Waterproof membrane failure

[Pulse]

I have just finished throwing things around after my deck's WPM failed. I did it all according to the rules and was hoping for some advice as to why it failed and how to fix it.

Position: 4m elevated deck, northern side, partly over laundry

Framing: 190x45 TP joist hangers blocking at sheet ends 400mm spacing
15mm per m sloped from the dwelling.

Deck: 15mm hardie compressed sheet 5mm spaces, hardie jointing compound under screws (batten screws countersunk). 10mm polyethelene backing rod, hardies joint sealant. No leaks even prior to WPM.

WPM: Fuller WPM 300 primer 50% thinned, three top coats 20% thinned, 50mm bondbreaker tape, 100mm reinforcing tape, two layers reinforcing tape over some joins, most only one. ( I did thin the wpm slightly because it was toothpaste consistency and impossible to roll on)

was waterproof for 6 months until I tiled it!!

Tiled with 300mm tiles and davco ultraflex on the diagonal (no cuts for movement joints - longest dimension 6m) Only the centre done, no edging or grout yet.

It down the other day and about 3 major leaks developed. All at joints between the sheets. Against the house was OK. The membrane had 2-3 months sun exposure prior to tiling.

Why did it leak?
was it perished from the sun?
was it thinned too much (can recommends 2 coats only, unthinned)
was it too much too expect on a northern side with 43ºC days.
did the lack of expansion joints cause it (no grout yet though)

How to fix it?
If I pull up the tiles over the cracks will I be able to effectively reseal the membrane where it butts against the other tiles. (laid diagonal)
Should I just pull off all 30m2 and get pros to WP? What do they do differently?
I've heard of a thick clear membrane that can be applied over the top.
Should I put up a large waterproof shade sail? or a roof?
Would grouting and clear coating with grout sealer help?(without grout the whole deck become a shallow swimming pool)

Anyway, thanks for any help.. the deck was the more ambitious of my projects so I'm pretty disappointed.

Cheers
Pulse

[ThePope]

Oh nooo, what a bitch...

You seem to have done everything Hardies recommends for installation. But you've had two failures by the sound of it, the wpm and the sealant on the sheet joint.

Assuming it was all installed correctly, only thing I can think of is a reaction between the wpm and the joint sealant but that's just a stab in the dark. As removing tiles and redoing it is a last resort I'd be running this past the tech guys at Hardies and Fuller and seeing what they recommend. The clear membrane on top sounds like a good idea, if it works.

[Pulse]

Thanks for the help ThePope, I'm thinking that maybe leaving the expansion joints out was the critical step.

Anyone else have any ideas?

I'm leaning towards bloody huge waterproof shadesail(roof)

Pulse

[ThePope]

Could be right, seems leaving no expansion joint directly over the sheet joins may be your problem. Seems odd that if failed before you grouted though, but I'm no tiler.

Can't see why you can't redo the tiling over the sheet joints, redo membrane and sealant and re-tile leaving the expansion joint. Maybe try one and setup a 'dam' over the joint and flood it, see if it holds ok.

From Hardies tech manual:
"A 5mm wide control joint is required in the thickness of the
tiling over all HardiPanelTM Compressed Sheet joints. Use the
marked battens described in Step 1 to locate the control joints
over the membraned deck joints. Seal the tile control joint with
a sealant recommended by the tile supplier, applied over a
suitable bond breaker such as a tape or backing rod where
the tile is very thick. (See Figure 21).
All other joints in the tiling can be filled with grouting
recommended by the tile supplier for external decking and the
selected tiles, preferably a flexible variety."