Top 10 Deck-Building Mistakes
Avoid these common problems, and build a longer-lasting, safer deck.
The process of building decks is not nearly the same today as it was a decade or more ago. While the outdoor environment and the range of design options have remained relatively constant, emerging technologies and new products require a deeper understanding of industry codes and best practices.
The list of materials that can be used to build decks has certainly grown, or in some cases changed significantly. There are new varieties of pressure-treated material, for example, that can affect the choice of fasteners. High-performance building designs that include a continuous layer of exterior insulation complicate the attachment of deck ledgers.
As a deck builder, inspector, and plans analyst, I have seen a lot of inferior deck-building practices from professionals and do-it-yourselfers alike. I’ve also seen an abundance of bad information that perpetuates problematic designs and poor construction practices. A badly built deck is not only more likely to fail, it’s also dangerous for those who use it.
Here, I highlight the most common errors I see in deck building and offer some suggestions to ensure that your next deck is safe and durable.
Mistake 1: Failing to install a continuous handrail on stairs
The post in the middle of this flight of stairs interrupts the top of the railing, which was designed to serve as the handrail. A new continuous handrail, albeit an unsightly one, had to be added.
The error: For construction or aesthetic purposes, builders regularly interrupt handrails with newel posts. It’s also common to see a guard’s top rail used as a handrail.
The solution: Code provision R318.104.22.168 requires that a continuous handrail be installed on any set of stairs that has four or more steps. A continuous guard free of midspan posts extending through the top can be used as a handrail, but only if it meets specific geometric requirements. To be considered a handrail, the guard’s top rail must be graspable by those walking up and down the stairs. If a post interrupts a guard’s top rail, a true handrail must be added to the guard running along the stairs.
Mistake 2: Installing hardware incorrectly and using the wrong fasteners
Always follow manufacturer guidelines for appropriate fastener types and sizes, and use stainless-steel or galvanized fasteners if you are using pressured-treated lumber.
The error: Incorrect fasteners in hangers are a notorious mistake. For example, deck screws are not a proper way to attach joist hangers, and using 1-1⁄4-in.-long 10d nails where 3-1⁄ 2-in. 16d nails are required is a sure sign that manufacturer instructions were not followed.
Fasteners that don’t have the correct corrosion-resistance rating will fail quickly when installed in treated lumber. Also, using only one-half of a two-part post-to-beam connector and installing under-size bolts in 6×6 post bases are common installation errors
The solution: For hardware to work as the manufacturer claims it will and the way the inspector expects it to, follow the manufacturer’s installation instructions. Proprietary hardware is not specified in the code; therefore, it is considered an alternative. Alternatives are approved via testing or engineering, and that information must be provided to the building official. The only way to be sure hardware will perform as expected is if it is installed as it was tested or designed. Beyond code compliance, valid product warranties depend on proper installation.
Mistake 3: Bolting beams to the sides of posts
The error: A tragedy brought to us from the aisles of big-box stores: directions to deck builders to bolt deck beams to the sides of support posts. The average backyard deck has relatively few posts. Fewer posts result in greater loads at beam connections. It would take a huge load to shear a 1⁄ 2-in.-dia. bolt, but long before that occurred, wood around the bolt could be crushed, potentially resulting in a failed connection.
The solution: Each ply of a multispan beam, whether
single or multi-ply, must have full bearing on intermediate
posts. This can be accomplished by notching a 6×6
to accept a 2-ply beam and bolting the beam to it, or by
the use of an approved post cap. With all the hardware
available to handle various direct-bearing applications of different-size beams and posts, there is no excuse for
disregarding this code requirement.
Mistake 4: Over-spanning composite decking
To meet the span tolerance of this diagonally installed composite decking properly, additional joists and hangers had to be added to the existing deck framing.
The error: The maximum span of wood-and-plastic composite decking generally depends on the type of plastic used in the product. It’s important to follow the span limits of a specific product as outlined in the manufacturer’s installation instructions, which some builders fail to review. Over-spanning composite decking is most commonly a problem when deck boards are run diagonally over joists or when they’re used as stair treads.
