Pergola Engineering
Timber frame pergola engineering: snow load, wind and member sizes
These packages are calculated to IBC 2018 and ASCE 7-14, Risk Category I for open structures.
Every capacity figure on this page is a bearing capacity at the support — compression perpendicular to grain at 625 psi times the member’s area. It is not a bending capacity, and bending is checked separately in the sealed package.
What design loads apply to a timber frame pergola?
A pergola roof is open, so the numbers behind it are not the ones people expect. The stamped calculations start from two ground snow figures, 30 psf and 50 psf, which become 23 psf and 38 psf once the roof shape is accounted for. Add 15 psf of dead weight for the timbers themselves and 20 psf of live load for anything that lands on them. Wind is entered at 115 mph as an ultimate design speed in Exposure B, which is a number the calculation starts from, not a storm the structure is promised to survive. Soil is assumed to carry 1,500 psf.
Shade slats matter here. The packages note that shade slats reduce wind area by 50 percent and load by 25 percent, which is why an open-slat pergola roof is sized differently from a solid roof of the same footprint.
What member sizes does a 16 ft by 20 ft pergola use?
Posts are nominal 10×10. At 30 psf ground snow the columns carry 7,400 lb, the edge beam is a 4×10 carrying 11,200 lb, and the rafters are 4×8 carrying 11,200 lb. At 50 psf the columns carry 8,300 lb, the edge beam steps up to a 4×16 carrying 12,600 lb against 40,000 lb bearing capacity, and the rafters step up to 4×10 carrying 13,000 lb.
What member sizes does a 20 ft by 25 ft pergola use?
Posts stay nominal 10×10. At 30 psf the columns carry 9,500 lb, the edge beam is a 4×10 carrying 14,200 lb, and the rafters are 4×12 carrying 13,900 lb against 30,000 lb bearing capacity. At 50 psf the columns carry 10,900 lb, the edge beam steps up to a 6×20 carrying 16,800 lb against 75,000 lb bearing capacity, and the rafters carry 15,200 lb.
Why do pergola members get bigger at 50 psf snow?
More snow on the roof means more load reaching each beam, so the beam has to be deeper to keep its bending stress inside the allowable value. On the 16 ft by 20 ft plan the edge beam goes from a 4×10 to a 4×16. On the 20 ft by 25 ft plan it goes from a 4×10 to a 6×20. The posts do not change size; they gain load but stay inside the 62,500 lb tabulated capacity.
How is a timber frame pergola held down in wind?
Uplift on a pergola is not held by any single part. It travels the post, its through-bolts, the quarter-inch aluminum knife plate running up inside the post, the anchors into concrete, the footing, and finally the soil. One shared footing and anchor detail is checked against a bounding uplift demand of 3,299 lb.
What joinery and foundation hardware are called out?
Timbers are Douglas Fir #1 or better / Select Structural S4S or #1 & Better FOHC, cut with tapered dovetail joinery. The drawings call out a 3×8 classic knee brace, 5/16 in. TimberLok screws at the post-to-beam connection, and (4) 1/2 in. x 9-1/2 in. anchor bolts at a pier, over soil assumed at 1,500 psf.
How is bearing capacity calculated across every structure?
One method, every package: 625 psi times the member’s bearing area. That 625 psi is the published compression-perpendicular-to-grain design value for Douglas Fir-Larch, and every member in these packages is Douglas Fir #1 or better. Sixty-two of the sixty-two bearing capacities published across our nine structures reproduce from it exactly.
| Member | Bearing area | Capacity |
|---|---|---|
| 4 in. by 8 in. | 32 sq in. | 20,000 lb |
| 4 in. by 10 in. | 40 sq in. | 25,000 lb |
| 4 in. by 12 in. | 48 sq in. | 30,000 lb |
| 4 in. by 14 in. | 56 sq in. | 35,000 lb |
| 4 in. by 16 in. | 64 sq in. | 40,000 lb |
| 6 in. by 16 in. | 96 sq in. | 60,000 lb |
| 6 in. by 20 in. | 120 sq in. | 75,000 lb |
| 10 in. by 10 in. | 100 sq in. | 62,500 lb |
The 625 psi figure is the published compression-perpendicular-to-grain design value for Douglas Fir-Larch in the American Wood Council’s National Design Specification for Wood Construction (NDS) Supplement, Table 4A. The snow conversion follows the flat-roof snow equation in ASCE 7-14 Chapter 7. Both are named here so the figures above can be checked against their source rather than taken on trust.
