Let’s Look Closer At Deck Failure!

There are just two basic reasons for decks failing. The first is overloading and the second is improper construction.

Overloading: deck failure usually happens unexpectedly. We have been using the deck for several years, even barbecuing for the family without a problem. But today we are having a graduation party. Fifty teenagers’ crowd the deck which has never carried more than seven people! The music starts, the teenagers jump up and down, and the deck tears off the rim joist with the deck collapsing towards the house. Unfortunately several people are hurt.

What happened? Floor loads in a house are limited to 40 pounds per square foot or a point load of 300 pounds. That limit is designed to carry some furniture and people walking or sitting in the room. Decks are usually designed to the same limit so our deck was grossly overloaded with a large number of people on it, and absolutely could not carry the extra shock load of the teenagers’ dancing . The reason the deck failed where it is connected to the house (or the rim joist) is that this is the point where the entire load is perpendicular to the direction of the fastener. It is like suddenly smacking the fastener with an ax.

Party on the deck, by all means, but have it checked for structural integrity to be sure that the extra load can be carried. Pay particular attention to the rim joist, which should be bolted in a manner that pulls the deck floor joist and the house floor joist together. Proper hardware and an installation guide are available from your local big-box store.

Improper construction: every existing deck should be inspected to see that it has been properly constructed. Unfortunately, many contractors and local building officials do not appreciate that proper deck construction is even more important than it is in a house. The deck at some point will carry greater loads than the house floor and is being continually attacked by the weather where it has to contend with sun, rain and snow loads.

Any home inspector can determine if your deck is capable of carrying the loads required. Most often a contractor will be able to reinforce the existing structure without the need of building a new deck.

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

Rob Leslie
Kaplan Professionals, Retired

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