How to Build a Deck: Prevent Rot at the Patio Door

Plan on building a deck?

Decking installed below doorsill to prevent rot

Plan on building a deck? Reduce the chance of water infiltration—install the decking a few inches below patio and entry door thresholds. It’s true that decking installed at the same level as the interior flooring looks great and makes for a smooth transition, but unless you live in the desert, it’s not a good idea.

When decking is installed up tight to the bottom of a door, that door is not only subject to falling rain, but also to the rain that splashes off the decking. And if you live in a northern climate, even a modest snowfall will expose your door to the powerful forces of water melting and refreezing.

Back when I was a siding contractor, water infiltration at doors with flush decks caused all kinds of problems. We frequently found rotten doorsills, damaged flooring and the beginnings of serious structural damage on houses only a few years old. There’s no magic number, but I would recommend a minimum of 2 in. from the top of the decking to the bottom of the patio door threshold. I opted for 3-1/2 in.

Many doors have a sill or threshold that needs to be supported, so don’t forget to install a kick plate under the door. I ripped down a piece of decking trim board and painted it the same color as the door. Kick plates do get “kicked.” Eventually, I am going to replace mine with a PVC or composite board—either will take a beating and look great without my having to stain or paint it every year.

— Mark Petersen, Contributing Editor

Need more advice about building a deck?

Rebuild an Old Deck With New Decking and Railings

Attaching a New Deck to a House the Correct Method

How to Build the Deck of Your Dreams