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When Will My Deck or Balcony Fail?

Assessing the probable failure point of a deck is complex, so the only real choice is to err on the cautious side. Here are the most important things to know.

I remember a couple of decades ago, when some of my female friends wailed mournfully at the sudden loss of hunky actor Kevin Smith. The story was that a balcony collapsed.

It turns out they had the story wrong . Smith, who famously played the role of Ares in the sword and sandals series Xena, actually fell three stories from a prop tower and landed on his head. Nonetheless, the balcony failure meme resonated with me. I’ve never set foot on an elevated deck or balcony since, without imagining poor Kevin, partying with some friends, then suddenly plummeting to his death.

But how and when do decks and balconies collapse? The simple answer is “when they’re not built right,” but other factors also come into play.

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Most major deck failures can be traced back to the ledger board, but all materials have a limited lifespan, so you need to recognize the warning signs. Source: Boston Decks and Porches

Decks built strictly to code tend to fail in particular ways (as the list below suggests). But far too many of them include DIY details and shoddy engineering.  I’ve seen some insanely rickety multi-story decks, particularly in Canada, often perched on 2x4 posts, with inadequate bracing, narrow and dangerous stairs.

Then there’s the question of material aging, corrosion, and rot. The multifamily home behind mine has a 30-year-old balcony, suspended by two angled 4x4s and a few steel plates to make the connections. How much longer will the blackened timbers hold? Should I warn the tenant that he may be risking his life out there?

Before I talk more about non-conforming decks, I’d like to borrow 7 key warning signs to look for prior to a failure, in the case of a more-or-less properly built deck (improper nailing patterns aside), from networx.com:

  1. Ledger board pulls away from the house: The leading cause of deck collapse in North America is ledger board failure. Since most decks are attached to the house via a ledger board, and not fully freestanding, a failure here can be devastating. In older construction, the ledger board was often just face nailed to the rim joist. Though more recent codes required lag screws or through bolts, today special brackets like Simpson’s “DTT2Z Deck Tension Ties” are required to meet the IRC stipulation of a 1500-pound rated connector. This bracket extends into the home, providing a solid connection from the house's floor joists through the rim and ledger boards to the deck joists.
  2. Ledger board splits: When a ledger board is attached to a house, a staggered bolt pattern should be used. This reduces the chance that a crack will form along the length of the board, and cause it to fail.
  3. Board rot: Many ledger boards rot prematurely due to lack of proper flashing or construction with standard lumber. All of a deck’s exposed framing lumber should be decay resistant, per guidelines established by the American Wood Council’s DCA-6 publication. Ledgers should also be flashed to prevent moisture attacks from above.
  4. Hanger/toe nail failure: I have seen some decks where the joists were simply toe nailed (nailed at an angle) to the ledger, and others where the joist hangers were falling out due to corroded fasteners or rotted wood. Both of these problems can lead to decks pulling away from the house. On most decks this is an area that can be easily inspected and should be checked regularly.
  5. Post and beam failures: Another leading cause of deck collapse is the failure of posts and beams, usually due to wood rot and undersized components. Posts set in soil, without protection, often rot to a point where they can no longer support loads. The same happens to beams that have not been properly maintained. Dynamic loads of many people out on a deck can lead to sudden and catastrophic failures; injuries and deaths are not uncommon.
  6. Joist failures: This type of failure usually does not cause the entire deck to fall down in an instant, and therefore is not as newsworthy as other types of collapse. However, injuries are still common as joist failures result in holes that you, your family, or your guests may fall through.
  7. Foundation failures: A foundation by its very nature is supposed to be strong and durable. Unfortunately, this is not always the case. I've seen decks merely resting on rocks or blocks, set in soil or on severely undersized pads. For tall decks the failure of foundation supports can cause enough drop to occur to rip even a properly attached ledger from the home. Uplift protection also needs to be provided at the foundation and at the deck level. Gravity may keep it looking secure now, but winds loads can lift even large decks.”

As Boston Decks and Porches points out, code compliance matters: “The building code requires that decks be designed and built to support a live load of 40 pounds per square foot. Live load refers to the weight of people and furniture. Forty pounds does not seem like much, you say. OK, but that’s forty pounds on every square foot. Thus an average sized deck, say, 14 ft by 16 ft, must support 45 people weighing 200 pounds each. When did you last have 45 people on your deck? Probably never. The point is that your deck is probably never fully loaded.”

