Tar paper over joists/mainbeams do or don't.

by Troy
(Saint John, New Brunswick, Canada)

With ACQ pressure treated lumber, is there any known benefit as per the manufacture to capping contacting surfaces with tar paper, for example, joists to mainbeam, or joist to decking?

Comments for Tar paper over joists/mainbeams do or don't.

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Jun 08, 2011
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ACQ Lumber and ZMAX Galvanized Coating
by: bones58gdi

The latest ACQ lumber will attack ZMAX in a damp environment.

Do this test by getting two ZMAX brackets three pieces of lumber. Connect two of pieces as joists using the joist connectors to the third piece acting as a band board or rim joist


As you connect the connectors and joists, line the one connector at all points where it contacts wood with Tar Paper. Leave the other without any paper.

Then wait one year. You will be sickened. My test was by accident. I pulled some brackets down after 6 months.

Hence forth I became an expert on ACQ and ZMAX.

Tar paper is highly recommended by Vycor. That could NOT be a better recommendation.

Oct 26, 2010
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Always a wise idea
by: Editor - Rich Bergman

I suppose if you live in a very dry climate, using felt paper, tar paper or other joist capping systems is not important. But if you are in a damp area, I do believe if done properly it will add years of life to the strucuture of your deck and also to the deck board materials. My thought is the best benefit would be to cover the joists where the decking sits on because that is where rain water has easiest access and has a tendency to pool. This is the first line of defence so to speak.

After that you could certainly cover the supporting beam where the joists rest on. However, this area will be less exposed to continuous moisture - it can still get wet. For sure, especially in heavy rain. But its a less critical area.

I have read a number of great articles in Fine Homebuilding and Journal of Light Construction and it seems like an ounce of prevention is worth a pound of cure.

Overall, I would say it is a wise thing to do and is so inexpensive that it is well worth it no matter how long you plan to live in your home. The next guy that buys your house will thank you when he has a deck that isn't going to rot out really quickly.

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