Thread: Coating Underneath the Deck

Hi all, long time viewer, first time poster,

Im building a deck (8 by 4m) with a carpenter and he's advised that coating underneath the deck is a good idea. The deck is low to the ground and he believes that a coating underneath protects against condensation and moisture buildup.

The wood is Kapur, and I live in Adelaide.

I was hoping to use Flood Spa n Deck. I was thinking to layout the wood, wet it, brush apply the bottom and sides, flip and repeat. Do about 3 planks at a time. I'm worried about the wood drying and the coating running around the edges?

Can anyone advise if coating underneath is necessary or worth it, and if so, what's the best way to apply Spa N deck if coating all round (brush, pad etc).

Thanks

Haroldholt
Your carpenter is telling you the good stuff. Timber is like a sponge it will take up any moisture from the surrounding environment including the ground below especially when your close to the soil. timber expands when wet so the timber will expand and shrink depending on the moisture content, this should be prevented as much as possible. So sealling the underneath side of the timber should be done. I am a bit concerned about your comment -I was thinking to layout the wood, wet it, brush apply the bottom and sides. Wetting the timber befor you coat is not recommended. Unfortunately if a job is worth doing, it is worth doing it properly the first time. That is lay the boards out one at a time or three at a time, but you need to coat all four side of the boards. Ideally two coats all round and then perhaps a final coat on the top after installation. The more coats the better the protection and the coating will last longer before recoating is necessary. Brush or pad is up to you, but I personally use the brush for the first two coats and then the pad for the top coats after installation and then recoating every 12 months.

Hi Hoppy, thanks for the quick reply.

My comment re coating wet is due to the Flood Spa N Deck product instructions. They instruct that the first coat be applied to wet boards.

Maybe this isn't the best product to use if you're coating all round?

Anyone else?

Thanks

I don't agree with Hoppy or your builder. In Adelaide I'm not sure why you'd bother as there is low rainfall and low humidity. If you really did want to bother then simply use a good external 'no prime' acrylic paint and whack on two coats and only on the flat underside not the edges- no need to waste an expensive decking coating on what will never been seen. But the life of your decking will be down to how you initially finish then maintain the top surface facing the sun and the weather. Do it right and a deck can last 30, 40, 50 years, do it wrong and it'll be buggered under 10. But since most people move houses every 10 years or other decks tend to get used by the owner who built it and the next owner - the 3rd owner does a reno and rips out the deck . . . even if it is in OK condition.

I would go with Blosses advice as the greatest degrader of the timber is the suns uv exposure. The underneath of the timber will get none. In saying that it is good to offer the timber somewater proofing underneath and it is easy to do. I use a good external acyrlic on the bottom of some of my decking boards and joists at my house. Partly for protection and partly for looks as you can see under a number of my decks. Once done you never have to do it again.

In relation to Flood Spa&Deck I have applied it to some 200 or so decks over the past 8 years. You are correct with the instructions as they want the first coating applied to wet timber. I would be reluctant to coat the tops of the boards until they are completely fixed as they will be scratched,pencil marked and obtain other bumps and bruises in the laying stages.
I prefer to use the Shurline 9 inch pad applicator that is easily found at Bunnings for 12.98 or so. Not sure how your fixing the boards but if you dig through the archives there is a fairly healthy disccussion on nails vs screws. My vote is on the countersunk stainless screw. However, my bias is from sanding decks on a weekly basis. Eventually most decks with weather exposure get to the point of needing restoring.

Not sure if it is of any interest but if you have a peek at my website and select view work all of the after photos are coated in Flood Spa&Deck ( Jarrah tint)

Good luck

jimj www.restore-a-deck.com.au

Thanks for the replies,

So that's 2 for underneath (my chippy and Hoppy) and 2 for it's not required but won't hurt (Bloss and Jimj).

Any others?

Cheers

Originally Posted by HaroldHolt

Thanks for the replies,

So that's 2 for underneath (my chippy and Hoppy) and 2 for it's not required but won't hurt (Bloss and Jimj).

Any others?

Cheers

Could be that time to flip the coin for heads or tails. Was wondering how the swimming lessons were going?

Jimj

Originally Posted by jimj

Could be that time to flip the coin for heads or tails. Was wondering how the swimming lessons were going?

Jimj

More to the point when did that old Russian sub resurface and drop Harold back to us

Yes boys, I'm moving back from China! The water wasn't as clean as Portsea.........

Anyhow, the deck....

Thanks again for all the advice.

I had a day spare, so I thought I'd undercoat the deck. If anyone else reads this post, then here's some hot tips.

1. I thought the idea of using cheaper self priming acrylic was a good idea, so I bought 4L to "roller on" the deck underside before laying. I got a colour called "brisket" trying to match the colour of the boards.
2. The cheaper acrylic, really wasn't that much cheaper, and I ran out, so needed another 4L and was left with 1L - which is really a waste.
3. When rollering on, you need to apply pressure to get into the grooves - this leads to bleed or running down the edge. This may be visible after laying - so you need to be careful, and/or spend time sanding or wiping the boards
4. You also need to be careful layering and moving the boards, so as not to get paint on the upper surface.

In hindsight, if I wasn't using Spa and Deck, I would have coated around the board using one product. As I was using Spa and Deck, it was problematic to coat all the way round with the manufactures recommendations suggesting wet wood.

Anyhow, the bottoms are coated and the boards are now being layed. I'll finish with Spa and Deck on top.

Thanks all.

Harold