A vaulted ceiling usually needs a structural ridge beam, because taking out the ceiling joists or rafter ties removes what stops the rafters from pushing the walls apart. A steel ridge beam is sized from its span between posts, the length of rafters on each side, the snow load for your location, the weight of the roof and any point loads, and in Ontario that sizing is done by an engineer because the building code's ridge beam table only covers built-up wood. The beam's cost follows its size, its connections and how hard it is to get into place.
Why vaulting a ceiling changes the structure
In a typical gable roof, rafters lean against each other at the peak and sit on the exterior walls at the bottom. Their weight, plus snow, tries to spread the walls outward. The ceiling joists are nailed to the rafters at the wall plate and act as ties that hold the bottoms together, so the ridge can be a light board.
Vault the ceiling and those ties come out. Without them, there are two ways to keep the roof up:
- Tie the rafters another way, with collar ties high up, steel tie rods, or a designed rafter connection to raised ceiling joists.
- Support the ridge, so each rafter is carried at both ends and no longer pushes outward. That's a structural ridge beam.
The 2024 Ontario Building Code puts this in Article 9.23.14.8. Except as provided in Sentence (4), rafters and joists must be supported at the ridge by a loadbearing wall extending from the ridge to suitable bearing, or by a ridge beam with at least 89 mm of bearing. Sentence (4) lets you skip ridge support only when the roof slope is 1 in 3 or steeper and the lower ends of the rafters are adequately tied to prevent outward movement. Where ceiling joists are raised above the base of the rafters, the connections have to be designed (Sentence (7)).
So a vaulted ceiling with no ties at the base of the rafters leads to a supported ridge, and that usually means a ridge beam.
What the code covers, and where steel comes in
Article 9.23.14.8 refers to Span Table 9.23.4.2.-L for ridge beam sizes. That table covers built-up lumber ridge beams and lintels supporting the roof and ceiling only, in three to five plies of 38 x 184, 38 x 235 or 38 x 286 mm, at snow loads from 1.0 to 3.0 kPa. The table can only be used where the supported rafter or joist length doesn't exceed 4.9 m and the roof carries no concentrated loads.
There is no Part 9 table for a steel ridge beam. The steel beam tables in Article 9.23.4.3 are written for beams supporting floors, or a roof and one floor through stud walls, as explained in our steel beam span tables guide. A ridge beam carrying rafters at the peak isn't either of those, so a steel ridge beam is designed under Part 4, which in practice means a structural engineer.
People often choose steel when:
- The span between supports is long, such as a great room with no interior posts.
- The snow load is high, so wood would be very deep.
- The rafters are long on one or both sides.
- The beam also picks up a point load, such as a post from a dormer or a hip.
- Headroom or appearance favours a shallow beam.
How a steel ridge beam is sized
Searches for "how to calculate steel beam size for roof" are common, but the calculation is the engineer's job, and getting it wrong is dangerous. What you can do is understand the inputs, so you can give the engineer what they need and read the result:
- Span: the clear distance between the posts or walls that hold the beam up.
- Supported length: for ridge beams, half the sum of the rafter spans on both sides (that's how Table 9.23.4.2.-L defines it for wood).
- Snow load for your municipality, from MMAH Supplementary Standard SB-1, plus drift where a higher roof sits next to a lower one.
- Dead load: roofing (asphalt, steel, tile), sheathing, rafters, insulation and the vaulted ceiling finish. Heavy roofing materials reduce allowable spans.
- Point loads from dormers, hips, valleys, chimneys or rooftop equipment.
- Deflection limits, which often govern ridge beams over finished vaulted ceilings, because a sagging ridge cracks drywall and shows on the roofline.
- Lateral support: how the rafters brace the top of the beam.
The engineer then picks a section, often a W-shape, and designs the end posts and the path for the load down to the foundation. For how steel and engineered wood compare on depth for the same span, see steel beam vs LVL strength and depth.
Posts, load path and connections
A ridge beam puts roughly half the roof load onto two points: the posts at each end. That load has to travel down through the gable walls or interior posts, through every floor below, to footings that can take it. In an existing house that often means:
- New steel or built-up posts in the gable walls or inside the house
- Blocking or posts through the floor framing below each post
- New footings or pads in the basement under each post
The rafters also need a connection to the beam. Common approaches are a wood nailer bolted to the top flange with rafters bearing on it, wood plates bolted into the web with rafter hangers, or welded seats and side plates. The method changes the beam's fabrication: hole patterns, welded plates, and sometimes a slope cut to suit the roof pitch.
If you're opening a bearing wall below to make the vault work, that's a second structural change. Our load-bearing wall removal cost guide covers that work.
What drives the cost of a steel ridge beam
- Section and length. Steel is priced by weight, so a heavier or longer beam costs more.
- Connections: nailers and bolt holes, rafter seats, end plates and post caps.
- Posts and footings to carry the beam's ends to the foundation.
- Splices: if the beam can't be carried into the attic or through the house in one piece, the engineer designs a splice and the beam comes in sections.
- Lifting and installation. Getting a beam into a roof often needs a boom truck or crane, partial roof removal or a careful route through the house, plus temporary support for the roof.
- Finish: shop primer if it will be hidden, or a paint finish if it stays exposed in the vaulted room.
- Engineering, permit, roofing work and finishing, which on a vaulting project usually exceed the cost of the steel.
Converting a ceiling to a vault changes the roof structure, so it needs a building permit in Ontario and drawings from a designer or engineer.
Ordering a steel ridge beam
Send us the engineer's drawing with the beam size, length, roof pitch, connection details and post details, plus notes on how the beam can get into position. We'll fabricate the beam with its plates and holes and the matching posts, mark the pieces and deliver to site.
Send your drawings for a quote, or contact us if you're still working out the design. See our steel beams and posts.
Frequently asked questions
What size steel beam do I need for a ridge?
It depends on the beam's span between posts, the rafter lengths on each side, the snow load for your location, the roof's weight and any point loads. The Ontario Building Code's ridge beam table covers built-up wood beams only, so a steel ridge beam is sized by an engineer.
Do I need a ridge beam for a vaulted ceiling?
Usually. Removing ceiling joists or rafter ties to vault a ceiling lets the rafters push the walls outward unless the ridge is structurally supported. Under the 2024 OBC, an unsupported ridge is only permitted on roofs of 1 in 3 or steeper where the lower ends of the rafters are tied.
What is the difference between a ridge board and a ridge beam?
A ridge board is a non-structural member the rafters are nailed to at the peak; the rafter ties or ceiling joists keep the roof from spreading. A ridge beam carries half of each rafter's load to posts at its ends, so the rafters don't need ties.
Is a steel ridge beam better than LVL or glulam?
Steel carries more load in a shallower depth and suits long spans or heavy snow areas. Engineered wood is lighter to handle and easier to fasten rafters to. The engineer picks based on span, depth, appearance and how the beam gets into the attic.
