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Extending joists on an existing deck

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Oct 03, 2013, 08:01 PM
Extending joists on an existing deck
I have been scouring the net and a number of boards and blogs but haven't quite got a definite answer to this. I think I know but I want to be sure before I proceed.

I am replacing all the top boards and railing on my 42' x 20' deck. This was built in the early 70's and everything has just rotted away over time. The joists and the girders look to be in excellent condition. No rot. No slip (the deck is built overhanging a hill at the end of my property). So far it seems like an almost purely redwood project to me...except one thing:

For some odd reason the builders chose to NOT build a true rectangle...they left a 2ft "chunk" off the end on the right half of the deck. So, instead of 42ft wide and 20ft deep all the way across it's 21'w x20'd for the left half and 21'w x 18'd for the right half. They left a 21'x2' "notch" off the far end of right half for some reason.

I would like to add that additional 2' back onto the right half completing the rectangle. This would require extending half the joists (11 of them) 2'. The ends of the joists extend out from the last girder the entire wide of the deck. Those on the left half extend out about 3'. Those on the right half about 1'.

Since there is no support at the end of the joists I've been working on ways of adding the last 2' and still maintain integrity (and hopefully pass an eventual inspection). The joists are 2x12 PT and I've thought of butting the ends together and sandwiching the joint between 2 1' pieces of 2x12 (one on each side) secured with 4 1/2" bolts (2 on each side of the joint through-and-through). I've also considered using some existing joist hardware but can't find anything specifically for this task, which tends to make me believe it's not supposed to be done.

I'd be interested in hearing from any of the experts out there. I'm a pretty handy guy and I tend to overbuild my projects (I once built a waterbed frame you could park a Mac truck on) but this one could hurt someone if I'm wrong. If the end I build fails the drop is about 20' onto a chain link fence and then a long way down a steep hill...not a pretty picture.

Any thoughts?

Thank you in advance...


This message has been edited. Last edited by: ntm91307,
Oct 03, 2013, 10:01 PM
Question: How are the joists supported? If the joists are attached to a double outer ribbon then extending things will be difficult. If the existing joists are directly bearing on top of a beam that is in turn supported by posts, then you are simply extending a deck and making it cantilevered.

If it's cantilevered, then the two main factors are how large is the support beam(s) and how far from the beam will the cantilever joint be. As a general structural rule, you'll want to have 2/3 of your extra joists attached to the existing joists and 1/3 for the addition - so you are looking at 6' long 2x12 material. This can be nailed, bolted or both to the existing joists.

Basically, as long as you have a beam underneath and supporting the existing joists a 2' extension should be possible, however it would be better if you could either post some pics to confirm the structure or get someone with some structural knowledge to take a look at what you plan to do.

Oct 04, 2013, 01:54 PM
The joists are anchored to the side of the concrete slab at one end (about a foot thick), that is my backyard essentially, and supported at the other by a beam (looks like roughly 4x16) supported by posts.

I see your solution and it's a simple one...kind of wish I'd thought of it myself. I didn't know the 1/3 - 2/3 ratio but it makes nothing but sense given the fact that at 2ft out a 260lb man (me) is going to exert about 520lbs of force on that joint. Put 5 or 6 of us out there (I throw big parties) and you've got few baby elephants stomping around out on the ends of a few measly boards. I get it...Smile

Thanks for your help...Smile