Why Floors Sag and Slope Over a Crawl Space

Most homeowners notice it the same way: something rolls across the kitchen on its own, or a piece of furniture that used to sit flat now rocks. A floor that has moved is telling you something about the structure underneath it, and the first useful question is not how to fix it but which of two different problems you have.

Sag and Slope Are Not the Same Thing

A sag is a dip. The floor is lower in the middle of a span and comes back up toward the walls, so a long level laid across the room rocks over a high point at each end. That points at the members doing the spanning — joists that are deflecting, or a girder that has bowed between its supports.

A slope is a tilt. The floor falls consistently in one direction across the whole room, with no dip in the middle. That points at what the structure is standing on — a pier or column that has settled, or foundation movement on one side of the house.

The distinction matters because the repairs are different. Sag is usually addressed by strengthening or supporting the span. Slope is usually addressed underneath the span, at the piers or the foundation. Guessing wrong means paying to reinforce joists that were never the problem.

What Is Actually Under There

In a typical Triangle crawl space home, the floor is carried by joists running in parallel, spanning from the foundation wall to one or more girders that run through the middle of the house. Those girders are held up at intervals by piers or columns, each sitting on its own footing. Every one of those elements can fail, and each produces a different symptom.

The Common Causes

Rot at the bearing points

Wood fails first where it stays wet, and in a crawl space that is usually where it touches something else — the ends of joists where they sit on the foundation, the band joist around the perimeter, the sill plate, and the base of any wood column where it meets its footing. A column that has gone soft at the bottom loses bearing and drops, and the floor above it drops with it. This is the most common cause we find, and it is a moisture problem wearing a structural costume.

Pier or footing settlement

Interior piers carry a surprising share of the house, and they often sit on footings that are smaller and shallower than the perimeter foundation. If the soil beneath one moves — because it was poorly compacted, or because water has been running under the house for years — that pier settles and takes a line of the floor down with it.

Joists that were always working too hard

Some floors have been deflecting slowly since the day they were built, because the joists were undersized for their span or spaced too far apart. Renovations create this too: removing a wall that was quietly carrying load, or adding weight the floor was never designed for, like a tile floor, a stone island, or a second-floor bathroom.

Notching and drilling

Plumbers, electricians and HVAC installers all need to get through the floor structure, and a joist can tolerate a certain amount of that if the holes are the right size in the right place. What it cannot tolerate is a deep notch cut into the bottom edge near mid-span, which is exactly where the wood is working hardest in tension. One aggressive notch for a drain line can take most of a joist's capacity, and it is a very common find.

Insect damage

Termites and wood-boring insects hollow out members from the inside, so a joist can look intact and have very little left. They are drawn to the same damp wood that rot is, which is why the two are so often found together. Note that treating the insects and repairing the damage they did are two separate jobs — extermination stops the colony but puts nothing back.

Notice how many of these trace back to water. Rot, insect damage and pier settlement all need sustained moisture to happen. Repairing the wood without fixing what is wetting it means doing the same repair again later.

How to Read Your Own Floor

Before anyone goes under the house, you can learn a lot from above. Lay the longest level or straightedge you own across the room in several directions and note where it rocks and which way it falls. Do it in more than one room. Then check whether the movement is confined to one area or shows up across the house — the same local-versus-systemic question that separates a lintel problem from a foundation one.

Other things worth noting, because they help whoever assesses it:

• Whether the floor feels bouncy underfoot, which suggests deflection rather than settlement
• Interior doors that have started to bind or swing on their own, and in which rooms
• Gaps opening between the baseboard and the floor, or between trim and the ceiling
• Cracks in drywall radiating from the corners of door frames
• Whether it has changed recently, or has been like that as long as you can remember

That last one carries real weight. A floor that has been gently out of level for thirty years and is not moving is a different situation from one that has changed noticeably in the last two, even if they measure the same today.

What an Assessment Involves

The answer is in the crawl space, and it is a hands-on inspection rather than a visual one: probing suspect wood to find out how much sound material is actually left, checking bearing points and column bases, looking at the piers and their footings, and taking moisture readings so the cause is established and not assumed. Measuring the floor's actual elevations from below gives a map of where the movement is, which is what determines whether the fix belongs in the span or underneath it.

Triangle Reconstruction has worked on crawl spaces and foundations across Wake and Durham counties, and every county bordering them, since 2012. If your floors have moved, call (919) 924-7600.