As a home inspector, I see more poorly built decks than properly built ones. Many are built in a way that failure will be inevitable. Give the right conditions and enough weight, a collapse will happen. It is estimated that 50 percent of decks built in the USA were constructed without a permit, therefore skipping the inspection process. When built at a height more than a couple of feet from the ground, deck failure can lead to serious injury. The following are frequently seen components of decks that have not been built properly and are commonly called out as a safety concern during a home inspection.
Railings- When a deck is more than 30" from the ground, the perimeter of the deck should be protected by a railing that is at least 36" tall. This railing should be able to withstand the force of a 200 lb load. The spacing between each baluster on the railing should not be greater than 4", as a child could pass through.
Ledger Board- Decks that are attached to the home should have a ledger board connecting the deck to the house. Assuming the siding is not stucco, EIF, or a brick veneer, the ledger board should be bolted (not nailed) to the home in several places. Ledger boards cannot be supported by a cantilevered (bump out) section of the home without additional supports placed under this area. A piece of flashing should be installed under the siding and over the ledger board to prevent water from entering the home where the ledger has been bolted. It is estimated that 90% of deck collapses occur where the ledger board connects to the home. I often see it installed improperly.
Support- Concrete piers, which support posts rest on and are attached to, are often placed above grade, allowing for movement from the settling or heaving of the soil. Piers should be installed on undisturbed soil and below the frost line. Posts should be properly connected to the piers to prevent separation. On taller decks, 6x6" posts should be used instead of 4x4", as the smaller posts can twist under heavy loads. Support beams should rest over the posts and be properly attached. Beam splices need to be supported by a post directly underneath. Beams and floor joists should be constructed with at least 2x6" boards to properly support the weight.
Hardware- Using the correct hardware is imperative when it comes to a properly built deck. Sometimes deck screws are used to attach joist hangers instead of 16d nails. Joist hangers are sometime even omitted completely, instead nailing into the end of a joist which is the weakest connection point. Rusted nails are found during inspections, indicating that the improper hardware has been used. On treated lumber, stainless steel or galvanized nails must be used because the high level of copper on the treated wood speeds up corrosion, weakening the hardware.
As you can see, there are many components of a deck that can be constructed improperly, leading to a major safety issue. Often, the decks are built close to the ground, preventing a complete inspection of the deck structure. While a deck failure from a couple feet off of the ground may not seem significant, many homeowners who have experienced it would disagree. With safety standards becoming more stringent on the building of decks, home inspectors around the country are calling out more safety defects than ever. While it is always up to the homeowner to address these issues, don't expect your home inspector to look over these deficiencies because the deck has been standing for years. Improperly constructed decks can fail dramatically and without warning, leading to serious injury and damage to the structures.
Stairs- Each step or riser height, should be around the same size. Often, the last step on the stairs is shorter than the rest due to the size of the planking. The maximum riser difference shall be less than 3/8", as a large difference between step height can be a trip hazard. Risers (between steps) should be framed in since they typically will have a space over 4". Handrails should span from the top step to the bottom and should be continuous and graspable, meaning you can run your hand up the railing without having to lift off your hand. Often the railing is separated by newel posts (support posts) and not considered continuous.
Deck Planking- The planking on the deck should have a small separation between each plank to allow water to pass through. Nail heads should not be under driven as they become a trip hazard, nor over driven as they allow water to pool in the impression left behind from the head of the hammer. Each plank should be staggered and bear its load on at least 4 joists. When heavier planking like composite is used, the joist span may be less than the typical 16" on center, based off manufacturer recommendations. When composite planking is used on stairs, additional stingers are often required to properly support the load.