Concrete Formwork Basics

What forms do

Concrete is fluid when it arrives and keeps its shape only if something holds it. Forms provide that support until the concrete has hardened. They also set the top surface level for screeding and the finished edge profile. A form that bows, moves or leaks changes the thickness of the slab and therefore the volume you need, which is why cheap forms often cost more in concrete.

Lumber sizes

A “4 in” slab does not fit a 2×4, since a 2×4 is only 3.5 in tall. Use a 2×6 and set it so the top is at the finished surface, and the slab edge is 4 in below. Straight boards are best. Check each for bow before use. Plywood, hardboard or flexible bender board form curves.

Staking and bracing

• Stake on the outside of the form every 2 to 3 ft, and at each side of a joint between boards.
• Drive stakes below the top of the form so that screeding is not obstructed, and fasten from the outside with screws.
• Brace corners and long runs with diagonal braces that bear against stakes or the ground.
• Use screws rather than nails when you want to take the forms apart easily.
• Backfill the outside with soil to keep the concrete from leaking under the boards.
• Check the form for position and level after bracing, because driving stakes moves boards.

Setting level and slope

Stretch a string line between stakes at the planned finished height and set the board tops to it. Slabs outside should slope away from buildings, commonly about 1/8 to 1/4 in per foot (1 to 2%). Set the high side first and measure down for the low side. Check with a long level or a laser. Water that pools on a slab freezes into ice in winter and speeds up damage.

Curves

To make a curve, use thin hardboard, plywood strips or flexible bender board. Kerf the back of a board with a series of saw cuts, or soak it in water so that it bends. Stake closely, every foot or less, to hold the curve. Curved edges mean the area is no longer rectangular, so use the circular slab calculator or break the shape into simple parts for the volume.

Release agent and removal

Coat the form faces with a release agent, such as a commercial form oil, so that the concrete does not stick. Do not let it run onto the base or the rebar. Remove forms after a day or two, once the edges have hardened, using a pry bar carefully so that the slab edge is not chipped. Backfill and compact the soil along the edge.

Forms for walls and footings

Walls are much harder than slabs, since fresh concrete behaves like a fluid and presses on the form with a pressure that increases with depth. At a depth of 4 ft, a fluid pressure of 150 lb/ft³ is about 600 lb per square foot at the bottom. Wall forms use plywood panels, studs, walers and ties, and need an engineered design if the wall is tall. A poured wall of 20 × 4 ft at 8 in thick needs 2.17 yd³ in the form, according to the wall calculator.

Footings in trenches can often use the sides of the trench as forms, provided the soil is firm and the trench is cut cleanly; this tends to use more concrete than the drawing. For tube forms, use the Sonotube calculator.

Mistakes to avoid

• Forms that are not braced and bow when loaded.
• Setting the tops without a string line or level.
• No slope for drainage.
• Gaps at corners that let the paste out.
• Using warped lumber.
• Removing forms too soon.

Form materials compared

When to remove forms

In cold weather leave forms on longer to hold in heat. Remove them carefully to protect the edges.

Checklist before the truck arrives

• Forms are straight, level to plan and braced.
• Release agent is applied.
• Stakes are below the top of the form.
• Corners are tight and gaps are sealed.
• The base is compacted and damp but not muddy.
• Reinforcement is in place on chairs.
• Tools, helpers and a curing plan are ready.

Example: forms for a 12 × 12 ft patio

A 12 × 12 ft patio needs 48 ft of form boards (four sides), so five 10 ft 2×6 boards cut to length, and about 24 stakes at one every 2 ft. The slab takes 1.96 yd³ with 10% waste. Stake the corners first, tie the boards with screws, then add the other stakes while checking the level. Drive stakes slightly below the top of the form, brace any bows and check the diagonals to make sure the patio is square: both should be equal.

Frequently asked questions