Nominal versus net
IBC Table 2304.11 (2021 and 2024) gives Type IV minimum sizes as width × depth in inches. Sawn lumber sizes are nominal. Glued-laminated (glulam) and structural composite lumber (SCL) sizes are net. Reading glulam sizes as nominal is a listed common error, and reversing the convention is just as wrong.
Members carrying floor loads
Columns carrying floor loads, or combined floor and roof loads:
- Sawn 8 × 8 nominal
- Glulam 6¾ × 8¼ in
- SCL 7 × 7½ in
Beams and girders carrying floor loads:
- Sawn 6 × 10 nominal
- Glulam 5 × 10½ in
- SCL 5¼ × 9½ in
A glulam girder under floor loads needs 5 × 10½ in. The 6 × 10 figure is the sawn size for the same member, 5¼ × 9½ in is the SCL size, and 6¾ × 8¼ in is the glulam column size.
Members carrying roof loads only
Columns carrying roof loads only:
- Sawn 6 × 8 nominal
- Glulam 5 × 8¼ in
- SCL 5¼ × 7½ in
Framed timber trusses, other roof framing and arches springing from wall tops: sawn 4 × 6, SCL 3½ × 5½ in. Upper half of arches carrying roof loads only: sawn 6 × 6.
An SCL roof-only column needs 5¼ × 7½ in. Do not grab 7 × 7½ in (the SCL floor column), 5 × 8¼ in (the glulam roof column) or 3½ × 5½ in (SCL roof framing).
The roof-only trap
The listed error is applying roof-only sizes to members that carry floor loads. A 6 × 8 sawn column holding up a mezzanine floor as well as the roof does not comply: combined floor and roof loads need 8 × 8 nominal. The 6 × 8 figure applies only when the column carries roof loads alone. The 4 × 6 figure is roof framing, not columns, and 6 × 10 is the floor beam and girder size, not a column minimum.