Age, Weather and Playing the Odds

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The balconies on this building in Berkeley California (area highlighted in red) collapsed. Source

What about the deck materials themselves? At what point do the boards, ledgers, posts, fasteners, and joists lose their physical integrity? What are the warning signs?

Most deck structures are built with treated lumber, so let’s start there. When does it fail? How long does it last?

The good news is that posts, at least, seem to last a very long time. According to this study of 125 chemically treated pine posts in Mississippi, only a handful failed within 50 years. In fact, the researchers estimated that the “time to 50 percent failure in the ACA-and creosote-treated posts were calculated as 96 and 78 years, respectively.” One weakness of this study, however, is that it only tested the vertical strength of the posts, not their lateral strength.

On other deck components, durability reports are mixed. Reading through discussions from deck builders , you’ll find that some treated framing fails in less than ten years, especially if repeatedly wet and dry. The big accelerator to rot and decay seems to be fungal growth.

Metal fasteners can also fail. As deck experts at Trex point out , changes in wood preservatives now require detailing that prior treatments did not. For example, alkaline copper quaternary (ACQ) can cause more rapid failure of metal fastenings if not installed properly. 

Trex suggests that “Because ACQ-treated wood contains much higher concentrations of copper, a barrier is needed between the joists and metal fasteners. When the zinc in galvanized fasteners touches ACQ, corrosion occurs. Joist flashing tape protects your wood with a thin membrane to guard against corrosive preservatives, and holds in place your metal fasteners and screws.”

Assessing the probable remaining life of a deck is an inexact science. What you can do, however, is identify clear warning signs, and either replace or repair them promptly.

  • Swaying. Does the deck move or sway in the wind? That’s generally a bad thing.
  • Cracks and signs of insects. Look for signs of insect damage. Have they found a cozy new home in a rotted
  • Green or black mold. Surface mold should wipe off easily with a mold cleaner. If it doesn’t, the damage may go deeper. Poke a screwdriver into the joist. It should sink more than a few millimeters into the material.
  • Corroded fasteners. Joist hangers and the nails that hold them on should not be rusted significantly.
  • Loose balusters. Tap each vertical baluster with a hammer. If it comes loose, check it for rot and decay. More people are hurt from railing failures than decks collapsing.
  • Cracked/damaged Ledger. Again, as mentioned in item 3 above, because of the key role it plays, examine the ledger board closely. If it has split down the center, replace it immediately. If the anchor fasteners have disappeared into the soft wood, the same caution applies.
  • Base of posts. Check the spot where the post contacts the cement footing or (in an older deck) the ground. Check for rot.

Code Breakers

Decks built with code violations or non-code variations pose the biggest challenge for estimating remaining life. First, these decks may hold only a fraction of the weight of a properly built deck, even when new.

For example, if the deck is framed with 2x8 joists 24 inches apart (not 16 inches), the deck boards should be 2x4s or 2x6, not the standard 5/4-inch decking. If you put the boards at an angle, the distance between joists should be even narrower.

At the same time, a two- or three-story deck built without sufficient cross bracing will be subject to years of stress from wind that might add up over time. For example, fasteners may loosen. Nails may weaken or break.

At the risk of dodging the question, if you own a deck that is clearly sub-code, spend the $150 or so to have a building inspector come over and assess its safety and health. He or she may be able to offer remedial steps you could take to add strength, such as additional metal fasteners or replacement of a few structural pieces.

Be prepared for bad news, however. If the assembly is 10 years old or more, you may be urged to tear it down and build the next one properly.

FAQ

Why do decks fail?

Most catastrophic deck failures can be traced to a problem with the ledger board connection between the deck and the building. This key structural board must be attached with staggered fasteners and flashed to prevent water damage. New building codes also require tension ties that prevent the deck from pulling away from the building.

When should my wood deck railing and balusters be replaced?

More injuries happen from railing failures than any other deck mishap. Often, galvanized metal fasteners fail before the pressure treated or cedar balusters and rails. Some reports also suggest that the new species of fast growing cedar may fail more quickly. Check fastener strength by tapping the balusters away from the rail with a hammer. If they loosen or come off, they probably need new, weather resistant fasteners or replacement. Check also for cracks and splits in the top rail.

Are my old deck boards safe?

More than just the age of your deck boards, the question is whether they meet the current standard for safe load bearing. Newer codes suggest that 2-inch dimensional (2x4- or 2x6-inch planks) be used, if the joist gap is 24 inches. For 16 inch joist spacing, a 5/4-inch plank will suffice, unless the deck boards are put on at an angle. For composite decking materials, the safe gap will typically be about 12 inches.