Ridge Beam Calculator

Calculate structural ridge beam size for cathedral and vaulted ceilings — LVL and solid lumber options with span tables, load calculations, and post sizing

Calculate required ridge beam size based on span, load, and rafter configuration

Quick presets

ft

Count

27 pieces

24" spacing • 16.1 ft length

PRO

Professional Calculator

Extended parameters for precise calculations

sq ft

Estimated Materials

60 bundles

Roof Area

1,792 sq ft

Squares

17.9

Detailed Breakdown

Roof Area1,792 sq ft
With Waste1,971 sq ft
Roofing Squares17.9
Bundles60
How to Use This Calculator
The Ridge Beam Calculator helps you determine the correct structural ridge beam size for cathedral ceilings, vaulted ceilings, and any roof design where ceiling joists or rafter ties are not present. A ridge beam is fundamentally different from a ridge board — it is a load-bearing structural member that must be sized by engineering calculations.

Beam Sizing tab: Enter your ridge beam span (the unsupported distance between posts or walls), the horizontal rafter span from ridge to wall plate, rafter spacing, roof dead load, live load (including snow), and roof pitch. The calculator computes the total uniform load on the beam in pounds per linear foot (PLF) and determines the minimum beam depth for various lumber types. The rafter span is critical because the ridge beam carries half the roof load from both sides — a 12-foot rafter span with 35 psf total load produces 420 PLF on the beam. Roof pitch affects the actual rafter length (and weight) but the load calculation uses the horizontal projection. For spans over 16 feet or loads over 500 PLF, engineered lumber (LVL or glulam) is almost always required.

Lumber Selection tab: This tab compares solid sawn lumber and engineered beam options side by side for your calculated load. Solid Douglas Fir #2 has an allowable bending stress (Fb) of approximately 900 psi, while LVL 2.0E offers 2,600 psi — nearly three times stronger per unit area. For a given load, an LVL beam can be significantly smaller (shallower) than a solid sawn beam. The tab shows required beam depth for each material type at your selected width. For exposed cathedral ceiling beams, glulam offers the best appearance in architectural-grade finishes, while concealed LVL beams are the most cost-effective. Beam width selection (single, double, or triple ply for LVL) affects lateral stability and rafter bearing.

Posts & Connections tab: Enter the point load at each post (approximately total beam load × span ÷ 2 for end posts), post height, and preferred materials. The calculator verifies post adequacy against buckling limits — a 6×6 Douglas Fir post can handle approximately 14,000 lbs at 8 feet of height, but only about 9,000 lbs at 12 feet due to increased slenderness. It recommends connection hardware (Simpson post caps, bearing plates, or concealed connectors for exposed applications) and footing size based on your point load and assumed soil bearing capacity of 1,500-2,000 psf. For new construction, concrete pier footings (Sonotubes) are typical, while renovations may bear on existing load-bearing walls.

The Formula
The ridge beam calculator uses these structural engineering formulas:

Uniform Beam Load (w) = (Dead Load + Live Load) × Rafter Span - Ridge beam carries half the load from rafters on each side; both sides sum to full rafter span - Example: (15 psf + 20 psf) × 12 ft = 420 PLF

Maximum Bending Moment (M) = w × L² ÷ 8 (for simply supported beam) - w = uniform load in PLF, L = beam span in feet - Example: 420 PLF × 16² ÷ 8 = 13,440 ft-lbs = 161,280 in-lbs

Required Section Modulus (S) = M ÷ Fb (allowable bending stress) - Fb for Douglas Fir #2: ~900 psi; for LVL 2.0E: ~2,600 psi - Example (LVL): 161,280 ÷ 2,600 = 62.0 in³ → 3.5×11.875" LVL (S = 82.3 in³) works - Example (solid DF): 161,280 ÷ 900 = 179.2 in³ → need 5.5×14" or larger

Deflection Check = 5 × w × L⁴ ÷ (384 × E × I) ≤ L/240 - E for LVL 2.0E = 2,000,000 psi; I for 3.5×11.875" = 488 in⁴ - Example: 5 × 35 × 192⁴ ÷ (384 × 2,000,000 × 488) = 0.58" vs L/240 = 0.80" → OK

Post Load = w × L ÷ 2 (for each end post of simply supported beam) - Example: 420 PLF × 16 ft ÷ 2 = 3,360 lbs per post

Footing Size = Post Load ÷ Soil Bearing Capacity - Example: 3,360 lbs ÷ 1,500 psf = 2.24 sq ft → 18" × 18" minimum footing
Example Calculation
Example: 16-foot Cathedral Ceiling in a Great Room — LVL Ridge Beam in Colorado

Jennifer is building a 16-foot-wide great room addition with a cathedral ceiling (no ceiling joists) and a 6/12 pitch. Her area has a 30 psf ground snow load. She needs to size a structural ridge beam.

