Building Forms for a Patio or Walk

This project will show you how to build strong, straight forms for a patio or walkway.

Intro

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.

Prestart Checklist

Time
About 4-6 hours to build forms for a 10x10-foot slab with curves

Tools
Sledgehammer, circular saw, hammer, carpenter's level, mason's line, cordless drill

Skills
Measuring, cutting, leveling, driving stakes, fastening

Prep
Lay out and excavate the site.

Materials
Deck screws or nails, bender board, expansion strip, construction adhesive, 2x4 and 2x6 lumber

Step 1

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

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 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

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.

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.


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