You’re researching house leveling (aka foundation repair) for your crawl space home, and you keep running into cedar shims. Steel is strong, steel lasts, steel is everywhere in construction . . . so why would a crew reach for little wedges of wood instead? It feels backwards.
At Anchor Foundation Repair, we’ve been leveling crawl space foundations since 1985, and we’ve refined our methods by watching what holds up under Brazos Valley homes. For crawl space work, that’s cedar where shims are concerned. But we’re not here to trash steel, because steel shims earn their keep on plenty of foundation repair jobs. Just not this kind.
In this article, we’ll explain what shims do, walk through the material options, and lay out an honest, side-by-side comparison of cedar and steel so you can see why one wins under a wood-framed home. Fair warning: even our favorite has a downside or two, and we’ll tell you those as well.
What Is the Purpose of Shims in Crawl Space Foundation Repair?
So what’s the point of a shim, and why does such a small piece matter so much? A shim is the fine adjustment at the very top of a support. Underneath it sits the stuff that does the heavy lifting and carries the weight of the home, but the shim is what dials things in to the exact right height for maximum support.
The support pier below handles the big, coarse steps in height. The shim handles the last little bit, the gap between “close” and ” all the way.” Without that top piece snugging everything up against the wooden beam, you’d be stuck with a support that’s either a hair too short or forced up too high using the thicker bits.
That’s why the shim carries more weight than its size suggests. It fills the final gap, spreads pressure against the wooden sill beam, and holds the home right where we set it.
A crawl space home is a wood-framed structure, and the shim is the one part of the support that presses directly into that wood. So the material you choose isn’t a footnote . . . it’s the piece doing the most delicate work, in constant contact with the part of your home most sensitive to pressure and moisture.
For why we land on cedar specifically, and the four jobs it pulls off up there, check out Why Use Cedar Shims for Your Crawl Space Foundation Repair? 4 Purposes.
What Shim Materials Are Available for House Leveling?
Now that you know what a shim is for, what can it be made of? A few materials show up in the shim world, but not all of them belong under a house.
Where contractors buy quality construction materials, you’ll find shims in three main types, wood, metal, and plastic. Each was designed with certain jobs in mind, and only some involve holding up a home:
- Wood (cedar): the tapered, natural-wood wedge we use under crawl space homes (and also for siding and occasionally shingles), built for pressing gently and precisely into a wood structure.
- Metal (steel): strong, flat, and available in fixed thicknesses. A workhorse on slab foundation repairs, where a rigid, long-lasting shim is exactly right.
- Plastic/composite: plastic shims are mostly for lighter tasks like setting windows and doors, not structural home support.
For a crawl space home, that narrows to two real contenders: cedar and steel. Plastic is built for other work, so we’ll set it aside and put the two that matter head-to-head.
Cedar vs. Steel Shims: The Pros and Cons
So how does cedar stack up against steel when you line up the pros and cons? Here’s the honest rundown, including where cedar’s downsides are, because it does have a couple.
The Case for Cedar Shims
We’ve previewed cedar’s strengths above, but we’ll lay it out in a nice neat list. It’s also only fair to give equal time to the downsides, because even our favorite comes with a couple.
Pros:
- Tapered edge for precise, gap-filling adjustment
- Large surface that spreads the load and supports the sill
- Lightweight, easy to handle, and flexible with the wood structure
- Kiln-dried cedar resists shrinking, rot, and termites
Cons:
- Costs more than steel, which is confusing and stings a little for a small piece of wood
- Can be harder to source in some regions, where quality cedar shingles aren’t as easy for a crew to get
Cedar’s durability isn’t just folklore. The wood carries natural compounds called thujaplicins that fight fungus and insects, which is why it earns a “durable” rating on the European EN 350 wood durability scale. In a Texas crawl space, where a little (or a lot of) moisture is always in the picture, that built-in resistance matters.
The Case for Steel Shims
Steel shims aren’t a bad product. In fact, they’re far from it. They’re strong, they last, and they’re our go-to on slab foundation repairs. The trouble is specific to crawl space homes, where a few of steel’s traits work against you.
Pros:
- Strong and long-lasting, and surface rust doesn’t meaningfully weaken them
- Excellent for slab repair, where we reach for them ourselves
Cons:
- Not tapered, so they come in fixed thicknesses (roughly ⅛ to ½ inch) and can’t fine-tune like a wedge
- Those fixed thicknesses can force some over-shimming, pushing small humps up into your floors over time
- The note-card footprint (about 3 by 5 inches) concentrates weight on a small area, which can crack blocks or telegraph humps into the flooring
- In a consistently damp crawl space, steel can develop more than cosmetic surface rust over the long haul
Here’s the balanced takeaway: metal isn’t the enemy, it’s the wrong tool for this job. On a slab home, steel shims are a great choice and we use them without hesitation. On a wood-framed crawl space home, cedar’s precision, footprint, and natural durability win out. Same craftsman, different tool for the material at hand.
If you want the full head-to-head, we broke it down in Wood vs. Metal Shims on a Crawl Space Home: Which is Best?
Ready to Look at the Whole Support, Not Just the Top?
Now you know what shims do, what they’re made of, and why cedar earns the top spot on a crawl space foundation while steel stays in its lane on slab work. The shim is small, but it presses right into your home’s wood, so the material choice truly matters.
After 40+ years under Brazos Valley homes, we’ve *settled* on cedar up top for good, field-tested reasons, and we’re always glad to walk a homeowner through why. If you’d rather we take a look in person, our house leveling team is a call away.
But the shim is only the tip of the iceberg. Underneath it sits the rest of the pier or support station doing the real structural work, and those deserve just as much thought as the wedge on top. See exactly what we *stack* under your home, and the top five reasons we trust it, in Top 5 Reasons Stacked Concrete Blocks Are Best for Crawl Space Homes.