Inspecting a Deck
Because decks appear to be simple to build, many people do not realize that decks are, in fact, structures that need to be designed to adequately resist certain stresses. Like any other house or building, a deck must be designed to support the weight of people, snow loads, and objects. A deck must be able to resist lateral and uplift loads that can act on the deck as a result of wind or seismic activity. Deck stairs must be safe and handrails graspable. And, finally, deck rails should be safe for children by having proper infill spacing.
A deck failure is any failure of a deck that could lead to injury, including rail failure, or total deck collapse. There is no international system that tracks deck failures, and each is treated as an isolated event, rather than a systemic problem. Very few municipalities perform investigations into the cause of the failure, and the media are generally more concerned with injuries rather than on the causes of collapses. Rail failure occurs much more frequently than total deck collapses; however, because rail failures are less dramatic than total collapses and normally don’t result in death, injuries from rail failures are rarely reported.
Here are some interesting facts about deck failure:
- More decks collapse in the summer than during the rest of the year combined.
- Almost every deck collapse occurred while the decks were occupied or under a heavy snow load.
- There is no correlation between deck failure and whether the deck was built with or without a building permit.
- There is no correlation between deck failure and whether the deck was built by a homeowner or a professional contractor.
- There is a slight correlation between deck failure and the age of the deck.
- About 90% of deck collapses occurred as a result of the separation of the house and the deck ledger board, allowing the deck to swing away from the house. It is very rare for deck floor joists to break mid-span.
- Many more injuries are the result of rail failure, rather than complete deck collapse.
- Deck stairs are notorious for lacking graspable handrails.
- Many do-it-yourself homeowners, and even contractors, don’t believe that rail infill spacing codes apply to decks.
This document does not address specific building codes, balconies, lumber species, grade marks, decks made of plastics or composites, mold, or wood-destroying insects.
A proper deck inspection relies heavily on the professional judgments of the inspector. This document will help improve the accuracy of those judgments.
The image above depicts an evenly distributed deck load. Building codes require decks to be designed to carry a uniformly distributed load over the entire deck. If evenly distributed, half of the load is carried by the deck-to-house connection, and the other half is carried by the posts.
The image above depicts a typical deck load distribution. People tend to gather near the railings of a deck, and so more load is likely carried by the posts.
Hot tubs filled with water and people are heavy and can weigh a couple of tons. Most decks are designed for loads of 40 to 60 pounds per square foot. Hot tubs require framing that can support over 100 pounds per square foot.
The image above depicts a free-standing deck (not attached to the home or building). A footing near a home must be on undisturbed soil. Some codes consider soil to be “undisturbed” if it hasn’t been disturbed in more than five years. It may be difficult to find undisturbed soil near the foundation of a new home.
The image above depicts a post base that is not attached to its footing. Posts should be connected to their footings so that the posts don’t lift or slip off.
The image above depicts a pre-cast concrete pier. Posts can lift out of pre-cast concrete piers, and piers can slide. Posts should be connected to their footings so that the posts don’t lift or slip off.
The image above depicts a proper post-to-footing connection. Posts should be connected to their footings so that the posts don’t lift or slip off their footings.
The image above depicts an adjustable post-to-footing connection. Posts should be connected to their footings so that the posts don’t lift or slip off their footings.
Wood can decay and degrade over time with exposure to the elements. Decay is a problem that worsens with time. Members within the deck frame that have decayed may no longer be able to perform the function for which they were installed. Paint can hide decay from an inspector and so should be noted in the report.
The pick test uses an ice pick, awl or screwdriver to penetrate the wood surface. After penetrating the wood, the tool is leveraged to pry up a splinter, parallel to the grain, away from the surface. The appearance and sound of the action is used to detect decay. The inspector should first try the pick test in an area where the wood is known to be sound to deterimine a “control” for the rest of the inspection. Decayed wood will break directly over the tool with very few splinters, and less or almost no audible noise compared to sound wood. The pick test cannot detect decay far from the surface of the wood.
Although deck inspections are visual-only inspections, inspectors may want to dig down around posts and perform pick tests just below grade level to look for decay
The most common cause of deck collapse is when a ledgers pulls away from the band joists of homes and buildings.
The two most common ways to correctly attach a ledger to a structure are with lag screws or through-bolts. The installation of through-bolts requires access to the back-side of the rim joist which, in some cases, is not possible without significant removal of drywall within the structure.
Most building codes state that, where positive connections to the primary building structure cannot be verified during inspection, decks shall be self-supporting (free-standing).
Determining the exact required spacing for the ledger fasteners is based on many factors, including:
- joist length;
- type of fastener;
- diameter of fastener;
- sheathing thickness;
- use of stacked washers;
- type of wood species;
- moisture content;
- band joist integrity; and
- deck loads…
…and so is beyond the scope of a visual inspection. However, the spacing of ledger fasteners is primarily determined by the length of the joists.
The deck receptacle should have a weatherproof cover.
Decks should not be located where they might obstruct septic tank accesses, underground fuel storage tanks, well heads, or buried power lines.
Egress openings under decks and porches are acceptable, provided the escape path is at least 36 inches (914 mm) in height, and the path of egress is not obstructed by infill or lattice.