Engineered cabana structures: stamped loads for four plan sizes

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

What design loads apply to an engineered cabana?

Every cabana size is calculated against both snow cases: 30 psf on the ground giving 23 psf at the roof, and 50 psf giving 38 psf. The frame contributes 15 psf dead load, with 20 psf live load on top. Wind is set at 115 mph ultimate in Exposure B, a calculation input rather than a rating, and the foundation work assumes soil at 1,500 psf.

Four cabana sizes have their own stamped packages: 16 ft by 16 ft, 16 ft by 20 ft, 20 ft by 20 ft, 20 ft by 30 ft.

What loads does each cabana size carry?

Each row below is read straight off the bearing table in the sealed package: the load the member carries, the member size, its bearing capacity at the support, and how much of that capacity is used. Capacity here is bearing — compression perpendicular to grain at 625 psi — not a bending capacity.

16 ft by 16 ft cabana

Columns carry 5,200 lb at 30 psf ground snow and 6,000 lb at 50 psf, on nominal 10 in. by 10 in. posts rated 62,500 lb in bearing. That is 8 percent of bearing capacity at 30 psf and 10 percent at 50 psf. Rafters are 4 in. by 10 in. carrying 3,000 lb and 3,200 lb against 25,000 lb, or 12 and 13 percent. Wind on the whole structure totals 1,484 lb, which the calculation resolves to 371 lb at each of four load points.

16 ft by 20 ft cabana

Columns carry 7,600 lb and 8,500 lb, using 12 and 14 percent of bearing capacity. Rafters carry 3,100 lb and 4,700 lb, or 12 and 19 percent. Wind totals 2,967 lb, resolving to 742 lb at each of four load points.

20 ft by 20 ft cabana

Columns carry 3,700 lb and 4,000 lb, using 6 percent of bearing capacity at both snow cases — the lowest utilization of the four sizes. Rafters carry 3,100 lb and 3,300 lb, or 12 and 13 percent. Wind totals 2,202 lb, resolving to 550 lb at each of four load points.

20 ft by 30 ft cabana

Columns carry 9,100 lb and 10,000 lb, using 15 and 16 percent of bearing capacity. Rafters carry 6,000 lb and 6,300 lb, or 24 and 25 percent — the highest in the set. Wind totals 4,451 lb, resolving to 1,113 lb at each of four load points.

What is the difference between an engineered and a non-engineered cabana?

An engineered cabana has a structural package sealed by a licensed professional engineer that states the design loads, the member sizes and the foundation for one building on one lot. A non-engineered cabana has none of that, so a permit office has nothing to review and no one has checked the members against a snow or wind load. Every cabana size listed above is covered by a sealed package.

What foundation does a cabana sit on?

Footings are sized against assumed soil bearing of 1,500 psf, with a geotechnical report requested. Each post sits on a quarter-inch aluminum knife plate, anchored into concrete and checked against a bounding uplift demand of 3,299 lb.

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 gazebo engineering and pavilion 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.