Why are the floors sagging over my crawl space?

Reviewed by Preston, Oklahoma foundation repair consultant · Updated 2026-10-14

In short: In a pier-and-beam house, the floor rests on wood beams and joists carried by interior piers or pads in the crawl space. When those pads settle, wood shims crush or rot, or beams and joists are too small or span too far, the floor sags, bounces or slopes toward the interior. It is a problem with the support system under the floor rather than with the perimeter foundation.

How is a pier-and-beam floor supported?

A pier-and-beam (crawl space) floor has three layers of support:

• Joists, the closely spaced boards that the subfloor is nailed to.
• Beams or girders, heavier members that carry the joists.
• Interior piers or pads, posts or masonry piers on small footings that hold up the beams, often with wood or steel shims between the pier and the beam to set the height.

A failure at any level shows up as a sag or slope in the floor above. Texas engineering guidelines treat pier-and-beam repairs, including shimming, framing repairs and added supports, as a distinct category from slab repairs ¹.

What goes wrong with the floor supports?

Pads moving with the soil

In Oklahoma, the active zone, the depth of soil whose moisture changes with the climate, has been estimated at roughly 0.5 to 4 meters (about 1.5 to 13 feet) ⁵, so a shallow interior pad on clay rests within the soil that shrinks and swells. Nearly all of Oklahoma has medium or high soil swelling potential ⁵. See shrink-swell and the active zone.

Shims and wood that decay

Wood decay is generally understood to occur above about 30% moisture content and not below about 20% ³. Decay requires moisture, oxygen, a suitable temperature (about 50 to 95°F) and the wood itself as food ⁴, and decayed wood loses strength ⁴. Wood shims and posts that stay damp in a crawl space are exposed to those conditions, and as they lose strength the beam above can drop. See crawl space moisture and wood decay.

Beams and joists that span too far

When a beam or joist spans too far for its size, or has been notched or cut for plumbing or ductwork, it bends more under load. Engineers often judge floor deflection against the L/360 live-load limit from the International Residential Code, meaning a span should not bow more than 1/360 of its length ²; the Foundation Performance Association uses the same L/360 ratio ⁷.

Debris and site conditions

The code calls for removing vegetation, organic material and wooden forms from the crawl space, and for drainage or a raised grade where groundwater can rise close to the floor ⁶.

What are the signs of interior support failure?

• sagging floor between supports
• bouncy or soft floors
• sloping floors toward the interior
• doors that bind and diagonal drywall cracks near the sag

Texas guidelines note that signs such as cracked drywall or racked door frames do not necessarily mean the structure is performing inadequately ².

How is interior sag different from perimeter settlement?

When the perimeter foundation drops, floors slope outward toward the exterior walls. When the interior supports fail, the floor sags in the middle or between supports while the perimeter stays near level. A floor elevation survey records the shape of the floor so the two patterns can be compared ²; see elevation surveys and perimeter drying settlement.

How is a sagging crawl space floor typically fixed?

The Texas ASCE guidelines list floor shimming, framing repairs, additional supports, restoring the open space under the floor and crawl space moisture control as pier-and-beam repair approaches ¹. Framing repairs replace or reinforce structural members that are damaged or distressed ¹. See crawl space repair.

Related

• Pier and Beam Crawl Space
• Crawl Space Moisture and Wood Decay
• Manufactured Homes on Piers
• Crawl Space Encapsulation
• Bouncy or Soft Floors