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Adding Joists to an Existing Deck

Posted: Mon Jul 08, 2019 4:18 pm
by dickie37
I bought a house with a deck made of pressure treated wood. It is wearing out and I want to replace the old decking with new Trex composite boards. The problem is that the existing joists are 19” apart. Trex needs 12 to 16” between joists. Is there any way to reinforce the deck with additional joists or braces that will help support the extra weight of their composite boards?

Re: Adding Joists to an Existing Deck

Posted: Mon Jul 08, 2019 4:42 pm
by A. Spruce
If the overall structure of the deck is still in good shape, then yes, you can add new joists between the existing. If there is rot in the existing framing, you're better off to start from scratch with all new materials.

Re: Adding Joists to an Existing Deck

Posted: Mon Jul 08, 2019 7:07 pm
by Shannon
yes I agree, add a joist between each of the existing ones and you are good to go!

Re: Adding Joists to an Existing Deck

Posted: Mon Jul 08, 2019 9:09 pm
by Aaron
Question on behalf of the OP: should the decking be removed before adding joists, or after?

Thinking before, so the joists can be tapped into place snug with the underside of the existing deck boards, and joist hangers fastened tightly and securely.

On the other hand if the deck boards are removed first there may be a better guarantee of levelness using a level. Thoughts?

Either way the new joists are cut slightly long, like a sawblade's width longer than a close measurement so they can be tapped in with a hammer and stay in place before a joist hanger is added. Sound right?

Re: Adding Joists to an Existing Deck

Posted: Mon Jul 08, 2019 10:15 pm
by A. Spruce
Adding joists will go a whole lot easier with the decking removed.

Crown the lumber so that the bow goes up. The new joists may need to be notched slightly or the hangers installed a bit lower to account for the difference in material height from old to new.

Cutting them just slightly snug will definitely help keep them in place while they're nailed or hangers are installed, though, a second set of hands or some creative blocking will do the same thing. A saw blade width will be too much, maybe a 1/16", just kind of depends on what you've got going on.

Re: Adding Joists to an Existing Deck

Posted: Mon Jul 08, 2019 10:41 pm
by Shannon
Yup after decking removed for sure. Just use the rim joist as your guild for height or if you prefer place a straight edge across the two joists on either side as you go.

Re: Adding Joists to an Existing Deck

Posted: Tue Jul 09, 2019 11:47 am
by dickie37
Thanks to ALL for your helpful replies and suggestions. Looks like I will remove the old decking BEFORE adding additional joists. I do have at least one existing joist that is pretty well rotted. Hoping there’s no more than a couple.

Cutting the new joists 1/16” longer makes lots of sense as I will be doing lots of the work by myself.

Trex composite boards are 1/6” (in theory at least). They actually measure 1”x 5 3/8”.

Trex also recommends joist tape to provide a good moisture barrier.

Re: Adding Joists to an Existing Deck

Posted: Tue Jul 09, 2019 12:44 pm
by A. Spruce
You will definitely want to closely inspect all framing members for rot. Minor rot can be repaired, major rot in a lot of components, just replace everything.

Joist tape is a good idea wherever there are supported members, whether it's joists on beams or decking over the joists. To be honest, I've never seen joist tape, however I have seen and used hand cut strips of 30# roofing felt for the same purpose.

Re: Adding Joists to an Existing Deck

Posted: Tue Jul 09, 2019 8:25 pm
by dickie37
Trex apparently sells their brand of joist tape so I plan to use it as they recommend. We live in the South in an area where we have lots of moisture.

Re: Adding Joists to an Existing Deck

Posted: Wed Jul 10, 2019 7:15 am
by Shannon
Yes the joist tape is fairly ne up here (maybe the last 3 yrs?) some of them are simply #30 roof felt in a 4" strip some are a stick on product. In reality anything is better then nothing