Parts for this step · tap one to find it
An older one-story house on a raised concrete foundation, with short wood cripple walls (about 2 ft) between the foundation and the floor. The model shows one 12 × 8 ft corner of the crawlspace; the existing framing is white. The retrofit adds anchors that bolt the wood to the concrete, plywood that braces the cripple walls, and clips that tie the floor to them. Nails, epoxy and connector fasteners are not listed.
≈3–5 days for a typical house
This guide shows the sequence, not a design. Anchor size and spacing, panel lengths, nailing and clip spacing all come from the approved plan for your house — a prescriptive standard plan where your building department allows one, or an engineer’s design.
Before any work, the crawlspace gets surveyed: the concrete checked for cracks and crumbling, the wood for rot and termite damage, and the cripple-wall height, stud spacing and any existing bolts measured. That survey decides which plan fits the house, and the permit is pulled on that plan.
≈2–3 hr on site; permit time varies · Cripple studs 2×4 · 16 in on center · wall 2 ft tall
Many Southern California building departments accept a prescriptive plan for qualifying houses, based on the California Existing Building Code (Chapter A3) or FEMA P-1100. Cripple walls taller than 4 ft, and some other conditions, need an engineer’s design. Rotted wood or damaged concrete gets repaired first.
New anchors go down through the mudsill into the concrete, one stud bay at a time. Each hole is drilled and cleaned out, then the anchor is set with epoxy or as an expansion anchor, as the plan specifies. A square steel plate washer and a nut clamp the sill down.
≈1 day for a typical house · 6 anchors in this section · about 4 ft apart as drawn · each set in a stud bay
Parts: 6 × Retrofit anchor bolt with nut; 6 × Plate washer, 3 × 3 × 0.229 in
Anchor size, spacing and embedment come from the plan and the anchor maker’s instructions. For a one-story house, the state’s prescriptive provisions call for ½ in anchors no more than 6 ft apart; many plans space them closer. Where there isn’t room to drill from above, the plan may call for steel foundation-plate connectors fastened to the side of the sill and the concrete instead. Adhesive anchors in an existing foundation may need special inspection — ask when you pull the permit.
Every edge of a plywood panel has to land on solid wood so it can be nailed. At this corner the front wall’s panel would end on open air, so a new stud (a nailer) goes in to catch it. The plan may also call for blocking where the mudsill is wider than the studs, behind any horizontal panel joint, or between floor joists.
≈1–2 hr · 1 nailer in this section · 2×4 cut to fit, ≈19½ in
Parts: 1 × Nailer stud, 2×4, cut to fit
Old framing is rarely right on layout. Checking where every panel edge will land before cutting any plywood saves rework.
Structural plywood, ripped to the wall height, is nailed to the inside face of the cripple studs so it covers the mudsill and the top plate. Vent holes are cut in each stud bay so the crawlspace keeps its airflow.
≈1–2 days for a typical house · 3 pieces from 2 sheets ripped in half (≈24 in), 1½ strips spare · 8d nails 4 in apart at edges, 12 in at studs between, per the plan
Parts: 2 × Structural plywood, 15/32 in, 4×8 sheet
The state’s prescriptive provisions call for one 2–3 in vent hole near the top and one near the bottom of each stud bay (just one if the studs are under 18 in), at least 1 in clear of the plates. For a one-story house they call for a braced panel at each end of every wall, bracing along at least 40% of its length, and each panel at least twice as long as the studs are tall; this model sheathes the whole run. Don’t overdrive nails through the face of the panel.
Framing clips fasten the top plate of each cripple wall to the floor framing above it — the rim joist along the front wall, and the end joist along the side wall where the joists run parallel — so the floor can’t slide off the walls in an earthquake.
≈3–4 hr · 5 clips in this section · 48 in apart as drawn
Parts: 5 × Framing clip, floor to top plate
The state’s prescriptive provisions space clips 48 in apart for a one-story house; clip type, rated capacity and fasteners come from the plan and the maker’s catalog, so use exactly the nails or screws it lists. Where the joists sit on the plate without a continuous rim, the plan may call for solid blocking between the joists instead.
The building inspector checks the work against the approved plan before anything is covered: anchors and washers, panel nailing and vent holes, and the clips. Some departments also inspect the anchors before the plywood goes on.
≈1 hr · Keep the permit card and approved plan on site until sign-off
Don’t insulate or cover anything that needs inspecting until it’s signed off.
That’s a crawlspace seismic retrofit: the mudsill bolted to the foundation, the cripple walls braced with plywood, and the floor tied down to them.