Before the Concrete Sets: Why a Footing Inspection Matters for Monolithic Slabs
When it comes to monolithic slabs, there aren’t a lot of second chances.
Once the concrete is poured, everything beneath it — the footings, vapor barrier, plumbing, even the soil — is locked in. That’s why I encourage folks building in Wake County or the Triangle area to consider a footing inspection, especially when working with a monolithic slab.
I’ve walked plenty of sites, stood on soft subgrades, poked holes in poorly compacted soil, and seen how a few missteps early on can lead to problems down the road.
Here’s what I look for during a monolithic slab preparation inspection—and why each one matters.
1. Bearing Soils
The first thing I do is probe the soil where the footings are planned. If I can easily drive a probe into the soil, there are soft spots that aren’t sufficient to bear load. Footings typically need to be able to withstand 2,000 pounds per square foot. Soft soil can lead to differential settlement, which might show up later as uneven floors or foundation cracks.
2. Footing Dimensions and Placement
Footings aren’t just a pad of concrete—they’re built to carry specific loads. That’s why it’s important they’re sized correctly and placed in the right spot, especially under interior bearing walls and point loads like columns. I’ve seen cases where lug footings, thickened slabs, or pier footings are either too small or off-center. When that happens, the structural load doesn’t bear evenly across the footing. Ideally, the weight of the structure should land within the middle third of the footing. If it doesn’t, that imbalance—called eccentricity—can cause the footing to rotate or settle unevenly over time.
3. Vapor Barrier and Capillary Break
Moisture control starts below the slab. Most plans call for a vapor barrier placed over at least four inches of clean, washed gravel. This gravel acts as a capillary break, preventing groundwater from wicking up into the slab. I also look for a vapor barrier that's at least 6 mil thick and overlapped a minimum of 12 inches. A proper barrier helps keep the slab dry, which affects comfort, reduces humidity-related wear on flooring, and limits vapor intrusion—including soil gases like radon.
4. Sewer Lateral in the Load Path
The sewer Lateral needs to be clear of the home’s “zone of influence”—basically, the area where structural loads are transferred to the ground. If plumbing runs through this zone, it could end up bearing weight it's not designed for. Over time, this can deform or even crush the pipe. It’s a surprisingly common issue and often hidden once concrete is poured.
Turndown footing in light grey. Capillary break in dark gray. Vapor barrier not shown. Red cone is the zone of influence where the loads of the home distribute through the soil.
5. Reinforcement Placement
Steel reinforcement plays a different role in footings and slabs, but both rely on it to do their job well. In footings, rebar helps resist tension forces from the structure above. In slabs, it helps control cracking from shrinkage and movement.
Where the rebar sits matters. In footings, it should be near the bottom—since that’s where tension tends to occur—but not too close. If the steel sits right against the soil or formwork without enough concrete around it (what’s called "clear cover"), it’s more likely to rust over time.
Spacing and overlap are important, too. Where one piece of rebar ends and another begins, they need to overlap by a certain distance—called the “development length.” Without that overlap, the bars can’t fully transfer the load between them, which weakens the connection.
6. Plumbing Interference
Plumbing sometimes ends up in places it shouldn’t—especially in footings. One common issue is a drain line running directly beneath a point load, like a column. When that happens, the weight of the structure ends up bearing on the pipe, which it wasn’t designed to handle. Over time, that pressure can distort or even collapse the line.
Another issue is plumbing being routed parallel through a footing, which can interrupt the concrete pour, weaken the footing and place the pipe under undue stress.
When plumbing has to pass through a footing or slab, it should be sleeved inside a pipe of larger diameter, and it should pass through perpendicular to the footing. This sleeve allows for slight movement of the pipe—like expansion, shifting, or settling—without cracking the concrete or damaging the line.
7. Drainage Slope and Plumbing Integrity
Where plumbing is exposed, I take a look at the slope and joints. Drainpipes need to slope at the proper rate (typically 1/8” to 1/4" per foot depending on pipe diameter) and the fittings should be secure and properly adhered. The bottom of the sewer lateral trench should be compacted, and the pipe should be continuously supported with sand or fine gravel to prevent bellies from forming when soil is dumped back into the trench.
8. Subbase Compaction
This is one I always check, and honestly, it’s often overlooked. If you walk across the slab area and your boots feel like they’re sinking into the soil or gravel, that’s a sign the subbase wasn’t compacted properly. Most plans call for 95% compaction, and that should feel firm underfoot. But the compaction note is often tucked away in the fine print—and sometimes missed in the rush to pour.
In Summary
A monolithic slab may look simple from above, but a lot is going on beneath the surface. A good inspection before the pour isn’t about making a project more complicated—it’s about making sure everything hidden inside that concrete is done right the first time.
If you’re building in places like Wendell, Angier, Holly Springs, or anywhere across the Triangle, I’d be glad to take a look and help make sure the slab you're about to pour has a solid start.
Interested in a pre-slab inspection or have questions about your build?
Feel free to contact us or book a service. I serve Cary, Raleigh, Apex, Pittsboro, and surrounding areas
