A Red Seal Carpenter sizing a beam on a residential main floor is not guessing at 2×10s versus 2×12s — the Ontario Building Code gives hard numbers in OBC 9.23.4.2 for built-up wood and 9.23.4.3 for steel, and the tables are laid out in a way that trips up framers who skim. Here is how to read them correctly, pick the right beam type, nail it off properly, land it on enough bearing, and recognize the point where you stop guessing and get an engineer involved.
The columns across the top of every Span Table 9.23.4.2.-H, -I, -J and Table 9.23.4.3. are indexed by supported joist length, not by joist depth and not by the clear span of the beam itself. Supported joist length is half the sum of the joist spans on both sides of the beam. A centre beam with joists spanning 4.8 m on one side and 3.6 m on the other is carrying (4.8 + 3.6) / 2 = 4.2 m of tributary floor. It does not matter whether those joists are 2×8s at 400 mm o.c. or 2×10s at 600 mm o.c. — the code has already wrapped the joist live load, dead load, and deflection limits into one number. That is why picking "2×10 beam because the joists are 2×10" is meaningless: a beam supporting 6 m of tributary floor carries nearly double the load of a beam supporting 3 m of tributary floor, even if the joists above look identical. Always calculate the supported length first, then walk into the table.
A built-up wood beam of 3-ply or 4-ply 2× lumber is cheap, field-assembled, and drilled and notched like any other stud — but Span Tables 9.23.4.2.-H through -J top out around 5 m of clear span for realistic supported lengths, and they drop fast once a second or third storey loads the beam. A steel W-shape under OBC 9.23.4.3 buys you longer spans in a shallower section: a W200×27 supporting a 3.6 m tributary under one storey spans 5.2 m, where a 3-ply SPF 2×10 in Table 9.23.4.2.-H covers roughly 2.8 m at the same tributary. Steel is the move when the client wants a clear basement without a teleposts down the middle, or when the beam has to tuck up into the joist bay to keep headroom. Glulam (typically 20f-E Douglas Fir per CSA O122) is the specified choice when the beam is exposed and has to look like architecture — a great room ridge, a timber-look kitchen island beam, a covered porch header. It costs two to three times a built-up, needs dry handling, and cannot be drilled freely, but you get a clean single member with factory camber and no nail lines showing.
A built-up beam only acts as a single member if the plies are fastened correctly. Going from 3-ply to 4-ply adds roughly one-third more capacity, and 4-ply to 5-ply another third — but only when the load is shared across all plies. OBC 9.23.3 and Table 9.23.3.4. govern the connections: for a 3-ply or 4-ply 2× beam, fasten from both sides with 3-inch (76 mm) common nails in two staggered rows, at roughly 300 mm on centre, with an additional nail at each end of each ply and within 100 mm of any butt joint. Butt joints in different plies must land over a support or well offset from each other — never stacked. Five-ply built-ups cannot be reliably nailed from two sides alone; use 1/2-inch through-bolts at the pattern specified by the engineer, typically 600 mm o.c. in two rows. Construction adhesive between plies is a belt-and-suspenders addition that dampens squeaks and helps plies act together, but it is never a substitute for the code nailing schedule.
Every span value in OBC 9.23.4.2 and 9.23.4.3 is a clear span between supports. The rough opening in the wall or the distance between teleposts is the clear span plus two bearing lengths. The code gets specific: for Table 9.23.4.2.-H (one floor, 3-ply built-up), bearing must be at least 76 mm when the supported length is 4.2 m or less, and 114 mm when the supported length exceeds 4.2 m. Table 9.23.4.2.-I (two floors supported) tightens those numbers, and Table 9.23.4.2.-J (three floors) commonly calls for 152 mm of bearing at the heavier tributary columns. Glulam beams require a minimum bearing length of 89 mm regardless of table. The reason is crushing: residential framing lumber has a perpendicular-to-grain compression allowable around 4.6 MPa, and a 3-ply beam on a 38 mm bearing is squeezing every pound of floor load through roughly 4,300 mm² of wood fibre. Short bearing leads to visible settlement, drywall cracks at the beam line, and doors that stop closing a year after occupancy. Post and pad sizing feeds off the same bearing length — a beam over a telepost still needs the full contact width or a steel bearing plate.
Designations read depth then mass per metre: W150×22 is 150 mm deep and weighs 22 kg/m. OBC 9.23.4.3.(2) requires a minimum of Grade 350W per CSA G40.21 — if the supplier slip says anything else, send it back. Sentence 9.23.4.3.(3) gives the only way to claim the table values: the joists must bear on the top flange at 600 mm or less across the entire length, the load must be transmitted through those joists, and 19 × 38 mm wood strips must be nailed to the flange on both sides to anchor the joist undersides. Without that lateral support the beam can roll or buckle laterally well before its flexural capacity is reached. The sill plate bolts to the top flange with power-actuated pins or 1/4-inch lag screws into nailer blocking — never a single row of nails straight down through the plate. For residential Part 9 houses, fire protection of the steel is not generally required, but check your local municipal amendments; some municipalities enforce 30 minutes of drywall cover on exposed steel in finished basements.
The Part 9 span tables assume uniform residential floor loading per OBC Table 4.1.5.3 — 1.9 kPa live plus standard dead — and nothing more. Step outside those assumptions and the tables no longer apply. Call a P.Eng. when: a bearing wall or column above drops a point load onto the beam; the beam cantilevers past a support; a concrete topping over 51 mm is planned (51 mm and under gets handled by the 0.8 multiplier in OBC 9.23.4.4); a roof load lands on the beam from a flush-framed cathedral or a truss girder (snow load is not in the floor tables); there is an opening or notch in the web; or the project is three or more storeys of wood frame or any steel in Part 4 territory. A stamped drawing costs a few hundred dollars and covers the builder, the client, and the inspector — a beam that deflects 25 mm at mid-span costs far more to rip out and replace once the drywall and flooring are in.