HOW TO BUILD A DECK: Avoid the #1 Deck Construction Mistake

how to build a deck overallQ: What should I do about a deck that’s bolted to my house without provisions waterproofing? The edge of the floor frame is attached to the house frame, but with no flashing for waterproofing the connection. If I lift the bottom of vinyl siding I can see the house wrap underneath. There’s got to be leaking going on.

A: Yes, leaking is almost certainly going on. Although thousands of decks are built quick-and-dirty like this, it’s still no reason to be complacent. You’re wise to pay attention to this detail and correct the shortcoming before wall rot sets in. Does the wood under the siding look rot-free? Assuming this is the case, start the repair by removing the row of deck boards closest to the house. This gives you room to slip some kind of flashing up underneath the siding.  Aluminum flashing is readily available, and though copper is more expensive it’s easier to work with because it’s softer. Also, the problem with aluminum is that it corrodes quickly in contact with pressure treated lumber. Since most decks are built with this kind of wood, copper is a much better bet. 24 gauge will work well. Extend this flashing up behind the siding as far as you can (4” would be a minimum), then down over the top edge of the ledger board and an inch or so down the outside face around each joist. When you replace the row of deck boards you removed earlier, cut them into shorter lengths, then place them on top of the flashing with an open joint at every couple of joists. This gap provides a place for water to drain down and out.

Work like this isn’t easy and it doesn’t happen quickly. The building who made your deck would probably have had to spend just as long completing the flashing detail as he would have installing half of the deck board. Just the same, waterproofing needs to happen to prevent rot of your house frame.

Click below for a 1 minute video tour of the problem and how to avoid it.

Does your deck need to be refinished? Click here for a detailed guide on how to get the longest lasting deck finish results for the least amount of effort. You’ll also find specific deck stain recommendations based on field trials I’ve conducted since 1990.


Posted on August 15th, 2019