The solution: Floor joists for a deck are typically installed at 16 in. on-center, which won’t properly support some composite-decking products when installed on an angle. In new construction, be sure floor joists are installed at the correct spacing. In existing decks, adding more floor joists is the only remedy. Similarly, additional stair stringers might have to be added to stairs where composite decking serves as the treads. Stair treads must be able to resist a concentrated load of 300 lb. over an area of 4 sq. in. This requirement puts a lot of pressure on the actual tread material to support concentrated loads. Some composite products are limited to an 8-in. maximum span when used as stair treads, which requires the support of six stringers in a 36-in.-wide stairway.
Mistake 5: Building stairs with incorrect riser heights
Once you have determined the rise and run of your stairs, stringer layout is straightforward. One detail, however, is easy to overlook. The bottom riser needs to be one tread thickness shorter than the rest.
The error: Often, the bottom step on a set of deck stairs is roughly 1 in. taller than the rest. Code allows a maximum variation of only 3⁄8 in. between riser heights. This guideline often confuses inexperienced carpenters, who insist that they cut every notch in the stringer the same.
The solution: Every notch cut into a stringer has an identical riser height except for the bottom one. The steps notched out of the stringer in the middle of the flight have treads placed above and below each step, effectively adding the same tread thickness to each riser height so that they remain constant. The bottom step doesn’t have a tread below it, though, so you must subtract the thickness of the tread from the height of the bottom riser, which is the bottom of the stringer.
Mistake 6: Ignoring clearances and inhibiting access
Some clearances around a deck are code-required, such as providing a minimum 36-in.-tall escape path from a basement egress window. Others are simply practical, such as ensuring access to hose bibs. Each clearance should be considered with equal diligence.
The error: Although well constructed, some decks are still code violations simply in how they interact with the house. For example, some stairs on multilevel decks end up near windows that the builder has not replaced with tempered-glass units. Other decks are built too close to the house’s main electrical service panel or the service conductors overhead—which need to be at least 10 ft. above a deck or 3 ft. to the side of a deck, according to code (E3604).
The solution: No matter what features exist on the exterior of a home—windows, air-conditioning compressors, low-hanging soffits, exterior lights, outdoor receptacle outlets, dryer vents—identify the required clearances before starting a deck design. While some features will influence the shape and location of the deck, other features may require only that appropriate access be integrated into the design of the deck.
Mistake 7: Attaching deck ledgers poorly
If sistering deck joists to floor joists isn’t an option, adding a beam, posts, and footings can help to relieve some of the stress placed on the fasteners connecting the ledger to the end grain of the cantilevered floor joists.
The error: The majority of deck plans end with a straight, continuous line at the ledger, rather than details as to what the ledger is connecting to. Unfortunately, the way a ledger attaches to a house is one of the most critical elements in deck construction, and many builders get it wrong. For example, they bolt ledgers straight to brick, stucco, or EIFS cladding. These practices violate the code. One of the more egregious ledger mistakes is connecting the ledger to a rim joist nailed to the end grain of cantilevered floor joists—those that support a kitchen bump-out, for example.
The solution: Detailing a ledger properly depends on the building type, the cladding material, and the site conditions. Of all the parts of a deck, the ledger can rarely be treated the same from job to job. Long before construction begins, considerations must be made as to, for example, whether stucco needs to be cut back with new weep screed installed or whether a few courses of lap siding need to be removed to bolt and flash the new ledger properly. Code requires that band joists supporting deck ledgers bear fully on the primary structure capable of supporting all required loads—in other words, they can’t be part of a cantilevered floor. A better option in that scenario is to build a freestanding deck that doesn’t rely on the cantilever to support it.
Mistake 8: Setting piers in disturbed soil
The error: When it comes to digging footings for deck piers, some builders are lazy. To avoid deck ledger failures, freestanding decks are becoming popular. But the piers nearest the foundation may be set atop backfill. In areas where the frost depth is not an issue and precast foundation blocks are used, they’re often set on top of the exposed grade — a code violation.
