Waterproofing top of joists

by Bill
(London, ON)

Twenty-one years ago I built a cedar deck. This year I'm replacing all the horizontal wood (I left it to the elements for all this time, but now the horizontal wood has worn out).

In order to screw deck boards from below (the deck is high enough for me to work under it) I nailed a 1X4 board on top of the joist and screwed the boards from below on both sides (I guess using 1x3s and securing boards on just one side would work as well). Twenty-one years ago, I just used cedar 1x4s, but I noticed that some of them have rotted away.

So, this time I'm looking for ways to waterproof the 1x4s before I put the decking boards on top of them. Would something like Black Knight Roof Sealer work? Or what about Blue Seal Waterproof Rubber Membrane? Or perhaps tar paper is a better choice? Looking forward to your comments!

Editor's Comments

The problem of deck boards or the joists below rotting can be more severe in some parts of the country than others. Certainly in more humid areas with frequent rain there will be a greater chance of rot taking hold between the gaps of the boards over the joists.

If you used 1x4 over top each joist in order to screw the boards from underneath you may have increased the chances of rot starting because instead of 1.5" long joist surface for rain and debris to settle on, it was increased to 3.5" or 4".

However, getting twenty one years of use out of your deck is beyond the average in North America.


You mentioned using two different kinds of water sealing compounds. A sealant that is painted on may work but I would shy away from that and go with something like an actual stripped membrane. Something that is only 2" wide. Grace Ice and Watershield is one idea or something from Blueskin is another.


Alternatively there is a cap system called Rot-Not that seems to go directly after this problem.

Felt Paper
And of course a good heavy 15lb felt paper has always been shown to work quite well. Although you have to staple it down or apply a roofing sealant on the edge of the joist and lay it over top. Stapling is easier.

Joist Clips

Lastly you could use a joist spacer that is domed and sheds the water and debris and locks permanently under each deck board.

At any rate, if you use one of the above solutions you should expect more than another twenty-one years of longevity from the deck. That makes at least forty-two years of service for two decks. Impressive.

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Sep 30, 2015
The deck is finished now
by: Bill

Thanks for the editorial comments.

I ended up topping the joists and the surrounding frame to which they're attached with 1x3 strips of Sienna p/t wood (flush on one side of the joist, leaving a 'lip' on the other side). Then I covered those with tar paper. I cut each strip so there's an overhang of an inch or two drooping down on each side to divert water away from the joists.

I used a few staples to keep them in place initially, but they really got fastened well once I screwed the deck boards in place from below (using the aforementioned 'lip'). That should go a long way towards prolonging the life of the supporting structure.

In fact, I fastened boards and railings such that when you stand on the deck there's not a single screw to be seen and thus there are no screw holes that can collect water and start to rot. All horizontal wood (boards, railings, steps) is screwed or bolted into place from below and all vertical wood (posts, balusters) is screwed or bolted into place from the outside of the deck, where each screw or bolt is countersunk into the wood so it's less likely to get wet.

Now, if I can only keep the ants from 'settling in' in the years to come!

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