Decks are one of the most common do-it-yourself projects. Unfortunately many deck builders fall into the category of shouldn’t-do-it-themselves.
It’s easy to hammer a few boards together, but it’s a lot harder to do with the best techniques, tools, equipment, and devices so that the deck has the best chance of safely lasting many decades. Almost every deck that I see has some problem, and many are built so poorly that I advise my client to just tear it down and rebuild.
A deck has many details to inspect, but there’s one deck problem in particular that drives me crazy. And it seriously weakens the deck structure so it’s worth looking at more closely.
Here’s the typical way to build a standard deck:
A ledger board is securely attached to the house, with bolts staggered up and down and spaced no more than about every 8 inches. Then the floor joists are installed perpendicular to the house and ledger board, and they’re attached to the ledger using joist hangers. At the far end the floor joists sit on a beam that’s supported on both ends by posts that sit on concrete footings.
Now let’s take a look at the load path takes to the ground. When you’re standing on the deck, your weight is shared among several joists (but we’ll look at just one joist in this example), and that load (green lines) moves horizontally out to the beam and in to the ledger board. When that load hits the ledger board it only has to move a very short distance horizontally before it hits the very secure connection of the ledger board to the house structure. At least we hope that connection is secure, but that’s a topic for another day.
When your load hits the beam it then might have to move quite a long distance horizontally before it gets to the posts and into the ground. Because the load might have to move a long distance horizontally the beam needs to be doubled – it needs to be made up of 2 pieces of 2x lumber. Everything’s fine with this design.
But here’s how some decks are built.
There’s a ledger against the house, but then there are boards at both ends of the ledger coming out perpendicular to the house. These boards are usually not attached very well to the ledger board — that’s the problem.
But now all the floor joists are attached to these side boards. Now let’s take a look at the load path. When you stand on the deck the load is shared among several joists, transferred horizontally through the joists over to the side board, and then horizontally again over to the ledger at the house and over to the post at the corner of the deck. But because the load is moving a long way horizontally these side boards should be doubled – they should each be made up of 2 pieces of 2x lumber. Otherwise they’re not strong enough to carry all that load. (Remember that the several floor joists share the load so they’re OK as single pieces of lumber.)
But when the load gets to the ledger board, the only thing holding this deck up is the single connection between the side board and the ledger (red oval in the figure). Often times this connection is only a bunch of nails. Now think about this. About half of the entire load on this deck – and that might be a whole deck-full of people if you’re having a party – is supported only by this bunch of nails. And then suppose that this deck is 10 or 20 years old, and it’s been subject to rain, sun, hot, cold, over and over and over. So that connection isn’t nearly as strong as it was when originally built.
This connection just isn’t strong enough, and this is the wrong way to build a deck.