How to Build Forms for a Patio or Walkway
Learn how to build straight forms for a strong and sturdy patio or walkway.
The biggest concern for any woodworking project should be safety—especially if you're building something that will see a lot of foot traffic like a patio or walkway. To build something that is structurally sound, you need to start with a solid foundation. Building straight forms for your patio is an important step and we are here to help. Take a look at our advice below to guide you in your construction. If done correctly, you'll build a safe patio that will be with your family for a long time.
Strong, straight forms make the best slabs. Slabs that bulge, tilt, or otherwise display sloppy construction mar the beauty of your project. There's no easy, inexpensive way to correct faults in concrete once it sets.
Inspect each piece of form lumber before buying it. Look for knots, splits, and other defects that could affect its strength. Wet concrete will push the form with a tremendous amount of force, so the forms need to be structurally sound.
Make your 2x4 stakes long enough to put at least 8 inches into the ground when the stake is driven to 1 inch below the top of the forms (this keeps the stake out of the way when you screed the concrete). Slabs wider than 8 feet require control joints. These cuts in the surface of the slab keep cracks from spreading randomly across the surface. You'll cut them after the concrete is poured, but you'll mark their location on the forms before you pour.
After you build the forms, pour and tamp a gravel base to the depth required by code. Then lay reinforcing wire mesh on dobies or bolsters on the gravel.
What You Need
- Circular saw
- Carpenter's level
- Mason's line
- Cordless drill
- Deck screws or nails
- Bender board
- Expansion strip
- Construction adhesive
- 2x4 and 2x6 lumber
Step 1: String Lines
Lay out and excavate the site. Then restring the mason's lines between the corner stakes. If you're going to use the top of the forms as a screed guide, restring the lines level with the top of the slab (for a structure with a slab base) or the sand base (for a sand-set installation). Then drive 2x4 stakes at about 2-foot intervals along the mason's lines, keeping their tops just below each line. Make sure the interior face of each stake falls directly under the mason's line.
Step 2: Install 2x6-inch Board
Place a 2x6 against the interior face of the stakes and, keeping it level with the mason's line (or a carpenter's level), fasten it to the stakes with 2-1/2-inch screws. When you add the gravel subbase, the 2x6 will let some gravel seep under its bottom edge but not as much as a 2x4 would. The wider form will be more stable.
Step 3: Continue With Boards
Continue fastening 2x6s to the stakes. Butt-join them and reinforce each joint with a 1x or a 3/4-inch plywood cleat screwed across the joint.
Step 4: Install Kickers
Screw and stake 1x kickers to each joint and at 4-foot intervals on the outside of the forms. Concrete is very heavy and without the kickers, its weight would push the forms out of alignment or snap them. If you are dividing a large patio with internal forms, now's the time to anchor them.
Dividing a Large Patio
If you pour a large patio slab or driveway in sections, one of your crew can start screeding one section while the next is poured. If the dividers will be temporary, you can use any straight length of lumber that's the same dimension as your perimeter forms. If the dividers are part of the design and will remain in the slab, use redwood, cedar, or pressure-treated lumber. Brush sealer on permanent forms and tape the top edge to keep wet concrete from staining the wood and to minimize scratches when you screed. Support the dividers with stakes driven 1 inch below the top so they will not be visible once the concrete is poured.
Getting the Slope Right
The surfaces of all outdoor hardscapes must slope 1/4 inch per foot to allow water to drain off freely. Build a slope gauge to get the right slant on your project. Put a 1/2-inch dowel or drill bit under one end of a 2-foot level taped to an 8-foot 2x4. This gauge will set the slope at 2 percent. The slope is correct when the bubble is centered in the vial.
What If the Project Requires Curved Forms?
If your patio or walk design incorporates curves, form the curve from 3-1/2-inch-wide strips of 1/4-inch hardboard or plywood. For strength, use two or three plies. Tack one end of one board temporarily with two 4d nails. Spring the material against the stakes on the curve, mark its length, and cut it. Cut the remaining pieces, then fasten them in place.