How to waterproof under deck?

Bubbles8712

We have a 5.0L X 3.0W X 1.7H deck which has an enclosed area underneath - except for the rain that comes from above!

I want to waterproof underneath the deck so I can have an additional storage space.

My thinking is to get 17mm polycarbonate sheeting and use 42 X 19mm H3 DAR Pine to help create a 1:10 fall across the length.

The deck has 13 joists. I’ll screw the 19mm pine underneath 3 of these joists, using spacers to help create a 60mm fall across the whole length. Photo below.

I’ll then attach the polysheets to the joists that have the 19mm pine screwed in. Then a gutter at the end to catch the water.

Questions

Is the 42 x 19mm H3 pine suitable to be used as described? It is quite cheaper than other H3 pine and want to ensure it’ll be okay to be damp for the most part of winter. The deck won’t dry much during winter.
Is a fall of 10mm every 1m sufficient? I’ve got about 60mm across 5m.
Any other recommendations?

cheers

Nailbag

Hi @Bubbles8712 the trays were custom bent on site by a roofing plumber running into again a custom gutter tray. The trays themselves are not fixed to or by the decking boards. Instead they are sitting on recessed yellow tongue flooring as per the photo. The trays are screwed into the side of deck joists, Then custom flashing sat over the joists/tray sides

The rubber strips on top of the joists are as @EricL rightly picked up is Protectadeck.

This provided a waterproof seal from there decking screws as well as under-deck protection

This section of the deck extension was over what was too be a single carport. The more complicated one, was a pointed section. on the front of the house separating the deck on each side. I wanted to make a room underneath waterproof while removing the pointed one above. These photos will give you a better understanding how this task was achieved.

Divided deckPointed section demolishedNew wall installedTrays installedExternal custom guttering installedTray caps installedWIP

MikeyDIY

Hi @Nailbag sorry to hijack this older thread but this solution is genius (!) and I was wondering if you have more photos of the system? Or have you written it up online somewhere? I’d love to see how the guttering worked. Is the colourbond work limited to the trays, side flashing and guttering? sorry again and thanks in advance!

Nailbag

Sorry @MikeyDIY these are all the photos I have. I utilised the expertise of a roofing plumber along with a my draft-person to achieve the end result in providing my end goal, in what I needed to build myself in preparation for the plumber and be within building regulations as an owner builder. This was a very complex project and was fortunate enough to have a young but very experiences roofing plumber to work with. I had the vision, and they gave the experience I needed to pull it off. My recommendations would be to do the same mate.

regards, Nailbag

Chappie4

G'day @Nailbag

looking to rejuvinate an old deck area and liked the under deck trays (colourbond sheeting). Is this custom made?

EricL

Hello @Chappie4

Let me tag @Nailbag as well to make sure they are aware of your kind words and question. I believe they are custom-made drain trays that were put under the deck. I'm sure Nailbag will confirm and give further details regarding its construction.

If you have any other questions we can help with, please let us know.

Eric

Nailbag

Hi @Chappie4

Yes mate the trays themselves were ordered to fit within 450mm centred deck joists. My plumber who specialises in roofing installed them and customer cut and formed the end to meet guttering also customer formed by hand on-site. I then measured the width across the top of the joists and ordered U-shaped flashing to run the length of each joist to complete the final seal. This was all to provide a waterproof barrier to a room underneath in the front of the house, and a carport to the side. As you can see by the workmanship, its extremely impressive for just a young tradie.

Nailbag