(continued)

Decking oriented in this manner has an increased resistance to bouncing or sagging. However the amount of decking material needed will greatly increase along with material costs. When this method is used, 1/4- to 3/8-inch spacers of treated marine-grade plywood or solid treated material should be placed at intervals of 2 to 3 feet between the decking members to increase stability.

This method also requires toe nailing the decking to the structural material below, and replacing single boards becomes very difficult after installation.

Avoiding Material Failures

Because the decking material is not only the most exposed material used in deck construction but also takes the most abuse over time, it is often the first material to deteriorate. Frequent applications of preservative treatments will extend its life span.

When possible, butt joints in the decking should be avoided. Where they are used, end grains tend to hold moisture, and the life of the deck will be shorter, with the chance of deterioration spreading to the structure below. The double joist system, described earlier, will reduce moisture entrapment.

Another cause of deck failure is the migration of water through the nail holes, particularly when the nails lose their withdrawal resistance and pop up. These openings leave a clear path for water to penetrate below the effective treatment penetration zone, causing the material to degrade from the inside out.

The rate of withdrawal among smooth shank nails is quite high in areas of heavy foot traffic, and spiral-groove or ring-shank nails or screws are recommended for attaching decking boards in these conditions.

The length of the connecting mechanism will also affect withdrawal, and the fastener should be 3-inch (10d) or greater when securing 1-inch thick decking and 3-1/2-inch (16d) or greater for 1-1/2-inch boards. If edge nailing can't be avoided, all holes should be predrilled to minimize splitting at the ends of the boards.

Bracing

When the deck is freestanding and is not held rigid laterally through a connection to an existing structure, the vertical structural members should be braced to remain stable. In a building, the framing is tied together with sheathing, providing lateral stability. In deck construction the structural members are exposed and in lieu of sheathing, bracing (diagonal cross members) is used to resist racking and possible structural failure.

For high structures, decks sited on sloping lots requiring long posts, for example, the need for bracing is critical. Any decks greater than 5 feet in height should be braced, particularly at the corners.

Bracing can take a number of forms. In most cases the simple Y type bracing should be sufficient for standard post-to-beam connections. This method of bracing, depending on the elevations, has an advantage in allowing easy access below the deck.

Where Y bracing is impractical or when the structure is designed with long beam spans or tall posts, X cross bracing can be installed at alternate bays, although some decks may require bracing at every bay for structural support. If the height of the posts exceeds 14 feet, two X braces or K braces, one on top of the other, may be required, and an engineer should be consulted.

If the length of the bracing is 8 feet or less, 2X4s are usually adequate; for longer unsupported lengths, 2X6s are recommended. All connections should be to the main framing with a minimum of 3/8-inch bolts used for fastening.

The number of bolts should be minimized to prevent moisture entrapment, and end grain exposure to water should be minimized. Both end grain and predrilled holes can be treated before attachment.

Stairs

Most decks contain at least some stairs, and in many cases the stairs are a feature, equal in visual impact to the decking. If the stairs include more than two risers (vertical steps), most codes require at least one railing at the side of the run.

All risers and treads (horizontal steps) should be of a consistent dimension within any run, and preferably within the whole design. If the design incorporates lumber of standard dimensions, there will be less waste and greater efficiency in material use.

For example a stair tread at 11-1/2 inches can be created from two 2X6s, or three 2X4s, and a rise of 5-1/2 inches can be achieved from a single 2X6. When nonstandard lumber is used, the members will have to be ripped (cut perpendicular to the grain), and end grain at the cut will need a field-applied treatment. Factory-applied preservative applications are always preferable.

Whatever the dimensions of the boards, the material for the treads should not be less than 5/4 inches, and 2X stock is the most common. When the riser is covered, the covering material can be thinner, typically 3/4 inch because it receives very little live or dead load. It is used to prevent toes from going under the tread and for aesthetic effect.

Stringers

The tread and riser boards are supported by a structural framing member called a stringer. The stringer is fabricated from a large board, often a treated 2X12 or 2X14 that has been cut to form a stepped pattern on which the tread boards are attached.

Each stringer is identical and must be placed vertically plumb and horizontally level. The number of stringers required depends on the width of the stairs. However if fewer than three are used, sagging can occur even on a stair with a width as short as 2-1/2 feet.

The boards are attached in the same manner as the decking with either nails or screws.

The stringer is attached to either the rim joist, a beam, or the blocking between the posts. Joist hangers or other metal connectors can be used to connect the upper stringer end to the joist or blocking.

The lower stringer end should rest on a concrete beam foundation or on footings below each stringer. This prevents direct contact with the ground and keeps the stairs from settling. The stringer should be attached to the footing with metal straps or tie downs to prevent horizontal movement or uplift caused by wind.

Next week we conclude our five-part series with Part V: Seating and Railings.

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.

Note: photos in the book are in black and white.

Small circular deck with bent decking members and an integrated overhead trellis provides a good example of the flexibility inherent in the materials. University of Washington Design/Build studio, 1992, instructor Steve Badenes. Photo: Daniel Winterbottom Bracing resists lateral sway. The most common are the Y, X, and K braces. Orient the end grain down or parallel with the post, and minimize bolt connections to prevent moisture infiltration. Image: Daniel Winterbottom The stringer connects the level of the deck to the grade, for the installation of stair risers and treads. Stringers can connect to the rim joist, to blocking between the joists, or to the beam. Image: Chad Wichers Stair stringer connected to a ledger with a joist hanger at the upper end and to a lower joist that also supports a landing between stair runs. Photo: Daniel Winterbottom Several options for resting the stringer at grade. A is the least permanent and prone to settlement. B provides a good connection, and the layout of connections allows some flexibility. C provides a good connection and, with a retrofit bolt, can be installed after the pad is poured, allowing the greatest flexibility. Image: Chad Wichers   Click on thumbnail images to view full-size pictures.