Timber frame carport engineering

Every timber carport we build is engineered as its own project. There is no standard carport package, and that is deliberate: a carport is usually taller than an open structure, often lands on a slab or driveway rather than separate piers, and normally has at least one open side. Those three things change the calculation enough that a package written for a pavilion would not describe your carport honestly. Shane Watson, PE prepares the calculation for your site, and the drawings he seals are what a permit office reviews.

Why is a carport engineered per project instead of from a package?

Three things move enough between carports that a shared package stops being accurate. The posts are usually taller, so wind acts on a longer lever arm at the base. The foundation is often an existing slab or driveway rather than piers sized for the load. And an open side changes how pressure builds under the roof. A per-project calculation answers those for your carport. A package cannot.

Who seals the drawings, and what if the state is different?

Shane Watson, PE. The license number we publish is Utah #5206456-2202, and he holds licenses in 21 states, listed on our engineering and permitting page. Sealed drawings for a permit are issued by an engineer licensed in the jurisdiction where the plans are submitted — through his license where he holds one, and through a coordinated seal by an in-jurisdiction engineer where he does not. The seal covers one structure on one lot and does not transfer.

What is covered, and what stays with you?

The engineering we prepare covers the timber structure we design and fabricate. Zoning, HOA approval, the permit application itself, the site plan, the foundation or slab, and anchorage into the ground stay with the owner or the contractor unless they are written into your contract. Saying which is which up front is how you avoid finding out at plan review.

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.

Do I need a permit, and does this page count as one?

The authority having jurisdiction where you build decides whether a permit is required, and we do not claim a structure will pass inspection anywhere. What a permit office reviews is the sealed drawing set prepared for your address. This page is a summary and is not a permit document.

See the full stamped design criteria on our engineering and permitting page, or compare with pavilion engineering and boat house 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. This structure is engineered per project, so nothing on this page is a sealed result. The sealed drawings prepared for your address control.