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

Jason

Hi @Bubbles8712,

Hope all is well.

My apologies but both our resident Bunnings D.I.Y. experts are not able to join us on the site today. Let me tag some other really helpful community members in @Nailbag, @JacobZ95 and @Jewelleryrescue to get their thoughts on your plan.

Thanks,

Jason

JacobZ95

Hi @Bubbles8712,

I think you may have gotten yourself mixed up when calculating the fall. To achieve 1:10 pitch over 5m you’d need to create a fall of 50cm, not 50mm which would be a 1:100 pitch.

Most roofing manufacturers advise a minimum of 5 degrees slope to allow water runoff which would be 88mm across every metre. So to achieve this you’d need a fall of 44cm over the 5m span. With this in mind, I think a more expensive material is a must unfortunately.

Also, knowing how flexible these polycarbonate sheets are, I’d be concerned that with only 3 attachment points over that length you’d risk the sheets flexing and the water pooling.

What you are aiming to achieve is definitely possible, but I think you’ve underestimated what is required to achieve it.

I’d have a look into the Siniat Suspended Ceiling System as a potential method of achieving this.

Jacob

Bubbles8712

Thanks for the insights, Jacob.

Thanks for correcting my math - I’ll ensure there’s sufficient fall to avoid pooling.

I had considered using traditional colorbond sheets instead of the polycarbonate, and having them screwed into each of the joists to support the extra weight. I could still screw the polycarbonate sheets to each of the joists to prevent the flexing. This will mean every 50cm or so the sheets will be secured.

I wasn’t aware of the siniat system - only the Underdeck system, which is quite pricey. I’ll get a quote from them anyway.

If the polycarbonate fails, I wouldn’t have spent that much on it!!

JacobZ95

Hi @Bubbles8712,

I think for this application colorbond sheets would be better just because of the increased rigidity over the polycarbonate sheets. I'd just be concerned that if there is a large downpour the weight of the water could cause some issues.

Keep in mind that Siniat are purely a manufacturer, they do not offer installation services. You'd likely have to find a contractor to install this for you.

Cheers,

Jacob

Nailbag

If the deck is screwed down, then the easiest solution is to remove them and get made up colourbond trays that will sit between the joists. They will need to be angled to allow for drainage. Then run a gutter along the length and have it connected to your storm water by a plumber.

I haver done the exact same project myself. as per the attached photos.

all the best.

EricL

Hello @Bubbles8712

Just to add to the suggestions made, I also recommend using larger timber piece such as 70x35 H3 and coating its entire surface with Gripset Betta 1L Waterproofing Membrane Bitumen Rubber. The more space you have underneath the better the airflow and any dampness or moisture will dry off quickly.

If you need further assistance, please let us know.

Eric

Bubbles8712

Thanks for the tips, Eric. I like the idea of more air flow and the membrane coating.

Bubbles8712

Hi @Nailbag

The Colorbond Trays you use, where do you purchase these from and are they custom made to size?

Our deck is 3000mm wide so I could flush the trays against the wall (left side of photo below), run them between each joist to the opposite side on an angle to a gutter. There is a stormwater pipe near which a plumber could tap into for us.

I notice the joists in your photo have black tape (?) around them. I assume this is to avoid moisture from accumulating in the joists?

...and my last question...how did you fix the trays to the deck? I can't see any screws!

Thanks heaps!

EricL

Hello @Bubbles8712

Let me tag @Nailbag to make them aware of your questions. The tape you are referring to is called CAgroup 50mm x 20m Bearer Protector G-Tape. It comes in a larger size of 100mm if you wish it to overlap the sides.

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

Eric