Why Hasn’t Your Deck Collapsed?


“Honey,” JoAnn called to her husband, “George stopped by yesterday.  He said our deck violates the building code.” 

Silence.

“You know George, he’s one of the building inspectors in town. ”

“Uh-huh “

“He says our deck violates the building code.”

Silence.

“Fred, listen to me.”

“I am listening,” he replied – as he continued playing with the TV clicker.

“Well, he says our deck violates the building code.”

Silence.

“FRED!”

“Well, he should know.  He’s one of the inspectors downtown.”

 JoAnn glared at her husband. “I just said that.”

“Uh-huh.”  Fred continued to scan the TV shows.

JoAnn walked to the TV, reached down, and unplugged it.

The screen turned black. Whatta ya doin? I was watchin’ that!”deck_collapse

“Not anymore.  Now you’re talking to me.”

 “Ok, ok.  So the deck violates the code.  It hasn’t fallen down.”

Interesting.  It has not fallen down.

Think about it:  if your deck does not meet the building code, why has it not fallen down?

There are several reasons that a non-compliant deck might not collapse.  Let’s take a look.

Non-structural requirements. The Residential Building Code1 is extensive, and its 800+ pages has a myriad of requirements to ensure that decks are solid and safe.  But many of those requirements address safety only, and not structural integrity.  Thus, deck rails may be too short (the code requires they be 36″ high).  Or the balusters on the railings may be Building Codetoo far apart (the code limits the space to less than 4″).  Or the handrail on your stairs may not be “graspable” (the code explains handrail requirements in ten tortured paragraphs2.  None of these violations impact the strength of a deck or its ability to remain upright.  So your deck may fail to meet some specific safety requirements, but its structure may still be solid.  Your deck is unsafe for some technical reasons, but it will not collapse.

Should you care that your handrail is not quite “graspable”?  Maybe it is a half inch too wide.  What’s a half inch?  Among friends?  Well, that half inch may become important when grandma slips on the stairs and cannot catch the handrail.  Or when a traveling salesman injures himself on your stairs and sues you.  Then you may care.3

Design Load.  The building code requires that decks be designed and built to support a live load of 40 pounds per square foot4. 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   e v e r y   square foot.  Thus an average sized deck, say, 14ft by 16, 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.

However, if your deck is structurally flawed, maybe with under-sized joists or a substandard connection to the house, it may fail when only partially loaded.  Perhaps as few as two dozen people would cause a catastrophe.  So far you’ve had only 18 people on it, and your deck is just waiting for more people to join the party.  And then it will collapse.

Skip Day deck-collapse BEFORE edited   Skip Day deck-collapse edited2Safety Margins.  Implicit in the building code are safety standards for materials and fasteners.  Industry organizations thoroughly test materials and fasteners under controlled conditions, and they determine each material’s breaking point and set a safe, “allowable” load for its use in the real world5.  Between each material’s breaking point and its allowable load lies its safety margin.

Wood is created by Mother Nature, who as we know, is notoriously fickle.  She twists grain lines, randomly inserts knots, and hides devious cracks in her wood.  Each of these reduces the strength of a particular board.  Engineers, of course, do not trust Mother and penalize her products with safety margins, like 2.5 to 16 or even 3 to 1.  Thus, the materials and connectors for your 14 x 16 deck could support 112 people in theory, but the building code resets the load allowed in the real world to only 45 people.  (Which is convenient, since you could not cram over a hundred people on a deck that size.). That 2.5 to 1 difference protects you and your guests from all the little issues that degrade even a properly build deck over time – shrinking wood, rusting nails, cracks in the wood, etc.  The deck feels safe because it is safe.  The safety margins guarantee it.

But what about an improperly built deck?  That was our original issue.  Suppose the code failures that George the inspector found were structural issues (like an undersized beam or missing joist hangers).  Maybe you’ve have 18 friends partying on your the deck, and it did not collapse.  Good news.  You could invite a few more people onto your deck, and it probably would not collapse.  A few more is still under the standard limit.  Right?  Your deck is OK….right?

See that little word probably four sentences ago?  Let’s read it again: it probably would not collapse.  Your deck has problems, and you’re depending on probably?  You and your flawed deck are floating in that safety margin — between when it may collapse and when it will collapse.  Indeed, it’s not a safety margin;  it’s a danger margin.

