Adding on existing deck

by Jim
(Mechanicsburg, PA)

I have a 12 x 10 deck. the deck comes off the house 12' and is 10' wide.

I would like to add on the deck and make it 18' off house x 16' wide. My question is about seams joining the new section of deck to old section.

It would be nice to try to make the deck addition consistent with existing. Any suggestions?

Editor's Comments

This is a good question and a common challenge many people face when they want to increase the size of an older deck.

The answer could get quite detailed head off in a number of directions. So I will just touch on a couple important points and then if there are other builders out there who would like to say something, please carry on where I leave off.

To make an addition seamless will require a bit more work than simply framing another rectangle and setting it up on some beams and bolting it on to the old deck.

In fact, that is not at all how I would approach this building project. At least if you really want it to look seamless.

Extending Beams

Let's start with the support of the structure. You should determine how to extend the length of the beam to carry the new area of deck.

This will entail more support posts. It may also mean trying to balance out the location of the support posts with the older posts. Otherwise it will have a tell tale appearance of an addition.


Once the beam is built you will have to carefully expose the band joist on the house, flash and examine it from inside the basement to see if you can bolt through it.

Extending Joists

To extend the deck you should sister some new joists to the old ones and over lap them safely - at least three feet and screw liberally. Even bolt them together.


Then lastly, to create a seamless look you will have to remove some of the old decking and intersperse new decking that is staggered back deeper into the old deck.

This will give the appearance that the new deck was meant to be this new size. None of the deck boards will show where the new framing begins because of the staggering effect.

This is a quick overview and surely not the final word but should help you out.

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