When it comes to foundation repair, not every home should be repaired using the same materials or methods. One of the biggest differences is between a concrete slab foundation and a crawl-space or pier-and-beam foundation.
A common example is the type of shims used during the repair.
Slab foundations typically use steel shims, while crawl-space and pier-and-beam homes may use wood shims or wood blocking in appropriate locations.
Why? Because the two foundations are built very differently.
A shim is a relatively small piece of material used to make final adjustments and establish firm contact between structural components.
During foundation repair, lifting or leveling can create small gaps between a support and the structure above it. Properly installed shims help close those gaps so the weight of the home is transferred onto the support.
Although shims are small, they can sit at an important load-transfer point, which makes using the correct material critical.
A slab foundation is primarily a concrete structural system.
When foundation piers are installed beneath a slab, the load from the concrete foundation must be transferred into those supports.
A simplified system may look like this:
Concrete Slab
↓
Steel Shims
↓
Foundation Pier/Piling
↓
Supporting Soil
Steel shims are well suited for slab foundation repairs because they provide:
High compressive strength
Precise final adjustments
Long-term durability
Resistance to crushing under normal foundation loads
No shrinking or swelling like wood
No wood rot or decay
A solid interface between the concrete foundation and the pier
Wood naturally behaves differently from steel.
Depending on the type of wood and its environment, it can compress, shrink, swell, split, or deteriorate from moisture and decay.
That makes wood generally inappropriate as the permanent shim material at the pier-to-concrete connection of a slab foundation.
When you’re supporting a heavy concrete foundation, you want the final load-transfer connection to use a material appropriate for that application.
That’s where steel shims come in.
This is where homeowners sometimes get confused.
If wood shouldn’t be used to shim a concrete slab, why would a foundation company use wood underneath a crawl-space home?
Because a crawl-space or pier-and-beam home is already primarily a wood-framed structural system above its supports.
Underneath many crawl-space homes, you’ll find components such as:
Wooden floor joists
Wooden beams and girders
Sill plates
Subfloor framing
Wood blocking
Piers, columns, or other supports
Unlike a concrete slab, the floor of the home is supported by a network of wooden framing members.
When we’re leveling or repairing one of these homes, we’re often making adjustments between components of an existing wood-framed system.
Think about the difference this way.
With a slab foundation, we’re dealing with something similar to:
Concrete → Steel Shim → Pier
With a crawl-space or pier-and-beam foundation, we may be dealing with:
Wood Beam → Wood Shim/Blocking → Support
The wood shim becomes part of an adjustment involving the home’s existing wood framing.
Properly selected structural wood can distribute loads and work with the beams and framing already supporting the floor system.
That is completely different from placing a piece of wood between a heavy concrete slab and a foundation pier.
A crawl-space foundation doesn’t function like one giant concrete slab.
Instead, loads are transferred through the home’s framing.
A simplified example is:
Flooring / Subfloor
↓
Floor Joists
↓
Beams / Girders
↓
Blocking or Shimming Where Appropriate
↓
Piers / Columns / Supports
↓
Footings / Soil
Each component works together.
When a crawl-space home becomes uneven, repairs may involve adjusting existing supports, adding new supports, replacing deteriorated beams or joists, or correcting gaps between structural components.
Wood shims or blocking can be appropriate in certain locations because we’re working with a wood-framed structure.
Absolutely not.
There’s a major difference between properly selected and installed structural material and simply wedging random scraps of lumber underneath a house.
A professional crawl-space repair should consider:
Condition of the existing beam
Condition of the floor joists
Size and location of the support
Moisture conditions
Wood deterioration or termite damage
Amount of adjustment required
Load being transferred
Whether additional structural support is necessary
Shimming cannot correct a rotten or severely damaged structural member.
If a beam or joist has deteriorated beyond an acceptable condition, the appropriate repair may involve replacement, reinforcement, or additional support rather than simply adding more shims.
Homeowners sometimes enter their crawl space, see wood between a beam and support, and immediately assume something was done incorrectly.
That’s not necessarily the case.
The important question is where the wood is being used and what type of foundation is being repaired.
Wood used appropriately within a wood-framed crawl-space system is very different from wood being used as the permanent load-transfer shim between a concrete slab and a foundation pier.
That’s why foundation repair methods need to match the construction of the home.
The easiest way to remember the difference is:
Steel shims are typically used at the pier-to-foundation connection.
Steel provides a strong, durable, precise interface between the concrete slab and the supporting pier.
Wood shims or blocking may be used in appropriate locations within the wood-framed support system.
Because beams, joists, and other components are already constructed from wood, properly selected wood materials can be appropriate for making adjustments and transferring loads within that system.
Seeing steel underneath one home and wood underneath another doesn’t necessarily mean one repair is better than the other.
They’re different foundation systems that require different approaches.
A concrete slab foundation and a crawl-space foundation transfer loads differently and are constructed from different materials. Foundation repair should account for those differences.
At Leon Foundation Repair, we believe homeowners should understand what’s being installed underneath their home and why.
Whether you have a concrete slab, pier-and-beam, block-and-base, or crawl-space foundation, the goal is the same: properly stabilize and support the structure using materials appropriate for that particular foundation system.
If you’re noticing uneven floors, cracks, sticking doors, soft spots, sagging floors, or other signs of foundation movement, contact Leon Foundation Repair for an evaluation.
Leon Foundation Repair
832-398-6280
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Serving the communities of Houston and the surrounding areas of Texas
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