Step 1: Determine Loads
• Roof dead load: 15 psf (asphalt shingles + plywood + insulation)
• Roof live load: 30 psf (snow load region — 30 psf ground snow × 0.7 exposure factor ≈ 21 psf, but IRC minimum is 20 psf; use 30 psf for the full ground snow value per local code)
• Total load: 15 + 30 = 45 psf
• Rafter span (horizontal): 12 ft each side (building is 24 ft wide)

Step 2: Calculate Beam Load
• Uniform load: 45 psf × 12 ft = 540 PLF
• Beam span: 16 ft (between gable end wall and interior post)
• Maximum moment: 540 × 16² ÷ 8 = 17,280 ft-lbs = 207,360 in-lbs

Step 3: Size the LVL Beam
• Using 2.0E LVL (Fb = 2,600 psi)
• Required section modulus: 207,360 ÷ 2,600 = 79.8 in³
• 3.5 × 11.875" LVL: S = 82.3 in³ → just barely adequate (103% of required)
• 3.5 × 14" LVL: S = 114.3 in³ → comfortable (143% of required) — recommended
• Deflection check for 3.5×14": I = 800 in⁴; deflection = 5×45×192⁴ ÷ (384×2,000,000×800) = 0.46" vs L/240 = 0.80" → OK

Step 4: Post and Footing
• Post load: 540 × 16 ÷ 2 = 4,320 lbs per post
• 6×6 Douglas Fir at 8 ft height: capacity ~14,000 lbs → adequate with large safety margin
• Footing: 4,320 ÷ 1,500 psf = 2.88 sq ft → 24" × 24" concrete pier (3.0 sq ft, 12" into undisturbed soil minimum)
• Connection: Simpson ABU66Z adjustable post base + Simpson LPC6 post cap

Bill of Materials:
• (2) 1.75 × 14" × 18' LVL beams (laminated together to form 3.5×14"): ~$280 each = $560
• (2) 6×6 × 8' Douglas Fir #2 posts: ~$45 each = $90
• (2) Simpson LPC6 post caps: ~$35 each = $70
• (2) Simpson ABU66Z post bases: ~$30 each = $60
• (2) Sonotube 24" × 48" + concrete: ~$60 each = $120
• Construction adhesive + through-bolts: $40
• Total materials: approximately $940
• Engineering letter (stamped calculations): $400-$600
• Total with engineering: approximately $1,340-$1,540

Frequently Asked Questions

What is the difference between a ridge beam and a ridge board?
A ridge board is a non-structural member (typically a 1x or 2x board) that serves only as a nailing surface where opposing rafters meet at the peak. It carries no load — the rafters are held in place by ceiling joists or rafter ties that resist the outward thrust at the wall plates. A ridge beam, by contrast, is a structural member that supports the upper ends of the rafters and carries approximately half of the total roof load down through posts to the foundation. Ridge beams are required when there are no ceiling joists or rafter ties to resist outward thrust — specifically in cathedral ceilings, vaulted ceilings, and any open-rafter design where the ceiling follows the roof slope. A ridge board might be a 1x8, while a ridge beam for the same span could be a 3.5x14 LVL — vastly different in size and purpose.
When do I need a structural ridge beam instead of a ridge board?
You need a structural ridge beam whenever ceiling joists or rafter ties are not present to resist the horizontal outward thrust of the rafters. The most common scenarios are: (1) Cathedral or vaulted ceilings where the finished ceiling follows the underside of the rafters and no horizontal tie is possible. (2) Open loft designs where the attic space is left open without any cross-framing. (3) Roof framing over large rooms with clearstory windows where rafter ties would block the window openings. (4) Any rafter roof where the rafter ties are placed higher than the lower one-third of the attic height (these become collar ties and do not resist thrust — they only prevent ridge separation). The IRC (Section R802.3) explicitly requires either rafter ties/ceiling joists or a ridge beam designed per the IBC to carry the full tributary roof load.
How do I calculate the load on a ridge beam?
The ridge beam carries approximately half of the total roof load from rafters on both sides. The uniform load in pounds per linear foot (PLF) is calculated as: (Dead Load + Live Load) × Rafter Span. The rafter span is the horizontal distance from the ridge to the exterior wall. Each side of the ridge contributes half the rafter span worth of load, and since both sides contribute, the total tributary width equals the full rafter span. For example, with a 15 psf dead load + 20 psf live load = 35 psf total load, and 12-foot rafters on each side: 35 psf × 12 ft = 420 PLF on the beam. For a 16-foot beam span, the total beam load is 420 × 16 = 6,720 lbs, with each end post carrying approximately 3,360 lbs. Always verify with your local building department, as snow loads, wind loads, and seismic factors may increase the design loads significantly.
What size LVL beam do I need for a 16-foot ridge beam span?
For a 16-foot span with standard residential loading (35 psf total = 15 dead + 20 live) and 12-foot rafters at 16" OC, the beam load is approximately 420 PLF. Using 2.0E LVL (allowable bending stress Fb = 2,600 psi), a 3.5×11.875" (double 1.75" LVL) beam is typically adequate, providing a section modulus of approximately 82 cubic inches against a required ~74 cubic inches. However, for the same span with 40 psf snow load (55 psf total), the load jumps to 660 PLF and you would need a 3.5×14" LVL beam. At 50 psf snow (65 psf total), a 3.5×16" or 5.25×14" beam may be required. These are approximate values — always have a structural engineer or your local building official verify the beam size, as LVL properties vary by manufacturer (Weyerhaeuser Microllam, Boise Cascade VERSA-Lam, etc.).
Do I need an engineer-stamped plan for a ridge beam?
In most US jurisdictions, yes. Because a ridge beam is a structural member carrying the roof load, building departments typically require engineered calculations stamped by a licensed structural engineer or architect. The IRC prescriptive span tables (Tables R602.7) cover only ridge boards and basic rafter-tie configurations — they do not include ridge beam sizing. Some jurisdictions accept prescriptive beam sizing from LVL manufacturer span tables (such as Weyerhaeuser or Boise Cascade published tables), but many still require a site-specific engineering letter. The cost of a structural engineer for a ridge beam design is typically $300-$800 for a residential project in 2026. This covers beam sizing, post and footing design, connection details, and a stamped drawing for the building permit. Given that undersized ridge beams can lead to roof sag, wall spread, and catastrophic failure, this is not an area to cut corners.

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