than maybe half a graduation line of seasonal movement. Gary's showed clear horizontal movement within weeks. The difference was obvious.
Vertical Movement
If the crosshairs shift vertically, there's shear movement. One side of the crack is moving up or down relative to the other. This can indicate settlement. My monitors have never shown vertical movement. Thank goodness.
What's Normal
Small fluctuations between readings are normal. Temperature affects concrete. A reading that's half a line different in winter versus summer is probably thermal, not structural. What matters is the long-term trend.
What's Not Normal
Progressive movement in one direction. Each reading farther from baseline than the last. That's what Gary's monitors showed. Clear progression over time. That's when you call an engineer.
Maintenance and Troubleshooting
Monitors need occasional attention.
Check the Bond
Once or twice a year, I gently try to wiggle each plate. They shouldn't move. If one feels loose, the bond is failing. Clean it off and reinstall with fresh epoxy.
Keep Them Clean
Dust accumulates. I wipe my monitors with a dry cloth occasionally so I can read them clearly. Don't use anything that might scratch the plastic.
If One Falls Off
It happens. Usually means the surface wasn't properly prepared, the concrete was damp, or the epoxy was old. Start over. Clean both the monitor and wall thoroughly. Let everything dry. Reinstall.
Sudden Big Changes
If a reading suddenly jumps from 0 to several grid lines, first check if the monitor is still firmly attached. A loose plate that shifted would show as a big reading change but doesn't mean the crack moved. Verify before panicking.
What Gary's Monitors Showed
The monitors told his story clearly.
Initial Reading
We installed three monitors on Gary's horizontal crack in May. All started at essentially 0,0. Centered crosshairs, no movement recorded.
One Month Later
The center monitor showed maybe half a grid line of horizontal movement. The ends showed less. We weren't sure if this was real or measurement error. Waited another month.
Two Months
Now all three showed clear movement. Center almost a full grid line. Ends about half. The wall was definitely bowing further. The monitors made it undeniable.
The Decision
Gary called an engineer with his monitor data. The engineer said the readings were exactly the kind of evidence he needed. Progressive movement documented over time. Recommended wall anchors immediately. Gary's $8,400 repair was justified by those readings.