Pavilion Engineering
Timber frame pavilion engineering: member sizes, snow and wind
These packages are calculated to IBC 2018 and ASCE 7-14, Risk Category I for open structures.
Capacity here means the bearing capacity printed in the sealed packet, which the packet calculates as 625 psi times the stated bearing area. It is not a bending capacity; bending, shear, deflection and the connections are checked separately in the sealed package.
What design loads apply to a timber frame pavilion?
Because a pavilion carries a closed roof, snow is the load that drives its sizing. Two ground snow cases are run, 30 psf and 50 psf, pairing with 23 psf and 38 psf at the roof. The timbers themselves count as 15 psf, and 20 psf is added for live load. Wind enters the calculation at 115 mph ultimate in Exposure B — an input value, not a survival claim — and the footings are checked against soil rated 1,500 psf.
A pavilion carries a solid roof, so there is no open-slat wind reduction. The package is sold as the 20 ft by 30 ft pavilion. The wind and snow sheets model it at 24 ft building length by 23 ft gable length, with a 9 ft eave, a 14.3 ft ridge, a 22.5 degree roof slope, and a mean roof height printed as 9.0 ft. Those are the sheet’s own analysis dimensions, not a second product size.
Which printed geometry figures do not reconcile?
The 9.0 ft mean roof height is what the sheet prints, and it does not reconcile with the eave and ridge on the same sheet: averaging 9 ft and 14.3 ft gives 11.65 ft, and ASCE 7 allows eave height to stand in for mean roof height only at roof slopes of 10 degrees or less. A second figure on that sheet does not reconcile either: a 9 ft eave and a 14.3 ft ridge across the 23 ft gable length work out to a 24.7 degree slope rather than the 22.5 degrees printed, and at a true 22.5 degrees the ridge would sit at 13.76 ft. We publish the printed figures rather than corrected ones, and the sealed calculation prepared for your address controls. Do not use these geometry values for design on their own.
What members does a 20 ft by 30 ft pavilion use?
Read straight off the bearing table in the sealed packet. Columns are full-dimension 10 in. by 10 in., rated 62,500 lb in bearing, carrying 12,300 lb at 30 psf ground snow and 14,000 lb at 50 psf — 20 and 22 percent of bearing capacity. The edge beam is 4 in. by 10 in. carrying 8,800 lb against 25,000 lb, or 35 percent. Rafters are 4 in. by 10 in. carrying 6,600 lb and 7,000 lb against 25,000 lb, or 26 and 28 percent. All members are Douglas Fir #1 or better, the species group whose published compression-perpendicular-to-grain design value is the 625 psi used here.
What ridge beam does the packet list, and why do two sheets disagree?
The bearing table lists a ridge beam at 6 in. by 16 in., rated 60,000 lb, carrying 12,300 lb at 30 psf ground snow and 14,000 lb at 50 psf — 21 and 23 percent of capacity. Note that the framing drawing calls out 4×14 Douglas Fir #1 ridge beams while the bearing table prints 6 in. by 16 in. At 625 psi the two do not carry the same bearing: 60,000 lb for the 6 in. by 16 in. and 35,000 lb for the 4 in. by 14 in. For bearing alone, both printed capacities exceed the 14,000 lb reaction. That comparison does not settle which size is correct, and it does not establish either beam’s bending, shear, deflection or connection capacity. We publish both as sealed; the sealed calculation prepared for your address controls and names the size that will be fabricated.
What tributary labels and line loads does the bearing sheet print?
Load figures in the bearing table are the reactions the sealed calculation reports for each member, on the tributary areas the engineer assigned. The sheet labels an 8 ft tributary length and a 600 sq ft tributary area, and that 600 sq ft is the full 20 ft by 30 ft roof plan rather than the area assigned to any one member. Applying the 8 ft length to the listed pressures gives 120 lb per foot of dead load, 160 lb per foot of roof live load and 184 lb per foot of roof snow at the 30 psf case.
Why is the load column higher than the plan pressures suggest?
All nine of our sealed packages print column loads above dead plus roof snow spread over the plan — between one and three and a half times higher. That pattern holds across every package rather than being particular to this one, so the load column carries inputs the plan pressures on this page do not show. The sealed calculation prepared for your address shows that work; this page reports its result.
What wind load does each pavilion post take?
The wind sheet computes its governing case on the 24 ft by 23 ft analysis model rather than on the sold 20 ft by 30 ft plan. On that model the case totals 5,059 lb across the structure. Divided across four load points that is 1,264.75 lb each, reported rounded to 1,265 lb. Read 1,265 lb as a calculation result for that model in Exposure B, not as a rating for a pavilion of any size.
How is a pavilion different from a pergola structurally?
The roof. A pergola roof is open slats, and the packages note that shade slats reduce wind area by 50 percent and wind load by 25 percent. A pavilion roof is solid, so its package evaluates the full roof area for wind and snow with no slat reduction. That is why the two structures do not share a member schedule. This page does not publish the pergola schedule, so read the difference as a difference in roof assumptions rather than a size comparison.
What foundation does a pavilion sit on?
The post base is a quarter-inch aluminum knife plate set inside the post and through-bolted, anchored into concrete over soil assumed at 1,500 psf. That detail is proven once against a bounding uplift demand of 3,299 lb rather than recalculated for each pavilion.
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 project address and accepted by the local permitting jurisdiction. 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 run two ground snow cases, 30 psf and 50 psf, at a 115 mph ultimate design wind speed. A site beyond those limits needs site-specific engineering sealed by a professional engineer licensed where the structure is being built. The engineer may change member sizes, framing spacing, connections, anchors or foundations as the project loads and soil conditions require. 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 pergola engineering and gazebo engineering.
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 pavilions page.