Garage wall on the slab
What's wrong here?
- The bottom plate on the slab is untreated lumber; it must be preservative-treated or naturally durable wood
- The anchor bolts are too far apart; a garage wall needs one every 3 ft
- Untreated wood is fine here because a plastic vapor retarder goes under the slab
- Nothing. This installation is compliant
Show the answer and the code reference
The answer
The wall's bottom plate rests directly on a slab-on-ground garage floor but is ordinary kiln-dried S-P-F (stamped KD-HT), not preservative-treated or naturally durable lumber.
Correct choice: 1. The bottom plate on the slab is untreated lumber; it must be preservative-treated or naturally durable wood
Concrete in contact with the ground wicks moisture into anything sitting on it, and a Minnesota garage slab also takes meltwater and road salt off the cars every winter. R317.1 requires sills and sleepers on a concrete or masonry slab that is in direct contact with the ground to be naturally durable or preservative-treated wood, unless an impervious moisture barrier separates them from the slab. This plate is the same pale color as the studs and its stamp reads KD-HT (kiln-dried, heat-treated), with no treatment marking. It will rot from the bottom, where nobody sees it, until the studs and anchor bolts have nothing solid to hold.
How to fix it
Replace the bottom plate with preservative-treated lumber rated for the use, using fasteners and anchor hardware compatible with the treatment (R317.3), and re-anchor it per R403.1.6. A common repair is to cut the nails, slide out the plate a section at a time and slip in treated stock.
Code references
IRC 2018 R317.1 — Location required
Protection against decay, by naturally durable wood or wood preservative-treated per AWPA U1, is required in listed locations. Item 3 covers sills and sleepers on a concrete or masonry slab in direct contact with the ground, unless an impervious moisture barrier separates them from the slab.
Drills citing R317.1IRC 2018 R403.1.6 — Foundation anchorage
Sill plates and sole plates must be anchored to the foundation and must be protected against decay and termites where Sections R317 and R318 require it.
Drills citing R403.1.6
2024 IRC:not yet checked Confidence: low
Minnesota
Minnesota adopts the 2018 IRC with Minnesota amendments as the Minnesota Residential Code (Minn. Rules ch. 1309). No Minnesota amendment to R317.1 is known. Minn. Rules 1309.0403 is reported to amend R403.1.6, but nothing found suggests it drops the R317 decay-protection cross-reference; the Minnesota text could not be checked directly. DLI is adopting the 2024 IRC as the next edition; see the 2024 IRC note.
About the Minnesota Residential CodeWhy the other choices are wrong
- The anchor bolts are too far apart; a garage wall needs one every 3 ft. R403.1.6 allows anchor bolts up to 6 ft on center, with one within 12 in. of each plate end. These are about 4 ft apart.
- Untreated wood is fine here because a plastic vapor retarder goes under the slab. R317.1 accepts only an impervious moisture barrier between the plate and the slab. Poly under the slab does not keep moisture in the concrete itself out of the plate, and garage slabs are often poured without it anyway.
- Nothing. This installation is compliant. Sills and sleepers on a concrete slab in direct contact with the ground must be naturally durable or preservative-treated wood (R317.1), and KD-HT spruce-pine-fir is neither.