Gambrel Roof Dimensions: Span, Rise, Run Reference Tables
Enter a building width to get every gambrel dimension. Reference table covers 10 ft to 60 ft widths.
Lower Run
6.00 ft
Lower Rise
10.39 ft
Lower Rafter
12.00 ft
Upper Run
6.00 ft
Upper Rise
1.61 ft
Upper Rafter
6.21 ft
Total Height
12.00 ft
Roof Area (20 ft length)
845 ft²
Standard Widths Reference (60° Sweep Angle)
| Width | Lower Rafter | Upper Rafter | Total Height | Lower Run | Upper Run | Roof Area (20 ft L) |
|---|---|---|---|---|---|---|
| 10 ft | 5.00 | 2.59 | 5.00 | 2.50 | 2.50 | 378 |
| 12 ft | 6.00 | 3.11 | 6.00 | 3.00 | 3.00 | 445 |
| 16 ft | 8.00 | 4.14 | 8.00 | 4.00 | 4.00 | 578 |
| 20 ft | 10.00 | 5.18 | 10.00 | 5.00 | 5.00 | 712 |
| 24 ft | 12.00 | 6.21 | 12.00 | 6.00 | 6.00 | 845 |
| 30 ft | 15.00 | 7.76 | 15.00 | 7.50 | 7.50 | 1046 |
| 36 ft | 18.00 | 9.32 | 18.00 | 9.00 | 9.00 | 1246 |
| 40 ft | 20.00 | 10.35 | 20.00 | 10.00 | 10.00 | 1380 |
| 48 ft | 24.00 | 12.42 | 24.00 | 12.00 | 12.00 | 1647 |
| 60 ft | 30.00 | 15.53 | 30.00 | 15.00 | 15.00 | 2047 |
How Each Dimension Scales
Every gambrel dimension scales linearly with building width in the half-circle method. A 12 ft gambrel and a 24 ft gambrel at the same sweep angle have identical proportions: every length doubles, every area quadruples. The two-pitch method breaks that proportionality because pitches are chosen independently.
Lumber Sizing by Span
For #2 SPF rafters at 16 in on center with moderate snow load (30 psf), the rule of thumb is: 2×6 up to 16 ft buildings; 2×8 from 16–24 ft; 2×10 from 24–32 ft; 2×12 from 32–40 ft. Beyond 40 ft, engineered trusses or LVL rafters become standard. Always cross-check against the IRC table for your snow load.
Field Tip
Treat the lumber sizing rule of thumb as a floor, not a ceiling. If your snow load sits anywhere near the upper end of a bracket, for example a 24 ft building at 40 psf ground snow, jump to the next size up rather than staying at the minimum. The cost difference between a 2×8 and a 2×10 rafter is small next to the cost of a sagging ridge line.
Frequently Asked Questions
›What is a standard gambrel building width?
Residential gambrels typically span 20 to 30 ft; agricultural barns span 30 to 40 ft. Anything under 16 ft is usually a shed or garage; anything over 40 ft requires engineered trusses.
›How tall is a gambrel roof?
In the half-circle method total roof height equals half the building width. A 24 ft gambrel is 12 ft tall from wall plate to ridge; a 30 ft gambrel is 15 ft tall.
›What is the maximum rafter length for 2x8 lumber?
IRC rafter tables list maximum spans for #2 SPF 2×8 at 16 in on center between 11 and 15 ft depending on snow load. For a 24 ft gambrel at 60° sweep, the lower rafter is about 12 ft, at the upper end of 2×8 capacity for moderate snow loads. Use 2×10 for snow loads over 40 psf.
›How do I read the dimension table?
Find your building width in the first column. The remaining columns give the matching upper rafter, lower rafter, total height, and roof area for the half-circle method at the standard 60° sweep angle. Override sweep angle in the calculator above for non-standard designs.