Wood in the Landscape: Decks Part III
by Daniel Winterbottom
Last week, Part II of this five part series covered the fundamentals of wood deck foundations. This week our series continues with a discussion of beams and joists, how to build with them, and how to avoid common problems.
The beams of a deck are intermediate structural members, transferring the dead loads of the joists (if used) and decking and live loads to the post or pier. They can be solid lumber, 4X6 or 4X8, or like the column, built up from 2X material.
Nailing the boards face to face, a common mistake, promotes premature material decay. In fabricating a built-up beam, spacers made of treated lumber or plywood are placed between the two or more pieces of 2X material at spacings of 12 to 18 inches depending on the depth of the material. The spacers are glued and/or fastened to the beam members allowing air circulation and creating a void for water to pass through.
The best method for supporting the beam is to rest it on top of the post. This allows the post to provide the best compressive resistance. This method raises the profile of the deck, and if this is aesthetically unacceptable, the beams can be bolted to the side of the column.
Side bolting places much of the shear stresses on the fasteners. If inappropriately sized, the fasteners can suffer metal fatigue leading to structural failure. Notching the post and bolting it to the beam is an intermediate solution.
A variety of galvanized metal connectors can form a mechanical connection between the beam and the post. The connectors are designed to be mounted on top of a wood post or to be embedded into the top of a concrete pier if the wood post is eliminated.
This article is the third in a five-part series. It was excerpted from Wood in the Landscape: A Practical Guide to Specification and Design, with permission of the publisher, John Wiley & Sons,
This ADA accessible ramp is framed with curved joists supported by sandwich beams, with blocking set between joists. University of
Washington Design/Build studio, 1998,instructors Daniel Winterbottom and Luanne Smith.
Photo: Daniel Winterbottom
The commonly used sandwich beam is less serviceable than the notched post or the traditional solid post. The loads are transferred to the post through the fasteners, placing severe shear stresses on the bolts. The bolts must be adequately sized.
Image: Chad Wichers
Click on thumbnail images
to view full-size pictures.