Lumber Formwork

Construction
Formwork

The general term lumber includes boards, dimension lumber, and timbers used in sheathing, studs, joists, stringers, walers, shores, sills, and bracing. Lumber formwork is easy to cut and adapt, which makes it useful for small jobs, irregular shapes, and details that do not suit a proprietary panel system.

However, a piece should not be selected only because it looks straight. Species or species group, grade, actual cross-section, moisture/service condition, span, spacing, load duration, bearing, connections, defects, and reuse condition all affect its capacity and stiffness.

Lumber and Timber Terminology

The original article used North American lumber terms. Under the current US Department of Commerce PS 20-25 standard, dimension lumber is nominal 2 in through less than 5 in thick and at least nominal 2 in wide. Timbers have a nominal least dimension of 5 in or more. These definitions and the size table below are US trade references, not universal metric sizes.

Term or mark Meaning Why it matters in formwork
Nominal size Trade designation rather than the finished cross-section Do not use it directly for section properties or clearances.
Dressed/net size Finished cross-section after surfacing Use the specified or measured actual dimensions in design checks.
S4S Surfaced on four sides More uniform dimensions and smoother handling; verify the grade mark and actual size.
S2E/S2S Surfaced on two edges or two sides, as stated by the supplier/grading rules The unsurfaced faces change actual dimensions and contact quality; do not guess from the abbreviation.
Rough Not dressed to standard finished dimensions Measure each required cross-section and check fit, bearing, and connection details.
S-DRY or KD For PS 20-25 lumber under nominal 5 in thickness, dry at manufacture/marking means no more than 19% moisture, subject to the applicable mark Dry trade size does not prove that in-service conditions will remain dry.
S-GRN Surfaced green; for lumber under nominal 5 in thickness, moisture exceeds 19% under PS 20-25 Dimensions and design adjustments differ; shrinkage, twisting, and joint opening require attention.
Grade mark Identifies grading agency, mill, species/species group, grade, and moisture condition as required Connects the piece to the design values used in the drawings.

Nominal and Dressed Lumber Sizes

Lumber is designated by nominal dimensions, which are larger than the minimum dressed sizes. The finished dimensions must be used in the formwork design. The following examples are taken from NIST PS 20-25 for US softwood dimension lumber. They are minimum dressed sizes at manufacture; project specifications, grading rules, moisture change, and actual measurement still apply.

US nominal size Minimum dressed dry size Dry metric equivalent Minimum dressed green size Green metric equivalent
2 x 4 1-1/2 x 3-1/2 in 38.1 x 88.9 mm 1-9/16 x 3-9/16 in 39.7 x 90.5 mm
2 x 6 1-1/2 x 5-1/2 in 38.1 x 139.7 mm 1-9/16 x 5-5/8 in 39.7 x 142.9 mm
2 x 8 1-1/2 x 7-1/4 in 38.1 x 184.2 mm 1-9/16 x 7-1/2 in 39.7 x 190.5 mm
2 x 10 1-1/2 x 9-1/4 in 38.1 x 235.0 mm 1-9/16 x 9-1/2 in 39.7 x 241.3 mm
2 x 12 1-1/2 x 11-1/4 in 38.1 x 285.8 mm 1-9/16 x 11-1/2 in 39.7 x 292.1 mm

Do not convert a nominal inch size into a rounded metric trade size and assume it is equivalent in another country. Order the material by the local grading and size standard named in the drawings.

Where Lumber Is Used in Formwork

Typical lumber wall formwork components and load path from sheathing through studs, walers, ties, braces, and supports
Typical lumber wall formwork components and load path. Member sizes and connections must follow the approved design.
Member Typical role Common checks
Sheathing or form face Contacts and shapes the concrete Panel grade/thickness, support spacing, deflection, joints, finish, coating, and edge condition
Stud or joist Supports the sheathing and transfers load to walers/stringers Bending, shear, deflection, bearing, spacing, orientation, and continuity
Waler or stringer Collects load from studs/joists and transfers it to ties or shores Bending, shear, deflection, splices, tie/shore bearing, and connections
Shore or post Carries vertical load to a sill or supporting structure Compression, buckling, plumbness, end bearing, bracing, extensions, and support below
Sill or sole plate Distributes shore or brace load at the base Bearing area, crushing, soil/floor capacity, drainage, and stability
Brace Maintains line, plumbness, and lateral stability Compression/tension, anchorage, connection eccentricity, wind, and erection sequence

For the broader system context, see types of formwork and formwork materials.

Selection and Design Checks

Formwork lumber members should match the species or species group, grade, size, and moisture condition stated in the falsework/formwork drawings. The current design values must come from the governing wood standard and supplement. Do not mix values and adjustment provisions from different editions.

  1. Confirm the grade stamp or inspection certificate and match it to the design species/group and grade.
  2. Measure the actual cross-section, particularly for rough, resawn, damaged, or repeatedly used pieces.
  3. Check the service condition. Wet concrete, rain, washing, and outdoor storage can make a dry-design assumption inappropriate.
  4. Use the approved formwork loads, spacing, spans, and connection details. Review loads on falsework and formwork.
  5. Check both strength and serviceability: bending, shear, compression/buckling, bearing, deflection, vibration, and connection capacity can govern.
  6. Follow the drawing for bracing, splices, blocking, shore extensions, and the stripping/reshoring sequence.
Why there is no span calculator here: a safe span depends on the actual species/group, grade, size, orientation, spacing, load combination, concrete pressure or slab load, duration, wet service, lateral support, bearing, connections, and deflection limit. A generic nominal-size calculator would hide these controlling inputs.

Inspection Before Use and Reuse

  • Reject or obtain an approved disposition for decay, insect damage, fire damage, severe splits, checks at connections, crushed ends, excessive warp, or missing/unreadable grade identification.
  • Check saw cuts, drilled holes, notches, nail damage, and previous concrete removal. Reuse does not mean unchanged capacity.
  • Clean without gouging the face or reducing the structural section. Apply release agent only as specified and keep it off reinforcement and construction joints.
  • Stack pieces off the ground with drainage and ventilation. Separate rejected or unverified lumber so it cannot return to service by mistake.
  • Inspect erected shoring immediately before, during, and after concrete placement as required by the applicable safety rules.

Many formwork failures begin with an unapproved substitution, weak support, missing bracing, damaged material, or a changed placing sequence rather than with the wood alone. The related causes of formwork failures page gives the wider prevention context.

References