Have you considered Scyon ( external ) waterproof as exterior balcony, tile with a high rated exterior tile

However

A structural engineers report on existing timbers ( $500 +) is a necessity, Safety, and expected life of all existing timber/ and ootings

Thanks for that. I'll have a look at Scyon. And yes we've had a structural engineer check the existing timber and footings. It's perfectly sound and once we waterproof the underneath, we intend to put panels of some description between each of the posts. This will strengthen the whole structure.

I don't know how the cost compares but NexGen has aluminium waterproof decking

 https://nexgendecking.com.au/products/aluminium-waterproof-deck/

Could you line the underneath with colour bond and the water would run to the edge. (It would need to have a slight slope of course)

Have a look at Underdeck at http://www.underdeck.com.au/ . There are other companies around that do the same thing.

My thoughts, Waterproof as a new structure on the existing joists/bearers ( check with structural engineer, if they can accept the additional weight, Scyon and Tiles )

Waterproofing under, will not protect your timber decking from weathering and early maintenance

and

You have an intention to " fill in between "

Do it as a professional, it will last for years and add value

We have waterproof options to suit.

Check out the below links or give us a call on 1300 661 801.

DryJoist - Three products in one. Waterproof Substructure, Joist system for Decking and a finished Ceiling underneath.

https://nexgendecking.com.au/products/waterproof-deck-system

Weatherstrip - NexGEN Weatherstrip is a proprietary extrusion uniquely crafted to fit snugly within the slotted edges of NexGEN decking, creating a watertight seal, preventing water from penetrating the surface of the deck

https://nexgendecking.com.au/products/waterproof-decking-weatherstrip/

Hi Geraldine

Your enquiry to waterproof the underside of your balcony is one we receive a lot of.

We build balconies, patios etc and line the undersides of such structures. From our experience in doing this for 25 years, we don't know of any system put into an existing structure that can be guaranteed 100% waterproof by the manufacturer of the product or the installer. I could be wrong however I'm sure there will be a disclaimer in your quote.

We have used the NexGen system with success in a Cottesloe balcony (WA) and used a different system in Mullallo and Peppermint Grove (WA) within 4 years of each other. With the Mullaloo balcony, we've had no issue after 4 years of install however the Peppermint Gove balcony waterproofing system installed in Janaury 2021, approved by the engineer, had to be cut on a two unusual angles to meet the 300 mm beam. With the first down pour, it did leak and we returned twice to rectify the problem with installing additional flashings (covering the beautiful Merbau beam).

It may be a case of starting over with removing the existing decking boards to install the waterproofing membrane and installing new decking boards on new framing, keeping in mind this will raise the existing floor level. The other consideration is that this is not a straight edge balcony so whatever waterproofing system is used has to be cut on a curve which may compromise the integrity of the membrane. You'll require a gutter and down pipes on the front of the structure also.

Whatever you decide to do, make sure you engage with a company who knows what to do.

Best wishes! Castlegate Home Improvements

Thank you for your detailed answer to my question. We've always known we would have to remove and replace the existing decking, but you're right, the shape of the deck could well be a problem. We're certainly looking for a company who knows what they are doing and who operate in Canberra. Can you recommend anyone?

Hi Geraldineone, we’re in WA and unfortunately don’t know anyone in the ACT. Beach Bathrooms are right, you want to ensure the balcony has all the right approvals. It might even pay to get an engineer involved. Best wishes!