Arbor Engineering
Timber arbor engineering: snow packets, member loads and footings
These packages are calculated to IBC 2018 and ASCE 7-14, Risk Category I for open structures.
What snow load is a timber arbor engineered for?
The arbor drawing sheet states 30/50 psf ground snow, the same range printed on every other structure in this set. The calculation sheets behind it print two cases: packet A at 30 psf ground with 22.5 psf on the roof, and packet B at 38 psf ground with 38 psf on the roof.
Packet B is worth flagging. Every other structure pairs 50 psf ground with 38 psf roof, a ratio of about 0.76. The arbor’s packet B prints the same 38 psf figure for both ground and roof, and it does not match the 30/50 range on its own drawing sheet. We publish both figures as sealed rather than picking one. The sealed calculation prepared for your address controls, and if your jurisdiction requires the 50 psf ground case, ask for it by name. The arbor drawings are dated July 9, 2026.
What loads do the arbor members carry?
The stamped arbor package models a 10 ft building length with no gable length and a 0 degree roof slope — a flat arbor, 10 ft eave to 10 ft ridge. The columns carry 3,000 lb against 62,500 lb of bearing capacity, about 5 percent, and the rafters carry 780 lb against 25,000 lb, about 3 percent. Capacity here means bearing at the support — compression perpendicular to grain at 625 psi times the member’s end-grain area — not a bending capacity. Bending is checked separately in the sealed package. The packet prints those same figures in both packet A and packet B. There is no ridge row in the arbor package; an arbor has no ridge.
Why are the member loads identical in both arbor packets?
The package does not state a reason, so we do not supply one. What the sealed sheets show is that the tabulated member loads are the same under both snow cases, while on the larger structures the loads step up between cases. At 5 percent and 3 percent of bearing capacity these members are far from their limits either way. The sealed calculations control.
What wind load is the arbor calculated for?
115 mph ultimate design wind speed in Exposure B, the same input as the rest of the set. That is a value the calculation starts from, not a survival rating for a built arbor. The arbor sheet resolves its main wind force total to 0 lb because the structure has no gable length and no roof slope, so we do not publish a per-load-point wind figure the way the pavilion and trellis pages do.
What foundation does an arbor need?
An arbor uses the same post base as the larger structures: a quarter-inch aluminum knife plate inside the post, through-bolted and 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 arbor.
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 trellis engineering and pergola 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 arbors and trellises page.