Beam Span Calculator

Sizing Beams the Way the OBC Actually Reads Them

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.

Code data verified 2026-07-06 by a Red Seal Carpenter (Ontario CofQ) against the Ontario Building Code 2024 (amended January 16, 2025). Part of the ToolFluency Ontario Building Code toolkit.

What is supported joist length in the OBC beam tables?

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. According to note (3) to Tables 9.23.4.2.-H/-I/-J, supported length means 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. Size the joists first with the OBC framing span calculator, calculate the supported length, then walk into the beam table.

Built-up 2× versus steel versus glulam — which beam should I use?

A built-up wood beam of 3-ply or 4-ply 2× lumber is cheap, field-assembled, and handled like any other framing lumber — but Span Tables 9.23.4.2.-H through -J top out around 5–6 m of clear span at light tributaries, 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: per Table 9.23.4.3, a W200×27 supporting a 3.6 m tributary under one storey spans 6.6 m, where a 3-ply SPF No.1/No.2 2×10 in Table 9.23.4.2.-H covers 3.22 m at the same tributary. Steel is the move when the client wants a clear basement without teleposts down the middle, or when the beam has to tuck up into the joist bay to keep headroom. Glulam (20f-E, conforming to CAN/CSA-O122 and CSA O177) is the specified choice when the beam is exposed and has to look like architecture — a great room ridge, 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. One coverage note: the OBC lists a fourth lumber group, Northern Species, with lower allowable spans than SPF — this calculator does not model it, so if your yard stocks Northern Species-stamped lumber, read its column in the code tables directly.

How are built-up beam plies fastened together in Ontario?

A built-up beam only acts as a single member if the plies are fastened correctly, and the OBC is specific. According to OBC 9.23.8.3.(1), the individual members must be 38 mm or thicker and installed on edge. Per 9.23.8.3.(7), plies are nailed together with a double row of nails at least 89 mm (3½″) long, spaced not more than 450 mm apart in each row, with the end nails located 100–150 mm from the end of each piece. Where that nailing is not used, 9.23.8.3.(8) requires 12.7 mm (½″) bolts with washers at not more than 1.2 m on centre, end bolts within 600 mm of the member ends. Butt joints in individual plies must occur over a support (9.23.8.3.(2)); on a beam continuous over more than one span they may instead land at or within 150 mm of the end quarter points, never stacked in adjacent plies at the same point (9.23.8.3.(3)–(6)). Construction adhesive between plies dampens squeaks, but it is never a substitute for the code fastening schedule. Full nailing tables live in the OBC nail & fastener schedule.

How much bearing does a beam need in Ontario?

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. According to OBC 9.23.8.1.(1), beams need even, level end bearing not less than 89 mm long, except as varied by the notes to Span Tables 9.23.4.2.-H to -K — and those notes get specific: Table -H (one floor) allows 76 mm, rising to 114 mm for 3-ply beams with supported lengths over 4.2 m; Table -I (two floors) requires 114 mm for all 3-ply beams and for 4/5-ply beams over 3 m supported; Table -J (three floors) requires 152 mm for 3-ply beams over 4.2 m and 114 mm for everything else. Glulam (Table -K) needs 89 mm minimum. Short bearing crushes wood fibre perpendicular to the grain — the symptoms are 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 number: a beam over a telepost still needs the full contact length or a steel bearing plate.

What are the OBC rules for steel beams in a house?

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, and it takes all three conditions together: (a) the joists bear on the top flange at 600 mm or less over its entire length, (b) the load is transmitted through those joists, and (c) 19 × 38 mm wood strips in contact with the top flange are nailed on both sides of the beam to the bottom of the supported joists. Miss any one and the beam is not "laterally supported" — it can roll or buckle sideways well before its flexural capacity is reached, and the table spans no longer apply. For residential Part 9 houses, fire protection of the steel is not generally required, but check your local municipal amendments; some municipalities want drywall cover on exposed steel in finished basements.

Worked example: sizing a basement centre beam

Bungalow, 8.4 m wide, centre beam down the middle: joists span 4.2 m on each side, so supported length = (4.2 + 4.2) / 2 = 4.2 m, one floor supported. Table 9.23.4.2.-H says a 3-ply 2×10 (38×235 mm) SPF No.1/No.2 built-up spans 2.98 m (about 9′ 9″) clear. At 4.2 m supported (not over 4.2), note (6) sets bearing at 76 mm each end, so each telepost-to-telepost rough span is 2.98 + 2 × 0.076 = 3.13 m. Want fewer posts? The same table gives a 5-ply 2×12 SPF No.1/No.2 4.46 m, or jump to a W200×27 steel beam at 6.3 m per Table 9.23.4.3 and clear the room in one shot.

When do I need an engineer instead of a span table?

According to OBC 9.23.4.1.(1), the beam span tables apply only where the floors serve residential areas as described in Table 4.1.5.3 — a 1.9 kPa uniform live load — 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 span multiplier in note (2) and OBC 9.23.4.4.(3)); a roof load lands on the beam from a flush-framed cathedral or a truss girder (snow load is not in the floor tables — for lintels carrying roof, use the OBC header & lintel sizing tool); the supported length exceeds 6.0 m, past the table's last column; or there is an opening or notch cut into the member. 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.

Running crews on jobs like this? ToolFluency's Ontario contractor suite handles timesheets, job costing, and invoicing for the same builds you're sizing beams for — and the permit checker tells you whether the beam swap needs a building permit before you open the wall.