So if you suspect your deck may be unsafe, what should you do?   Fred’s not going to help you, so I will.  Here are a few tips:

  1. Look at where the deck connects to your house. The majority of deck catastrophes Lag Screws labelbegin right there.  You should see big screws fastening the deck ledger to the house.  Not nails.  Nails are thin, smooth, and weak.  If you deck is attached with only nails, here’s the good news: you can add screws.  Half-inch diameter, galvanized lag screws are good, but you’ll have to pre-drill before you install them.  Better are Ledger Lok®  screws;  they do not require pre-drilling and install in a few seconds (really).  It is easy; here’s how.
  2. Each joist must be fastened to the ledger with a joist hanger — with every nail hole filled with a nail.  If you don’t know a joist from a ledger, see my explanation of deck terms (illustrated).
  3. Check for rot.  Use a screw driver and poke hard at all the framing members.  If the wood is soft, it is rotten and must be replaced.
  4. Tall decks are more vulnerable to structural problems.  Does your deck wobble?  Stand in your yard and invite a couple of teenagers to sway and dance on the deck.  No, not 12 teenagers.  They just might create a catastrophe.  Start with two.

For more information about structural issues, see my article about deck safety.

Hopefully, George the inspector will not award your deck this little prize:

Unsafe sign

But that warning would be better than the disaster in New Albany, Indiana a few years ago.

  ————————–

We at Archadeck of Suburban Boston offer professional design and build services for clients west and north of Boston.  Over the past 23 years we have designed and built over 800 projects.  We have enhanced the depth of our expertise by limiting our work to decks, porches, sunrooms, and patios.  To view some of these projects, visit our websiteTo learn how we treat our clients, check us on Angie’s List or read a recent article about us in Remodeling MagazineFor a free design consultation and a relaxed and rewarding design and construction experience, contact us via e-mail, subboston@archadeck.net or by phone, 781-273-3500.

© 2016 Advantage Design & Construction, Inc.

Maddie

Oops!  I forgot that the names of those prestigious alphabet soup organizations are registered and their documents are copyrighted.  Forgive me;  please sprinkle these wherever you like: ®®®®®©©®®©.

NOTES:


  1. The International Residential Code 2009. IRC-2009 and its updated version, IRC-2012, are the recognized residential building codes for most states in the US. 
  2. I’m sorry; is “tortured” too harsh a description?  You judge: “…The finger recess shall begin within a distance of ¾ inch (19mm) measured vertically from the tallest portion of the profile and achieve a depth of at least 5/16 inch (8mm) within 7/8 inch (22mm) below the widest portion of the profile. This required depth shall continue for at least 3/8 inch (10mm) to a level….”.  Nice, eh?  This is from para R311.7.7.3 (2) of IRC 2009. 
  3. Someone else who cares may work at your insurance company. I have seen an insurance adjuster who was investigating a roof claim notice a deck handrail issue and send the homeowner a written warning about the liability coverage on their deck. 
  4. In most states the live load for decks is 40 psf, per IRC 2009 (or the 2012 edition), table R301.5.  This live load includes snow.  In most northern states, the official snow load is 50 psf, which can be reached with four feet of snow.   For a full complaint about this dangerous loophole, see my article about snow on decks
  5. Testing lumber and determining allowable loads is complex and involves numerous organizations.  Pressure Treated (PT) Southern Yellow Pine (SYP) is the standard lumber for framing decks in eastern US.  In 2012 the Southern Pine Inspection Bureau (SPIB) tested thousands of SYP boards, in collaboration with the U.S. Forest Products Laboratory (FPL).   (Here’s a readable summary.)  The results were reviewed and certified by the American Lumber Standard Committee (ALSC) Board of Review.  From these results,  American Wood Council (AWC) engineers calculated the relevant design values and adjustment factors (like Wet Service Factor), and published very detailed tables in the American Wood Council’s widely respected National Design Specification (NDS) for Wood Construction, 2015 edition.  Building Code organizations, notably the International Code Council (ICC), will publish specific span charts in their updated 800+ page International Residential Code 2015 (IRC 2015), which contractors follow and local building officials enforce.  I should say strictly enforce  —  because who’s going to ague with the SYPIB/ FPL/ ALSC/ AWC/ NDS/ ICC/ and IRC?   Not to mention George. 
  6. Reference:  International Building Code 2012 (and other editions), para. 1710.3.1.  The IBC 2012 is available on-line;  here’s the paragraph that sets the 2.5 to 1 standard
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Categories: Deck safety, Deck Structure, Pressure Treated WoodTags: , , , , ,

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