A Waterproof deck question

Here's a question from Leigh WM from Bacchus Marsh in South Australia. He wants a waterproof deck so that he can use the space under.

We're wanting to find out if we can put a 'roof' under an existing deck to make storage/work space underneath.  I guess we'd sort of need to put battens of increasing thickness underneath to fix iron to to get run off?  Or some sort of upsidedown mini trusses?
Cheers n ginger beers,

Hi Leigh,
Yes, I reckon battens on the underside of the floor joist will do the trick.  Invariably you are after maximum head height so less is better.  I've seen a few done like that.  You don't need too many either as it is not a regular roof.
I'd be thinking of using trimdek or one of the other deeper profiles, so I don't need as much slope. They get fairly crappy up there after a few years too so the deeper pan helps.

They don't look real flash, but if you want to get a better look you are up for pulling up the boards, replacing with cement sheet, ceramic tiles etc.   A lot of expense but it has to be done in the case of a more permanent habitable room under.

Re: deck floor roof...
Ta mate. Just wondering with the cement sheet, how are they waterproofed at the joins? And would the deck then need a slight slope in case it ever rains again down here? Then I sppose any sort of normal ceiling boards underneath?

Hi Leigh,
You have to fix the sheets really solid, with the length of the sheet running across the joists.  Put noggins under unsupported joints and I like to see a row of noggins in the middle too.  Fix with countersunk screws and epoxy the joints.  (see Hardies tech sheets)
You should have a fall.   Sometimes you can lift one end up of the row of joists or otherwise drop the other.  Depends on your layout and how the joists are fixed.

The waterproofing is the same as for a bathroom. Not as bad as it sounds.  Water based emulsion stuff that you can apply with a paint roller,(it's thick, tilers sometimes trowel it on) with a thin fibreglass tape over the joints of the sheets and at the wall.  Messy but easy to do.  One coat, tape the joints, do a another coat.(again see manufacturer's tech sheets.)  consult with tiler first or if you do your own, buy the waterproofing, adhesive and tiles from one proper tile shop, that way they are all compatible.

I have always took a lot of care at the joint up to the wall. Once again it depends on your layout, what the wall is made out of.

As I said it is expensive, but done well it creates good usable space under.
Good luck with it, and with the rain too.

A couple more thoughts on a waterproof deck.

  • The sheets we are talking about here are 16mm thick compressed cement sheet.   Made by James Hardy & Co.   They have fairly detailed fixing instructions, they sell the epoxy putty for the joints etc.
  • Usually these sheets are laid on the floors of bathrooms in elevated houses.
  • In a bathroom situation the size is mostly small enough to manage without expansion joints, but on a deck that could be fairly large it is a different matter.
  • Whether using timber joists or metal "C" section purlins, I'd be putting in an expansion joint at say every 3600 or 4200 (using 1200 sheets).
  • This is of course a lot more often than would be used if the deck was poured concrete.  This along with the extra rows of noggins is to control the movement that is inherent in this type of floor.
  • I would be taking advice from a good tiler, as to whether to use a rubberised tile adhesive.
  • I have never seen a deck using cement sheets that wasn't finished with ceramic tiles, but I guess it would be possible to give it a two pack epoxy paint finish.

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