How to Form a Concrete Slab: Layout, Stakes, Slope

How to form a concrete slab comes down to four things: laying out square corners, setting form boards at the right height and slope, staking and bracing them so they cannot move, and checking the depth before the truck arrives. Most residential slabs use 2x4 forms for 3.5-inch slabs and 2x6 forms for 4 to 5.5 inch slabs, held by wood or steel stakes every 2 to 3 feet. Good forms make screeding easier and keep your concrete order close to the math.

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Materials

2x4 or 2x6 lumber, straight and free of big knots.
Wood or steel stakes, 12 to 24 inches long.
Duplex (double-head) nails or screws.
String line, line level or laser, tape measure.
Form release oil, optional.
Hardboard or bender board for curves.

Step 1: Lay out the slab

Drive temporary stakes at the corners and run string lines. Check square using the 3-4-5 method: measure 3 feet along one side and 4 feet along the other; the diagonal should be 5 feet. For larger slabs, use 6-8-10 or measure both diagonals of the rectangle; they should match.

Step 2: Excavate and base

Dig deep enough for the base and slab. A 4-inch slab on 4 inches of gravel needs about 8 inches below finished grade, minus any height above grade. Compact the base. See gravel base under a concrete slab.

Step 3: Set the forms

Set the first board along the string at the finished height.
Drive stakes on the outside every 2 to 3 feet and at each joint.
Fasten stakes to boards with duplex nails or screws from the outside.
Cut stakes flush or below the top of the form so they do not block the screed.

Step 4: Build in slope

Patios and walks need drainage. A common slope is 1/8 to 1/4 inch per foot away from the house. On a 12-foot patio, drop the far form 1.5 to 3 inches. Use a string line and level or a laser. See how to check slope with a level.

Step 5: Brace and check

Kick braces at corners and every few feet on tall forms resist the pressure of wet concrete. Check:

Form tops are straight with no sags.
Depth from string to base is consistent.
Diagonals still match.
Isolation joint material is in place against the house.

Curves

Use 1/4-inch hardboard, plywood strips, or plastic bender board for curves, with stakes every 1 to 2 feet.

Removing forms

Strip forms after a day or two once edges are firm, being careful not to chip them. Lightly oiled forms release more easily.

Bottom line

To form a slab, square the layout, excavate and compact the base, set straight 2x4 or 2x6 boards at height with slope for drainage, stake every 2 to 3 feet, brace corners, and check depth and square before the pour.

Questions people ask

What size boards do I use to form a 4-inch slab?

Many use 2x6 forms set on the base, or 2x4s for a 3.5-inch slab.

How far apart should form stakes be?

About every 2 to 3 feet, plus at every joint and corner.

How do I make sure forms are square?

Use the 3-4-5 method or check that both diagonals are equal.

How much slope should a patio have?

About 1/8 to 1/4 inch per foot away from the house.

When can I remove concrete forms?

Usually after 24 to 48 hours, once edges are firm.

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