Slab and Pad Forms

Written for Canada. Plans are metric, but many crews and suppliers work in feet and inches, so this site gives both: millimetres and metres, with feet and inches beside them. Codes are the National Building Code of Canada 2020 (NBC 2020) Part 9 as your province adopts it. A number marked Confirm has not been checked against every province, so check it with your own authority before you act on it.

Reviewed by a working tradesman with 17+ years in the field.

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The base, edge forms, steel and joints that set a slab's level, thickness and cracking.

The big idea

Slab forms hold the edges and the level of the concrete. A slab shows every mistake: a 10 mm (3/8 in) high spot, a wrong slope, a bar sitting on the dirt. Get the base, the edges and the steel right before the truck arrives.

Types of slabs and pads

• Slabs on grade: basement floors, garage floors, and crawl-space or utility slabs.
• Structural slabs: engineered, with rebar mats or beams. Built to the drawings.
• Exterior slabs: sidewalks, patios and driveways. These see frost, salt and weather.
• Equipment pads: pads for heat pumps, generators and pumps, set level.
• Thickened-edge slabs: slab edges thickened to act as footings.

Under the model code (NBC Part 9, Section 9.16, read in the Vancouver Building Bylaw 2014):

• Concrete slabs are at least 75 mm (3 in) thick, not counting any topping (9.16.4.3).
• A drainage layer is required below floors-on-ground, except below slabs in garages, carports or accessory buildings (9.16.2.1). The text I read gives no thickness for it, so take it from the drawings and your province's code.
• Dampproofing and waterproofing follow Section 9.13 (9.16.1.4).
• Where floor drains are required, the floor must slope so no water can accumulate (9.16.3.3). No slope value is given.

Base preparation

A slab is only as good as what is under it.

• Strip topsoil, roots and organic material.
• Compact the subgrade. Replace soft spots with compacted granular fill.
• Place and compact the granular base to the thickness in the drawings.
• Fine-grade the base so slab thickness is even.
• Place the vapour barrier where required, lapped and taped at joints.
• Place insulation where the drawings call for it (heated, frost-protected or energy-code slabs).

Forming materials

• Dimensional lumber: the form board must be at least as tall as the slab is thick. A 38 x 89 mm (2x4) is only 89 mm (3 1/2 in) tall, so for a 100 mm (4 in) slab use a 38 x 140 mm (2x6).
• Stakes (wood or steel), tight to the form and driven below the top.
• Form pins and clamps for commercial work.
• String line and laser for elevation.
• Form release (optional) so boards come off cleanly.

Building slab and pad forms

1. Snap lines. Mark the edges and the joint layout.
2. Set the form boards. To the line and the elevation.
3. Stake them. Tight, so they will not move under screeding.
4. Check the elevation. Laser or level along the full perimeter.
5. Set the reinforcement. Mesh or rebar on chairs, at the height the drawings specify.
6. Install isolation joints. At walls, columns and anything fixed.
7. Final walk. Thickness, slope, reinforcement, joints and bracing.

Reinforcement

Welded wire mesh or rebar, as the drawings specify. Many house slabs on grade are unreinforced, so do not add or leave out steel on your own.

• Rebar mats for structural slabs
• Extra bars at thickened edges and around openings
• Chairs, not bricks. Steel on the ground does nothing, and mesh walked into the base does nothing either.

Control, isolation and expansion joints

Concrete shrinks as it cures. Joints decide where it cracks.

• Control joints: a weakened line that makes cracking happen where you want it. The Portland Cement Association guidance says maximum spacing is 24 to 36 times the slab thickness, with panels square or nearly so (length no more than 1.5 times width), and the groove at least 1/4 of the slab thickness and not less than 25 mm (1 in) deep.
• Isolation joints: a separation around columns, walls and anything that moves differently from the slab, using a premoulded filler such as asphalt-impregnated fibre or compressible foam.
• Expansion joints: used between large exterior slabs and fixed structures, as drawn.

Worked example. A 100 mm (4 in) slab: 24 × 0.1 = 2.4 m and 36 × 0.1 = 3.6 m (8 to 12 ft). On a 6 m × 6 m (20 ft × 20 ft) slab, cut joints at 3 m (10 ft) both ways to make four 3 m × 3 m panels. The groove is at least 100 ÷ 4 = 25 mm (1 in) deep.

Saw-cut timing depends on the method: early-entry dry cutting 1 to 4 hours after finishing, conventional wet sawing 4 to 12 hours after, per the same guidance. Keep joints straight, clean and continuous.

Elevation and drainage

• Garage slabs are sloped toward the overhead door or a floor drain.
• Exterior slabs are sloped away from the building.
• Basement slabs are sloped toward floor drains where they are installed.
• Equipment pads must be level.
• Check elevation at the corners, mid-spans and edges.

Your drawings give the slope. As a guide, 1% is 10 mm per metre, about 1/8 in per foot. Example: a 6 m (20 ft) garage slab sloping 1% to the door is 60 mm (about 2 3/8 in) lower at the door end.

Pouring considerations

A 6 m × 6 m slab, 100 mm thick: 6 × 6 × 0.1 = 3.6 m³, about 4.7 yd³, before waste.

• Place evenly so forms are not pushed out of line.
• Screed to the form tops to hold the level.
• Consolidate structural slabs with a vibrator or by rodding.
• Watch the forms for movement during the pour.
• Keep mesh or rebar at its set height.
• Do not place concrete on frozen ground, and do not add water to the mix on site.

Common mistakes

• Wrong elevation or slope
• Weak staking
• Mesh or rebar lying on the ground
• Missing isolation joints
• An uneven base, giving a slab of uneven thickness
• Poor drainage planning
• Pouring onto a wet, soft or frozen base