Wood in the Landscape: Decks Part III
When using a wood post, it is important that the width of the beam be equal to or greater than the post so water doesn't flow down the face of the beam and into the end grain of the post. If the post is cut, the exposed end grain should be treated with a preservative before the beam is set in place.
Wood post connectors are either H shaped or of a twisted H shape, where the direction of the U at the top and bottom are perpendicular to each other, allowing the bolts to be perpendicular in the respective posts and beam.
Each form has predrilled holes providing locations for carriage or machine bolts. There are also U straps that extend up from the sides of the post and over the top of the beam. Some designers use steel flat bar with predrilled holes as straps and connect one to each side of the beam and the post.
The form of the beam depends on where the point of contact is between the posts and the beam. The following are beams common in deck construction:
A: Simple beam, a single span supported at each end.
B: Cantilever beam, supported only on one end.
C: Overhanging beam, extending beyond one or more supports.
D: Continuous beam, supported by three or more structural members.
E: Fixed beam, fixed to supports at each end.
One other structural member, a ledger, isn't technically a beam but acts in a similar manner. A ledger supports the same loads as a beam, but instead of resting on posts or piers, it is mechanically attached to an existing structure.
When the deck extends out from a building, a 2x or 4x member is connected to the existing rim joists or vertical framing (studs). The ledger can be attached with either two 3/8" diameter lag bolts every 24 inches to the rim (band) joist or to wall studs or expansion-bolted to the foundation wall. Thus the ledger is continuously mechanically supported.
Because the dressed ledger member is often only 1-1/2 inches wide, little surface area is available for nailing. Thus, the deck nailing must be done very close to the ends or sides of the decking boards, causing a splitting of the boards. To resolve this, a flat 2X4 can be attached flush to the top of the ledger, providing an additional 1-1/2 inches or a total of 3 inches support and nailing area.
When attaching the joists to the ledger, toe nailing should be avoided. The large nails required will split the ends of the joists and allow water to collect where it shouldn't. To eliminate a toe nail connection, joist hangers can be nailed to the ledger to receive the joists.
An alternate method that reduces but doesn't eliminate toe nailing uses a ledger 2 inches taller than the joists. Nail or screw a 2X2 piece of wood (cleat) flush with the bottom of the ledger, and lay a bead of sealant at the joint to prevent water infiltration. The joist can now rest on the cleat. The joist will still need to be toe-nailed into the ledger, but smaller nails can be used, thus avoiding the splitting.
This method should only be used if other options are not possible because water will sit on the cleat causing debris and fungi to accumulate, and most of the loads will be supported by the cleat.
Whatever the method of connecting the joists, the ledger should also receive a "Z" flashing strip that tucks under the wall siding, runs 90 degrees out over the top and 90 degrees down the face of the ledger, preventing water from penetrating the rim joist or sill.
The joists, in the platform method of framing, transfer the live and dead loads of the decking to the beams. They are positioned perpendicular to the beam and can be set either on top of the beams or hung flush with the top of the beam using joist hangers.
The first method relies on the integrity of the beam to transfer the loads. The second method depends on the integrity of the connector, the joist hanger, to transfer the loads to the beams.
The spacing of the joists depends on the span capability, which is determined by the dimension of the decking. Typical joist spacing ranges from 16 to 24 inches on center, when supporting 5/4-inch or 2X decking.
The first method elevates the joist, increasing the profile of the deck by the vertical dimension of the joist lying above the beam. The connectors used in this application are hurricane ties; these are twist straps that connect the joist to the beam in order to resist uplift by wind pressure.
With the second method, the joists are set flush with the beam using a joist hanger or ledger. The joist hanger is a galvanized metal stirrup into which the beam is nailed. The joist fits into the U-shaped stirrup which is then nailed to the joist.
Joists can span long distances relative to their vertical dimension, and twisting, or curving, is a potential problem. To counter this tendency, place solid wood blocking the same dimension as the joists, or 1X2 wood or metal bridging, between the joists at the midpoint of the spans and nail it to each member to resist movement.
Another common problem to avoid when attaching the decking to the joists is nailing close to the end grain of the decking boards. When the joints are staggered this is hard to avoid. If a pattern requires the decking to change direction at particular points, or if joints line up in a running pattern, a double joist system can be used.
If a 2X spacer is used between the two 2x members, the nails at the end of the decking boards can be set approximately 1-1/2 inches back from the edge. This provides good drainage and ventilation to the decking end grain, instead of the joist below trapping water, as in the single joist method.
Both beams and joists should be oriented vertically to the cross section's longitudinal axis. All board lumber has a "crook", a bend that is counter to the greatest dimension. In some members this is obvious in others it may be more difficult to see.
When placing the joists, its important to orient the "crowned" side up. This allows the weight of the structure to provide a counter resistance that will straighten the crown instead of increasing pressure on the natural curve as it would if the member is placed crown side down.
Next week we continue this five-part series with Part IV: Decking and Stairs.
Daniel Winterbottom teaches in the University of Washington's Department of Landscape Architecture. His interests focus on urban and community landscape design, vernacular landscapes, therapeutic gardens, sustainable design, and the craft and detailing of built forms. He is principal of Winterbottom Design, a Seattle-based landscape architecture and site planning firm.
Wood in the Landscape: A Practical Guide to Specification and Design, copyright 2000 by Daniel Winterbottom, is available at bookstores and at Amazon. To order from the publisher, visit John Wiley & Sons or call 800-225-5945.