How Thick Should a Shop Slab Be in Spokane? 4 vs. 6 Inches, Car Lifts and 2026 Costs

Short answer: most Spokane shop and garage floors should be at least 4 inches of concrete, and 6 inches is the safer choice if you'll park heavy trucks, RVs or equipment, or install a heavier car lift. Code sets the floor at 3.5 inches, but a lift manufacturer's spec, your vehicles and your base usually decide the real number. Going from 4 to 6 inches adds about $2 per sq ft in HomeGuide's 2026 pricing.

What the code requires

The International Residential Code, which Washington adopts for houses and residential accessory buildings, sets the minimum for a concrete floor slab on the ground:

• Minimum thickness: 3.5 inches (IRC R506.1)
• Base: a 4-inch layer of clean graded sand, gravel or crushed stone under the slab, unless the slab sits on well-drained or sand-gravel soils (IRC R506.2.2)
• Vapor retarder: 6-mil polyethylene (or an approved equal) between the base and the slab in heated spaces (IRC R506.2.3)

That's a minimum for a house floor, not a recommendation for a shop that carries a loaded pickup or a tractor.

Car lifts set their own minimum

If a two-post lift is in your plans, the lift's install manual matters more than the code. BendPak's 2025 manual for its 9APF and 12APF two-post lifts calls for:

• 9,000 lb lift (9APF): at least 4.25 inches of steel-reinforced concrete
• 12,000 lb lift (12APF): at least 6.5 inches of steel-reinforced concrete
• Both: 3,000 psi minimum strength, cured at least 28 days
• Placement: not within 6 inches of cracked or defective concrete

BendPak's concrete requirements page adds that anchors must stay at least 6 inches from expansion seams and control joints, that lifts never go on hand-mixed concrete, and that existing floors should be test-drilled for thickness and core-tested for strength.

So a "standard" 4-inch garage floor is already too thin for even BendPak's 9,000 lb lift. If a lift is a maybe, decide before the pour. Thickening the slab where the posts will go is far cheaper than cutting out and repouring later.

4 inches vs. 6 inches: what it costs

HomeGuide's 2026 installed prices by thickness:

• 4-inch slab: $6 to $8 per sq ft
• 6-inch slab: $8 to $10 per sq ft
• 8-inch slab: $10 to $12 per sq ft

For a 30x40 shop floor (1,200 sq ft), that's about $7,200 to $9,600 at 4 inches and $9,600 to $12,000 at 6 inches. In concrete volume, 1,200 sq ft at 4 inches is about 14.8 cubic yards, and at 6 inches about 22.2 cubic yards (square feet times inches, divided by 324). For local ready-mix pricing and short-load fees, see our Spokane concrete cost per yard guide.

Which thickness fits your shop

• Storage, a workbench and cars or half-ton pickups: 4 inches on a compacted gravel base
• A 9,000 to 10,000 lb two-post lift: at least the lift's minimum (4.25 inches for BendPak's 9APF), reinforced, and we'd thicken the post areas
• Three-quarter and one-ton trucks, RVs, trailers and small tractors: 6 inches, reinforced
• Heavier lifts, skid steers or loaded equipment: 6 inches or more, sized to the lift's manual or an engineer's design

Spokane conditions that matter more than an inch

Thickness won't save a slab that sits on a bad base or gets wet underneath. In Spokane County:

• Frost depth: footings and thickened edges go at least 24 inches below finished grade under the joint City and County frost-depth policy
• Drainage: water that gets under a slab freezes and lifts it, so grade the site and the gravel base so water runs away from the building
• Steel buildings: on a pre-engineered metal building, the slab is often poured with the foundation, and the column footings are sized from the manufacturer's reactions, not the slab spec. Our PEMB foundation guide covers what to ask before that pour
• Coatings later: if you want an epoxy or polyaspartic floor, plan the finish and vapor retarder now. See our epoxy vs. polyaspartic shop floor guide

Questions to ask before your shop slab is poured

• How thick is it everywhere, including where a lift or heavy equipment will sit?
• What reinforcement goes in, rebar or wire mesh, and how is it held up in the middle of the slab?
• What's the concrete strength (psi), and will they wait the 28 days a lift needs before anchoring?
• How deep and how compacted is the gravel base?
• Is there a vapor retarder, and where will the control joints go relative to your lift and workbench?

Get your shop slab priced

Raven Buildings pours shop slabs, foundations and the steel and pole buildings that sit on them across Spokane County. Call 509-350-2016 or request a free quote, and tell us the size, what you'll park on it and whether a lift is in the plans. See our concrete services and steel building services.

Sources

• International Code Council, 2021 IRC Chapter 5, Section R506 Concrete Floors (On Ground)
• BendPak 9APF and 12APF Two-Post Lift Installation and Operation Manual (2025)
• BendPak, Car Lift Concrete Slab Requirements
• HomeGuide, How Much Does a Concrete Slab Cost? (2026)
• City of Spokane / Spokane County Joint Policy BUILD-07, Official Frost Depth