what to do about a rotten deck installed above a water tank?

[ginb]

Hi All.  Our front deck - 1.5m x 3.2m - was springy so we took up a few planks and saw the timbers supporting the decking were completely rotten and had dry rot.  Water had got in from above, and all sides.  There had been battons beneath the decking planks, then a layer of cement board (not sealed along edges) then timber frame underneath which seems to have been made out of H3 timber.  The whole structure is above a below ground water tank.  Rian water had got in between the decking timbers, through the gaps between the concrete, over the timber frame (from above but also from the sides which were also unsealed).  The flashing along the wall adjoining the neighbour was also not stopping water from getting in down the wall on that side. 

We have to replace the structure and I'm wanting advice on the best way to make sure the same thing doesn't happen again.

Should I consider tiles around the edge (or tile the whole thing) with bullnose around the side to stop the water? Or go back to the same timber frame but protect it from rain in some way?  

[MitchellMc]

Welcome to the Bunnings Workshop community @ginb. It's terrific to have you join us, and many thanks for your question about deck installation.

How old was the deck? Typically, decks are fully exposed to the weather, and with the right materials—like H3-treated pine for the frame—they should last 15 years or more. If yours has rotted sooner, there may be other factors at play, such as poor drainage, lack of ventilation, or water getting trapped in areas where it can’t dry out properly.

It sounds like water has been getting in from multiple points, including the sides and gaps in the cement board. Addressing these weak points will be key to preventing the same issue from happening again. Depending on your preference, tiling with proper waterproofing could help, but it’s also important to consider ventilation beneath the structure to prevent moisture build-up.

If you can share more details on the deck’s age and any signs of early damage, we can offer better suggestions on how to rebuild it for longevity.

Please let me know if you have any questions.

Mitchell