Foundations & Slab-on-Grade Forms

Foundations & Slab-on-Grade Forms

Quick Answer: Footings transfer load into soil; stem walls rise from footings; slabs-on-grade are ground-supported floors formed at the perimeter. Build forms from form ply, studs, stakes, and kickers, square and level to plan elevation, apply release agent, and place keyways/dowels where pours must lock together. Under slabs use a continuous vapor retarder; separate slabs from walls with isolation joints and layout control joints for shrinkage.

Module 27307 (Foundations and Slab-on-Grade) sits in the Foundations and Formwork domain (about 10 items on Commercial Carpenter AEN27CCARP04). Journey-level carpenters are expected to form continuous and pad footings, stem walls, and slab edges; control line and grade; and understand how under-slab and joint details protect the finished floor. Placement, rebar, and finishing of concrete itself are covered in the Concrete chapter—here the focus is building the mold correctly and safely.

Foundation System Parts

Continuous (strip) footings run under load-bearing walls. Pad (isolated) footings support columns or posts. Depth and width come from the structural drawings and geotechnical criteria—not from habit. Frost-depth requirements in cold climates push footings below the frost line; warm climates and slabs-on-grade may use different frost and soil-treatment details. The carpenter’s job is to form exactly what the plans show: width, thickness, steps, and elevation.

Stem walls sit on the footing and create a crawl space, basement, or raised grade for the first floor. They may be formed with plywood or modular panels, braced, and poured monolithically with the footing or as a second pour with a keyway and dowels at the cold joint. Monolithic pours need forms that hold both the footing step and the wall height in one setup; two-pour systems need clean construction joints and correctly placed projecting steel.

Layout: Line, Square, and Elevation

Foundation work fails inspections and creates crooked buildings when layout is sloppy. Sequence:

1. Establish building lines from control points, hub stakes, or batter boards set to the site plan.
2. Square corners with the 3-4-5 method (or multiples: 6-8-10, 9-12-15) or by checking equal diagonals on rectangles.
3. Set elevation with a builder’s level, laser, or total station from a known benchmark. Mark form tops or pour lines so the finished concrete hits plan grade.
4. Check footing width and thickness against the section detail before closing forms.
5. Verify rebar clearance (cover) from form faces and soil—forms that are too tight crush the cover the engineer designed.

Batter boards hold string lines for excavation and form faces; they sit outside the work so digging does not destroy the reference. Once forms are up, re-check strings or offsets so both faces (or the outside face and a thickness mark) match the plan.

Elevation control is non-negotiable. A footing poured low forces extra stem height or shims; poured high eats into floor-to-floor clearances. Mark the top of footing (TOF) and top of wall (TOW) on stakes or form faces. Commercial jobs often require surveyor sign-off before pour—know when your crew must hold for that hold point.

Form Materials and Hardware (Foundation Scale)

Foundation forms are usually simpler than tall wall or slab-shoring systems, but the same vocabulary applies:

Footing forms may be 2× boards, plywood, or proprietary plastic/metal edge forms. Stake them firmly; loose boards blow out under fluid concrete. For stem walls, use studded plywood, modular aluminum/steel panels, or job-built forms with walers (horizontal stiffeners) and form ties that hold the two faces together through the pour. Ties are detailed more fully with vertical formwork; at foundation height, the principle is the same—ties take the lateral pressure of wet concrete so the wall does not balloon.

Release agent goes on clean, dry form faces before steel is congested in the way, and never in quantities that puddle into the pour and stain or weaken the surface. Follow the product label; some agents are restricted on surfaces that will receive coatings or architectural finishes.

Keyways and Dowels

When a footing is poured separately from a stem wall, the joint must transfer horizontal shear and keep the wall from sliding. Common details:

• Keyway: A longitudinal groove formed in the top of the footing (wood key, beveled strip, or proprietary former). The wall pour fills the key and creates a mechanical interlock.
• Dowels: Vertical or hooked bars left projecting from the footing into the wall zone, matching the wall reinforcement layout. Bars must be the correct size, spacing, embedment, and cover—and protected from mud and bending damage before the second pour.

Exam scenario: A footing is poured without the key strip or with dowels bent flat “to keep them out of the way.” The wall has no reliable shear key or continuity—reject that practice. Always install keys and dowels as detailed before the first pour sets up past the point where they can be placed correctly.

Slab-on-Grade Edge Forms and Under-Slab Work

Slab-on-grade floors rest on compacted subgrade and usually a granular subbase. The carpenter’s formwork role is typically the edge form: boards or metal forms that set slab thickness and perimeter line. Thickened edges, grade beams, and integral footings need bulkheads and stepped forms matching the section.

Vapor retarder (vapor barrier): Polyethylene or specified membrane is placed under the slab (on top of the prepared base, often under or over a sand blotter per spec—follow the drawings). Lap seams the specified distance, tape or seal as required, and repair tears. The membrane reduces moisture vapor rising into the slab and floor finishes. Penetrations (pipes, columns) need careful sealing. Do not leave large gaps “because the concrete will seal it.”

Isolation joints (expansion/isolation): Separate the slab from walls, columns, and fixed objects so the slab can shrink and move slightly without cracking at those hard points. Isolation is often a strip of compressible material (fiber, foam, or asphalt-impregnated board) against the wall or around columns. This is different from control (contraction) joints, which are planned weakened planes in the slab to manage shrinkage cracks in a grid pattern.

Control joint layout purpose: Concrete shrinks as it cures and dries. Without joints, cracks form randomly. Control joints (tooled, saw-cut, or formed) create preferred crack locations at regular spacing so the floor remains serviceable and look acceptable. Spacing rules come from the design/spec (often related to slab thickness and mix)—carpenters may form bulkheads or mark saw-cut lines; they do not invent joint spacing from memory when the engineer or ACI-style guidance in the project documents governs.

Construction joints are stopping points for a day’s pour; they need clean vertical bulkheads and often dowels or keys for load transfer across the joint when the next pour continues.

Pour-Day Readiness and Safe Stripping

Before concrete arrives:

• Forms on line, plumb/level, braced, and at elevation
• Release agent applied; debris cleaned out
• Rebar and embeds supported with correct cover (chairs, dobies)
• Keyways, dowels, sleeves, and anchor bolts located
• Access for trucks and pump; washout area designated
• Crew clear of crush zones if forms shift

Stripping: Foundation side forms may come off after concrete can support itself without edge damage—always follow project specifications, engineer direction, or approved standards. Do not memorize a single “strip after X hours” for every climate and mix. Cold weather slows strength gain; hot weather and high-early mixes gain faster. Premature stripping chips edges, spalls faces, and can collapse unsupported walls. Leave braces until the structure or temporary shoring can take the load.

Safety Notes for Foundation Formwork

• Struck-by / crush: Forms and stakes under pressure can kick out; stay out of the line of fire during placement.
• Excavation: Footing trenches are excavations—cave-in protection and access rules apply; do not work in unprotected trenches.
• Rebar impalement: Cap unprotected vertical bars.
• Manual handling: Form panels and wet concrete loads are heavy—use team lifts and mechanical help.

Master foundation vocabulary, square and elevation layout, edge forms, vapor retarder placement, joint types, and keyway/dowel continuity, and you cover the bulk of 27307 items that pair with concrete placement knowledge from Module 27304/27305.