Read every bearing percentage on this page as crushing at the bearing surface only. It is not a member’s axial, buckling, bending, shear, deflection or connection utilization, and any of those can govern at a far higher percentage. Those checks are separate calculations in the sealed package.
How does ground snow become roof snow in these packages?
Every package runs two ground snow cases and converts each to a roof snow load. The conversion is identical across all nine structures.
| Ground snow | Roof snow | Ratio |
|---|---|---|
| 30 psf | 23 psf | 0.767 |
| 50 psf | 38 psf | 0.760 |
Both ratios sit within one percent of each other, which is what a single set of exposure, thermal and importance factors produces. A site outside those two ground snow cases gets its own sealed calculation rather than an interpolation from this table.
What wind load does the package calculate at each load point?
Wind enters every package at 115 mph ultimate in Exposure B — the figure the calculation starts from, not a survival rating. Each package resolves its governing wind case to four load points.
| Structure | Plan modeled | Total | At each load point |
|---|---|---|---|
| Trellis | 18 ft by 3 ft | 457 lb | 114.25 lb |
| Gazebo | 14 ft by 14 ft | 2,493 lb | 623.25 lb |
| Cabana 16 by 16 | 16 ft by 16 ft | 1,484 lb | 370.90 lb |
| Cabana 20 by 20 | 20 ft by 20 ft | 2,202 lb | 550.40 lb |
| Cabana 16 by 20 | 16 ft by 20 ft | 2,967 lb | 741.79 lb |
| Pergola | 16 ft by 20 ft | 3,387 lb | 846.78 lb |
| Cabana 20 by 30 | 20 ft by 30 ft | 4,451 lb | 1,112.69 lb |
| Pavilion | 24 ft by 23 ft | 5,059 lb | 1,264.75 lb |
Each packet prints both the total and the per-load-point figure. On five of the eight the per-point figure does not equal the printed total divided by four — 370.90 lb times four is 1,483.60 lb against a printed 1,484 lb, for example — because the packet rounds the total for display and computes the per-point figure from the unrounded value. We publish both as printed. Read them as calculation results for the plan each package models, not as ratings for a structure of any other size. The arbor package resolves its main wind force to zero because that structure has no gable length and no roof slope.
Does this page work as permit documents by itself?
No. A permit office needs the sealed package prepared for your building and your county. Packages are site-specific, single-use, and non-transferable, and they are void without a proper engineer’s seal. Bigger spans, a different exposure, higher snow or wind, or soil that does not match the assumption get new site-specific calculations rather than a copy of this summary.
What if my site needs more snow or wind than the standard package?
The standard packages are stamped for 30 psf to 50 psf ground snow and a 115 mph ultimate design wind speed. A site that needs more capacity is normally handled with larger members, closer framing, or both, together with site-specific engineering sealed by a professional engineer licensed where the structure is being built. We fabricate to sealed drawings; we do not perform structural design through this website.
See the full stamped design criteria on our engineering and permitting page, or compare with pavilion engineering and gazebo engineering.
One rafter row in the sealed packet prints a 4 in. by 8 in. member. At 625 psi that 32 sq in. bearing area is 20,000 lb, which is the capacity we publish for a 4 in. by 8 in. member. We publish both figures exactly as the packet prints them rather than reconciling them ourselves. The sealed drawings control.
Shane Watson, PE (Utah #5206456-2202) seals our structural engineering packages and holds professional engineer licenses in 21 states. Sealed drawings for a permit are issued by an engineer licensed in the project state — Watson where he is licensed, or a coordinated in-state engineer where he is not. Western Timber Frame has completed jobs in all 50 states. Figures on this page are summarized from the stamped engineering package (Project 5C 25-022, calc job SSE WTF-026-03) and are calculated to IBC 2018 and ASCE 7-14. The sealed PDF prepared for your address controls.
See sizes, options and pricing on our timber frame pergola kits page.