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To make the space under my deck more usable when it rains, I've installed rubber screen spline between the seams on my composite deck and also installed gutters at the low ends to channel rain water away...The deck is built with a small slope, away from where it attaches to the main building...I can see that rain water does flow down the 'sealed' seams and into the gutters.

This has reduced the amount of water that flows between the deck seams to the lower 'porch' area by 98%...but there are still some areas where water will penetrate through the seams (small drips) instead of being directed away.

So I'd like to know what type of additional sealer (see picture below) - either a paint or a self-leveling fluid caulk I could apply into the seams (on top of the spline) to eliminate the small leaks that still occur in places where there are very small gaps between the deck boards and spline.

(Note - the screen spline was rolled into each seam and it sits about 1/4 inch below the deck surface. The seams are about 1/8 inch wide. Each deck board is one piece, 7/8" x 6" x 11 foot) Ideally I'd like to apply some kind of sealer that is about as viscus as water so it would flow down the slope of the seams and into the small gaps that are currently allowing rain water drips...

I'm thinking a thinned house paint or a semi-harding, enameled deck sealer might do the job - - setting up somewhat flexible to allow for expansion and contraction...I understand that I would probably need to redo this every couple years...I really don't want to use regular caulk if I can avoid it.enter image description here

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  • Hello, and welcome to Stack Exchange. I'd be worried about the decking rotting over time.
    – Daniel Griscom
    Commented May 7, 2016 at 14:19
  • Thx for your comment - the composite decking is over 10 years old with no issues - even the color hasn't changed much. I like the no maintenance aspect! This link link should go to a picture I drew of what I 'hope' sealing the screen spline does...
    – sharpshorts
    Commented May 7, 2016 at 15:48
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    I wasn't clear: using the decking as a roof might capture water against/in the decking with no chance of it drying out. Being soaked constantly could cause rotting that the intermittently-wet conditions before hadn't caused.
    – Daniel Griscom
    Commented May 7, 2016 at 15:52
  • I considered drainage - the deck has a slight pitch to it and with the spline installed, water now drains** down the channel created by the seams and exits out the far end, where I want it to go. Underneath, the decking is still exposed to air movement...so if the underneath never gets wet, either will the wood framing that supports it...
    – sharpshorts
    Commented May 7, 2016 at 18:46

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Sooner or later the splining will deteriorate, and whatever sealant you put in may as well. Also, you are creating little pools in the gaps that may be breeding grounds for mosquitos in wet weather.

Have you considered a waterproof underlayer instead of sealing the seams? You could use any of various roof panels, such as pvc, polycarbonate, asphalt composition or even metal. These would only need to be sealed at their edges, a relatively few linear feet that are easily accessible from underneath. And many of them are relatively cheap.

This layer can be easily sloped and you could direct drips in whatever direction you choose.

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  • Thx for your commentsI'm OK with deterioration of the sealant over the years, if I uses house paint as the 'sealant' it should last as long as the rest of the exterior paint...The splining has withstood for 20 years in my screened porch...
    – sharpshorts
    Commented May 7, 2016 at 15:18
  • @sharpshorts trying to trap the water from above is a bad idea. Decks are meant to drain so the components can properly dry. It works on your screened porch because I assume your screen porch has a roof...so no water nor sun wear. Even composite needs to dry out--but the bigger issue is the structural members the composite is attached too. Bib's advice is good...the typical way to handle this is UNDER the deck using a system as described.
    – DA01
    Commented May 7, 2016 at 16:01
  • The deck has a pitch to it and water does drain down the channel created by the spline in the seams and now water exits where I want it to...it's just that there still are areas that drip-drip-drip when it rains, thus the need for a sealer that's viscus when it is applied over the spline and then set's up, filling the minor voids between the composite and spline...Underneath, the decking and wood framing is still exposed to drying air movement but if it never gets wet underneath, so much the better...(the screen porch is exposed to the same sunlight and weather conditions as the newer deck)
    – sharpshorts
    Commented May 7, 2016 at 19:14
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Been there done it. Caulking between the composite deck boards works if sloped right. I sloped mine 1" on 6' On one deck I have plastic panels under it and that was a bad idea cause the mains and underside cant breath and is rotting. I am about to replace the 8 year old rotted treated boards with composite like the others and caulk it too. The minor amount of pooling dries in no time. The peal and stick under the deck boards is worst of all cause it rots the deck boards really fast. They should make a deck board with a channel on the edge that locks with the adjacent board with an 1/8" of play for expansion and contraction. If you caulked that, it would work like a champ and small cracks would drain down the channel (inside the board) to drain. My idea. I get half of everything when you make it. ha

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They do make deck boards that lock together. It’s aluminum decking and it’s far superior to composite decking. It’s made to use as a water tight deck above patios or other decks but we used it everywhere including our front porch. Don’t worry about the aluminum getting hot in the summer, it doesn’t get any hotter than wood or composite that’s laying in the sun. They have many different colors and it looks great. They have railing too and there’s many trim pieces that make it look great. From a distance our looks like wood. Up close it looks great and has a textured skid resistant surface.

I’ll never have to do anything else to our decks again. The framing I wrapped in metal just like the fascia boards on our house and under the deck I used vented vinyl soffit material. This deck is maintenance free and will stand for a lifetime. Go metal, you won’t regret it. It’s not much more than composite is will be cheaper in the long run.

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Not sure if you still have this problem or if this is a solution (but it might be for others):

Dexerdry came out in the last couple of years. If you pitch the deck and do proper water management, with a good install, it should work.

https://dexerdry.com

Dexerdry Insert

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