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Wood Deck Failures

What happened to pressure treated deck lumber?

Pressure treatment is a process that forces chemical preservatives into the wood. The wood is placed in a closed cylinder and pressure is applied to force the preservative into the wood. Preservatives protect wood from decay and insect damage. There are 3 classes of wood preservatives; the waterborne method is typically used in residential construction. For decades the lumber industry successfully used CCA, chromated copper arsenate, as the preservative of choice. However negative publicity regarding the arsenate caused the wood product industry to voluntarily transition away from CCA to alternative preservative systems. CCA has no longer been used residentially since December 2003. A number of alternative preservatives are available. The most recognized are ACQ, alkaline copper quad, and CA, copper azole.

Testing has indicated that ACQ and CA are more corrosive to steel and the protective coatings applied over steel.

In other words, the old galvanized nails, screws, metal hangers and connectors were no longer approved for use with the ‘new’ pressure treated wood. Unfortunately, either nobody told the builders and lumber suppliers or they just didn’t care. Hot-dip galvanized or stainless steel fasteners and hardware are the recommended products to use. The structural stability of the wood deck depends upon using these approved fasteners.

When you see rusted nails, bolts, and other metal hardware on a wood deck, it is a big problem.

 

Doug Hastings
MN Home Inspector, Minneapolis & St. Paul
ASHI certified inspector, ACI
Kaplan University, Home Inspection Lead Instructor

‘Pops’ Rob Leslie
Kaplan Professionals, Retired

 

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Decks Can Be Dangerous

Deck failures are becoming more common. Why do decks fail?

The most common reasons are:

  • Old age
  • Poor design
  • Improper materials
  • Overloading

Decks should be inspected annually. All lumber should be inspected for rotting, particularly where posts meet the ground, at joist unions, and the ledger board connection to the house. Most of this rotting is caused by old age, moisture, and lack of maintenance.

However, much of the rotting is coming from fasteners and connectors that are incompatible with the type of wood preservative the deck lumber was treated with. Rusted heads on nails, screws, or bolts connecting metal joist hangers are an indication that these fasteners or connectors have failed. This was caused by a chemical reaction between the metal hardware and the pressure treated wood. This form of failure is the number one reason for decks collapsing.

Unfortunately, there is great confusion as to which fasteners and connectors are to be used with what type of lumber. The best choice is stainless steel, but it is also the most expensive and difficult to obtain. The most commonly used nails, screws, bolts, and metal hangers are hot dip galvanized. These are a good, available, and lower cost choice. The thicker the galvanized coating the better and longer the metal hardware will perform. Other approved materials, similarly priced to hot-dipped galvanized, are silicon bronze and copper.

Deck collapses are often related to overloading or lateral movement. This two conditions place a great amount of stress on the deck fasteners and connectors. Therefore, whenever rust is seen on nails or screws, they should be removed for inspection or replaced.

Avoid a disaster inspect your deck each year.

Doug Hastings
MN Home Inspector, Minneapolis & St. Paul
ASHI certified inspector, ACI
Kaplan University, Home Inspection Lead Instructor

‘Pops’
Rob Leslie
Kaplan Professionals, Retired

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