The solution: Foundation systems are required to extend a minimum of 12 in. into undisturbed soil (R403.1.4). In cold climates, where the ground freezes in winter, a pier foundations for non-freestanding decks must extend to a depth below that which is likley to freeze—in some places deeper than 48 in. This prevents the soil below the pier from freezing and heaving the pier upward
Piers must bear on undisturbed soil as well as set below the frost line. This could mean the need for 3-ft.-deep footings in some areas. However, if the piers are in a backfill region, as is the case with piers nearest the house on a freestanding deck, the footing depth may have to be as deep as 10 ft. to reach undisturbed soil and to comply with code.
Precast foundation blocks must be set at least 12 in. into the ground. However, even in the middle of a lot, the topsoil is tilled roughly 6 in. prior to seeding, so it’s likely that the footing needs to be at least 18 in. deep to comply with code. Assume that all deck piers and foundation blocks require some digging.
Mistake 9: Incorrectly attaching guard posts
The error: Connecting a guard post to a deck incorrectly is among the most dangerous deck-building errors. Fastening guard posts to deck rim joists or floor joists with wood screws is not acceptable. While some builders get the guard-post-to-rim-joist connection right, they don’t always make sure that the rim joist is attached to the deck framing properly.
The solution: The code (table R301.5) requires the top of a guard to be capable of resisting a concentrated load of 200 lb. in any direction. Depending on the design of the guard assembly, a stout guard-post-to-deck connection can be accomplished with blocking and through-bolts or with horizontally oriented hold-down hardware. In some rail designs, most of the load resistance is handled by the post connection to the deck. In those instances, the post should be attached to the joists, not the rim, because the rim is not usually fastened to the joists in a manner capable of transferring the load. Rims are typically nailed into the ends of the joists, the weakest possible connection for withdrawal resistance.
There’s more to consider than just the post-to-deck connection. The assembly must be able to resist a concentrated load at any point along the top of the rail. Posts are typically spaced 5 ft. to 6 ft. apart. When a continuous top cap runs across the posts, it acts like a horizontal beam to help distribute the load over a larger area. When the top of the guardrail is interrupted by a post that runs long, however, there is a considerable increase in the leverage the post puts on its connections.
Mistake 10: Making beam splices in the wrong places
The error: When a long built-up beam spans multiple posts, many builders run one 2x long so it extends beyond the supporting post. They apparently believe this practice is good because splices of opposing beam plies are separated rather than being only inches apart on top of a post. Unfortunately in these cases, an engineer’s evaluation or a rebuild of the beam is required.
The solution: Beams are under two stresses: bending and shear. Shear forces act perpendicular to the length of the beam and are greatest near the bearing ends.
Bending changes the beam’s shape, a force called deflection, and is greatest in the center of the beam span. The code lists maximum allowable limits for deflection. In deck beams, the deflection limit is typically reached long before shear limits are a consideration. Any reduction in bending resistance also increases deflection potential, which could lead to a code noncompliance.
Beam splices that miss the bearing point by a small amount don’t greatly affect bending or deflection, and the shear strength of one fewer ply is likely still sufficient. In these cases, the cost of an engineer’s review might just get you the OK to build. But don’t put splices in the center of a span — build the beam so that splices land on top of supporting posts.
Drawings: Dan Thorton
More on Decks:
Ultimate Deck Build – In this video, Fine Homebuilding’s Justin Fink gives an introduction on how the deck will be built. This Ultimate Deck Build series includes step-by-step instructions on how to build a better deck
Is Your Deck Safe? – Protect yourself from collapse, rot, and nasty splinters. Inspect these eight critical areas every season.
How to Build a Deck: Video Series – These four videos demonstrate the entire process, from planning to staining.
First Aid for a Failing Deck – The life span of most residential decks is 20 years or less. In this article, veteran carpenter Rick Arnold tunes up an existing deck. He removes the old decking and railing, saving the original deck frame while taking steps to safeguard users from a future deck collapse.
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