The size of your ridge beam depends on three things: how far it spans between supports, how much roof load it carries, and what material you use. A typical residential ridge beam ranges from a doubled 2×8 for short spans around 8 feet up to engineered lumber like triple-ply LVL or glulam beams for spans of 20 feet or more. Getting the size right matters because an undersized ridge beam will sag over time, cracking finishes and compromising your roof structure.
Ridge Beam vs. Ridge Board
Before sizing anything, make sure you actually need a ridge beam. Many roofs use a ridge board instead, which is just a non-structural plank at the peak where opposing rafters meet. In a conventional roof with ceiling joists or collar ties connecting the rafters, the triangular shape created by the rafters and joists is self-supporting. The rafters push against each other at the top, and the ceiling joists prevent them from spreading apart at the bottom. The ridge board is simply a nailing surface.
You need a structural ridge beam when that triangle is broken. The most common scenario is a cathedral ceiling or vaulted space where ceiling joists are removed. Without horizontal ties holding the rafter feet together, nothing stops the ridge from dropping under the roof’s weight. The ridge beam takes over, carrying the vertical load from the rafters and transferring it down to posts or bearing walls at each end. Building code also requires a structural ridge beam (not just a board) when the roof pitch is less than 3-in-12.
How Ridge Beam Loads Are Calculated
A ridge beam carries roughly half the total roof load on each side. The key concept is “tributary width,” which is the horizontal distance from the ridge to the midpoint between the ridge and the exterior wall. For a simple gable roof, the tributary width is half the horizontal span of one rafter. If your building is 24 feet wide and the ridge runs down the center, each rafter spans 12 feet horizontally, and the tributary width for the ridge beam is 6 feet.
To find the load per linear foot on the beam, multiply the tributary width by the combined roof load (dead load plus live or snow load). Dead load for a typical residential roof is around 10 to 15 pounds per square foot (psf), covering the weight of sheathing, shingles, and framing. Live load or ground snow load varies by location, commonly 20 to 50 psf in most of the U.S. but reaching 70 psf or more in heavy snow regions.
So for a 24-foot-wide house with a 30 psf snow load and 10 psf dead load, the total roof load is 40 psf. Multiply that by the 6-foot tributary width and your ridge beam carries 240 pounds per linear foot. That number, combined with the beam’s unsupported span, determines the required size.
Common Beam Sizes by Span
The material you choose affects the size dramatically. Solid sawn lumber (like doubled or tripled 2x stock) works for shorter spans but runs out of capacity quickly. Engineered lumber, specifically laminated veneer lumber (LVL) and glulam, handles longer spans in a smaller cross-section because it’s stronger and more consistent than natural wood.
For a moderate roof load (30 psf snow, 10 psf dead) on a 24-foot-wide building, here’s a rough sense of what’s typical:
- 8 to 10 foot span: Doubled 2×10 or 2×12 in Douglas Fir No. 1 or better. This covers small additions, dormers, or short runs between posts.
- 12 to 16 foot span: A 3.5×11.25-inch LVL (single or doubled depending on load), or a 3-1/8 x 12 glulam beam. This is a common range for single-car garages or modest cathedral ceilings.
- 18 to 24 foot span: Triple-ply 1.75-inch LVL (giving a 5.25-inch width) in depths of 14 to 18 inches, or a 5-1/8 x 13.5 to 16.5-inch glulam. At these spans, engineered lumber is essentially required.
These are ballpark figures. A heavier snow load pushes you to a deeper beam. A steeper roof pitch reduces the tributary width slightly and can allow a smaller beam. The Southern Forest Products Association publishes sizing tables for Southern Pine and glulam ridge beams organized by snow load (20, 30, 40, 50, and 70 psf) and span length, which are useful starting points before an engineer finalizes your design.
Why Deflection Matters as Much as Strength
A beam can be strong enough to hold the load and still be too small. The other limit is deflection: how much the beam bends under load. Code sets the maximum allowable deflection at L/240 for total load and L/360 for live load, where L is the beam span in inches. For a 16-foot (192-inch) beam, that means no more than 0.8 inches of sag under full load and no more than 0.53 inches under live load alone.
In practice, deflection often controls the design more than raw strength, especially on longer spans. A beam that’s technically strong enough to carry 240 pounds per foot might deflect enough to crack drywall on a cathedral ceiling or create a visible dip at the ridge. If your ceiling is finished with drywall or plaster, you’ll want to stay well within those limits. Some builders target L/360 for total load in cathedral ceiling applications for this reason.
Supporting the Ridge Beam
A ridge beam is only as good as what holds it up. The beam must transfer its load to posts at each end (and at intermediate points for longer runs), and those posts must bear directly on a foundation, a bearing wall, or a structural column that reaches the foundation. You can’t just rest a ridge beam on a non-bearing partition wall.
Post sizes depend on the total load they carry and their height. For most residential ridge beams, 4×6 or 6×6 posts are standard. A 4×4 post can buckle under heavy loads or at taller heights, so it’s rarely appropriate for ridge beam support. Posts should connect to the beam and to the bearing point below with metal connectors or hardware to prevent lateral movement. If a post sits on a double top plate of a bearing wall, blocking between the studs below is necessary to transfer the load straight down through the wall framing.
Intermediate posts reduce the effective span of the beam, which can significantly reduce the required beam size. If you have a 24-foot ridge line, adding a post at the midpoint turns it into two 12-foot spans, potentially cutting the beam depth by several inches.
Connecting Rafters to the Beam
Rafters must hang from or bear on the ridge beam with adequate hardware. Unlike a ridge board where rafters are simply toenailed, a structural ridge beam needs positive connections that prevent the rafter from pulling away or sliding. The most common approach uses engineered metal hangers sized to the rafter dimensions and the load.
For steel-framed roofs, code specifies clip angles (minimum 2×2 inches) fastened with No. 10 screws, with the number of screws increasing based on building width and snow load. A 36-foot-wide building in a 50 psf snow zone, for example, requires 5 screws per leg of each clip angle. Wood-framed roofs typically use manufactured joist hangers rated for the specific rafter size and load, which is simpler and more common in residential work.
Getting Your Beam Sized Correctly
Online beam calculators and span tables can get you in the right neighborhood, but a structural ridge beam typically requires engineering review. Most building departments want to see a stamped engineer’s calculation or a reference to an approved span table before issuing a permit for a structural ridge. This is partly because the variables (snow load, roof pitch, rafter spacing, beam material, support conditions) interact in ways that a single table can’t fully capture.
When you talk to an engineer or pull up a span table, have these numbers ready: building width, ridge beam span between supports, roof pitch, rafter spacing (16 or 24 inches on center), and your local ground snow load (your building department can tell you this). With those inputs, sizing the beam is straightforward. Without them, any answer is a guess.