Timber frame gazebo engineering: snow load, rafters and permits

These packages are calculated to IBC 2018 and ASCE 7-14, Risk Category I for open structures.

What snow load is a timber frame gazebo engineered for?

Gazebo packages are run twice, once for 30 psf ground snow and once for 50 psf, which translate to 23 psf and 38 psf on the roof itself. Dead load for the frame is taken at 15 psf and live load at 20 psf. The wind figure in the calculation is 115 mph ultimate, Exposure B; it describes the design input, not a wind speed the gazebo is guaranteed to survive. Footings assume 1,500 psf of soil bearing.

The gazebo set is the largest in the group: 8 calculation packets covering 16 ft by 16 ft, 20 ft by 20 ft, 24 ft by 24 ft, 30 ft by 30 ft plans, each run at both snow options.

What rafter size does a gazebo use at each plan size?

Posts are nominal 10 in. by 10 in. on every plan, rated 62,500 lb in bearing. Rafter depth is what changes. Percentages below are how much of each member’s bearing capacity the load uses.

16 ft by 16 ft

Columns 3,500 lb at 30 psf and 4,000 lb at 50 psf, using 6 percent. Edge beam 4 in. by 10 in. at 3,700 lb, 15 percent. Rafters 4 in. by 8 in. at 3,700 lb and 6,100 lb, 19 and 24 percent.

20 ft by 20 ft

Columns 5,000 lb and 5,200 lb, 8 percent. Edge beam 4 in. by 10 in. at 6,000 lb and 6,400 lb, 24 and 26 percent. Rafters 4 in. by 8 in. at 5,900 lb and 6,400 lb, 30 and 26 percent.

24 ft by 24 ft

Columns 7,500 lb and 7,900 lb, 12 and 13 percent. Edge beam 4 in. by 10 in. at 9,000 lb and 9,800 lb, 36 and 39 percent. Rafters are 4 in. by 8 in. at 9,000 lb at 30 psf, and step to 4 in. by 14 in. at 9,711 lb at 50 psf.

30 ft by 30 ft

Columns 8,600 lb, 14 percent. Edge beam 4 in. by 10 in. at 10,100 lb, stepping to 4 in. by 12 in. at 11,100 lb at 50 psf. Rafters 4 in. by 8 in. at 10,100 lb, stepping to 4 in. by 16 in. at 11,100 lb.

How much wind load does a gazebo post carry?

Wind on the whole structure totals 2,493 lb on the 16 ft by 16 ft plan, 2,181 lb on the 20 ft by 20 ft, 3,666 lb on the 24 ft by 24 ft, and 5,939 lb on the 30 ft by 30 ft. Each total is resolved to four load points, giving 623 lb, 545 lb, 917 lb and 1,485 lb. The 20 ft by 20 ft total sits below the 16 ft by 16 ft because roof pitch and height, not footprint alone, set the area the wind acts on.

Why does a solid gazebo roof need bigger beams than a pergola?

A solid roof has no open-slat reduction. The packages note that shade slats reduce wind area by 50 percent and load by 25 percent on a slatted roof, and a gazebo gets none of that. It takes the whole wind and snow load, so beams and rafters are sized up for the same footprint.

Do I need a permit for a gazebo?

That is decided by the authority having jurisdiction where you build, not by us and not by this page. Most jurisdictions ask for a sealed structural package for a permanent roofed structure on footings. The sealed package prepared for your county is what gets submitted. We cannot tell you your county’s threshold, and we do not claim a structure will pass inspection.

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.

Bearing capacity by member size, at 625 psi

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 to roof snow, all packages

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.

Governing wind case by structure

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 cabana engineering.

Two rafter rows in the sealed packet print 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.