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Pergola vs Gazebo — Structural Differences
Pergolas and Gazebos are all classified as accessory outdoor structures, but they differ significantly in how they are framed, loaded, and regulated.
Pergolas: open or slatted roof structure designed to provide partial shade. Because they do not carry a solid roof load, pergolas generally have lower dead loads and reduced snow accumulation compared to roofed structures.
Gazebos: a freestanding structures with a solid roof. The roof framing must be sized to support dead load, snow load, and wind uplift based on local building code requirements. Gazebos roof framing is often designed using the same load assumptions as residential roof construction.
Heavy Timber vs Big-Box Store Pergola Kits
Heavy timber pergolas and gazebos are constructed using large-section wood members intended to carry structural loads over long spans. Posts are commonly 8x8, or larger, with beams and rafters sized based on span length and applied loads.
Big-Box Store pergola kits are typically made from thin dimensional lumber, engineered wood, or aluminum components. These kits are primarily decorative and rely on exposed metal brackets and fasteners for assembly rather than structural joinery.
Because of their lighter construction, many big-box pergola kits are not designed to support significant snow loads or resist high wind forces without additional reinforcement.
Douglas Fir #1 FOHC Material Standard
All pergola and gazebo structures are built exclusively from Douglas Fir No. 1 Free of Heart Center (FOHC) material.
Douglas Fir is widely used in structural framing due to its high strength-to-weight ratio, stiffness, and predictable performance in large timber sizes. It is well suited for exterior structures where structural capacity and long-term durability are required.
Free of Heart Center (FOHC) timbers are milled so the center of the tree is excluded from the section. This reduces internal stress, limits excessive checking, and improves dimensional stability compared to heart-center timbers.
No. 1 grade Douglas Fir provides tighter grain structure and fewer structural defects, making it appropriate for exposed structural applications where appearance and performance both matter.
Timber Checking, Shrinkage & Movement
Large solid-sawn timbers naturally experience checking, shrinkage, and minor dimensional movement as moisture content changes over time. These characteristics are inherent to wood and are expected in heavy timber construction.
Checking refers to surface cracks that form as wood dries from the outside inward. In Douglas Fir timbers, checking typically follows the grain and does not reduce structural capacity when members are properly sized.
Shrinkage occurs primarily across the width and thickness of a timber as it acclimates to ambient conditions. Lengthwise shrinkage is minimal. Free of Heart Center (FOHC) timbers reduce internal stress and help limit excessive checking compared to heart-center timbers.
Minor seasonal movement is normal and should be anticipated in connection detailing and installation. These characteristics are cosmetic in nature and are not indicators of structural failure.
Typical Sizes, Spans & Post Spacing
Pergola and Gazebo sizing is governed by structural span limits, timber size, and applied loads rather than appearance alone. Proper post spacing and beam sizing are required to control deflection and maintain long-term structural performance.
Pergolas: Most commonly built in square configurations such as 12×12, 12×16, 14×14, 14×16, and 16×16, depending on patio size and desired coverage. Because pergolas do not carry a solid roof load, post spacing is typically driven by beam span limits and rafter layout.
Gazebos: Most commonly built in rectangular configurations. Common footprints include 12×16, 14×16, 16×20, 16×24, 20×20, and larger configurations where required. These sizes support symmetrical roof framing and provide adequate space for roof load transfer through posts and beams.
As span lengths increase, beam depth and width must also increase to maintain structural capacity. Post spacing, beam sizing, and roof framing must be coordinated as a complete structural system. Undersized members may result in excessive deflection or long-term movement even if the structure appears visually adequate at installation.
Foundations & Mounting Methods
Pergolas and Gazebos are anchored to concrete using an concealed steel knife plate system designed to transfer vertical loads, lateral forces, and uplift into the supporting foundation.
A 1/4-inch steel knife plate mechanically fastened to a concrete slab or concrete pier using structural concrete screws sized and spaced to meet anchorage requirements.
The timber post is connected to the concealed knife plate using 3/4-inch solid oak dowel. This connection provides a traditional timber-to-steel load transfer while maintaining a wood-only visible appearance at the post base.
Posts are installed approximately 1/4 inch above finished concrete to prevent prolonged moisture contact at the base of the timber. This clearance reduces the risk of decay while preserving a direct and continuous structural load path.
Foundation type, anchorage requirements, and fastener specifications vary by jurisdiction and should be verified with the local building department prior to construction.