Site prep before a shop slab

The short answer

A good shop slab starts with the dirt under it. Before any concrete, the site gets cleared of brush and roots, stripped of topsoil, built up with compacted fill into a level pad that sits above the ground around it, and graded so water runs away from the building on every side. Then the slab gets formed to the right thickness, with anchor bolts set exactly where the building plan puts them. Skip any of that and you get cracks, standing water, and out-of-square walls down the road. We do the dirt work and pad and put up the metal building on the same place, so the pad gets built for the building that sits on it.

Start with where it sits

Where the building goes matters as much as how it's built. Look for:

• High ground. A pad in a low spot fights water forever.
• Drainage paths. Don't set a shop across a draw or anywhere water sheets off after rain. If water has to cross the site, give it a way around. Our post on water gaps shows how much force that water carries.
• Access. Steel trucks, concrete trucks, and later your trailer all have to get in and turn around. Plan the drive with the pad.
• Doors and wind. Big doors facing the weather fill up with rain and dust.

Know what's under you

Soils change across a single place, and they change a lot across Parker County. The USDA soil survey uses Parker County as the type location for the Windthorst soil series, which has a clay subsoil, roughly 35 to 50 percent clay, with moderately slow permeability. The same survey notes that some nearby soil series have high shrink-swell (USDA OSD).

In plain terms: parts of the county have clay subsoils that hold water and move as they wet and dry. Clay that swells under one corner of a slab and shrinks under another is how slabs crack. That's why fill, compaction, and drainage matter.

How much fill, what kind, and how tight it has to be compacted isn't something a website should tell you. Compaction specs come from a soils report for your site and from your slab design, not a blog post. If your lender, building supplier, or county asks for one, get it before the pad is built.

Clear and strip

The pad area, plus a margin around it, gets cleared down to dirt. Brush, cedar, mesquite, roots, and stumps come out. Roots left under a slab rot and leave voids. We clear mechanically, by dozing and grubbing. See land clearing.

Then the topsoil comes off. Topsoil is full of organic matter that settles. It's good for grass around the building and bad under a slab.

Build the pad

The pad gets built up in layers, not dumped in one pile. Each lift gets moisture added or worked out as needed, then compacted before the next one goes on. The finished pad sits above the surrounding grade so water has somewhere to go.

The pad should run past the slab edge on all sides. That leaves room for forms, for the apron, and for grading away from the building.

Slope it away

Water is the enemy of a slab on clay. AA Metal Buildings, a metal building vendor, recommends grading the ground to slope about 5 percent away from the building (AA Metal Buildings). On a shop, that means water off the roof and the pad runs away from the slab edge, not under it.

Think about where roof water lands, too. A metal roof dumps a lot of water fast. Gutters, a rock apron, or a shaped swale keep it from cutting a ditch along the slab.

Slab thickness and concrete

Slab design belongs to your building supplier and the people who design it. As a reference point, the same AA Metal Buildings article describes common vendor practice (AA Metal Buildings):

• 4 inches minimum thickness for a light-use slab.
• 5 to 6 inches where you park equipment or heavy trucks.
• 3,500 to 4,000 psi concrete.

Your building may call for more, including thicker edges or footings under the columns. Ask your building supplier for the manufacturer's stamped drawings and foundation loads, and build the slab to those.

Anchor bolts

Metal buildings bolt to the slab at every column, and the bolts have to land where the building plan puts them. AA Metal Buildings puts the tolerance at 1/16 inch of the plan (AA Metal Buildings). Miss by more than that and a column won't fit, and you're drilling and epoxying bolts after the fact.

That's a big reason to plan the pad, the slab, and the building together. Whoever sets the bolts should be working from the building's drawings and know exactly what's going on top of them.

Permits

Inside Weatherford city limits, the city requires a permit for accessory structures over 200 square feet, which covers most shops and barns (Weatherford). Unincorporated Parker County doesn't issue building permits, but it does regulate septic and floodplain work (Parker County). Other towns and counties have their own rules, so check with yours before the pad goes in.

Barndominium pads

A barndominium pad follows the same rules with more riding on it, because people live on that slab. Plan where water, septic, and electric lines come in before the pour. On barndominiums, our scope is the shell, the slab, and the site. Interiors, electrical, and plumbing aren't part of our scope and are handled by your own finish contractors, so bring them into the conversation before the pour.

One crew for the pad and the building

When the same crew builds the pad, the drive, and the building, the pad gets built for that building: the right size, the right height, and drainage running the right way, with the slab formed and poured to the building's drawings so the bolts land where the columns go. We walk the site with you, lay out the pad and the building, and price it on site. See dirt work and pads and metal buildings for how we do it.

Get a quote

Call or text 254-716-5760, or send the building size, the town, and photos of the site with the form below. We'll text or call back to set a site visit.