Deck Flashing and Basement Water Problems

How to Install Deck Flashing the Correct Way

By Mark J. Donovan




Years ago I decided to finish the basement in my home, however I first had to deal with a water problem associated with my deck. When the deck was installed the deck contractor used 1” Z door/window flashing between the house clapboards and the deck boards. The 1” height was nowhere near high enough to prevent the rain water from slipping up behind the clapboards and going behind the 1” high flashing. To make matters even worse, the deck contractor had put nails through the flashing as he fastened the deck boards to the deck ledger board / floor joist.

As a result, every time it rained water would filter through the nail holes in the deck flashing and slide behind the house clapboards below the deck. As a result, all the rain water that seeped behind the clapboards worked its way down into the basement.

To resolve this problem I had to remove the deck boards nearest the home, and replace the Z window/door flashing with 6” wide deck flashing that slid up 3” behind the house clapboards and down over the deck ledger board that attached to the house. In addition I added an additional deck floor joist about 2” away from the deck ledger board to use as a nailing surface for the deck boards. Finally, I re-installed the deck boards.

So if you have a basement water problem that does not appear to be associated with ground water seepage, check along the basement sills for water or moisture.

 Examples of deck flashing.

Chances are you will find that water is seeping in behind the house clapboards. If there is a deck along that side of the home where you see the basement water problem, most likely you have a similar deck flashing problem.


For help on building a deck, see HomeAdditionPlus.com’s Deck Installation Bid Sheet. The Deck Installation Bid Sheet will teach your how to hire the right deck contractor, and help to ensure that your deck project is completed on time and budget.

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