Waterproof deck - leak risk?

[Nick G]

My architect sent through initial concepts for a remodel on a 2 flat property. My plan is to free up more internal space on both floors by replacing the internal stair access to the top flat with external access to a deck and then the entrance.

The deck was to sit over a bit of roof from downstairs that juts out further than the downstairs space.

My architect pointed out we'll need a waterproof deck. "Unfortunately with the relative first floor to roof levels, the proposed deck would be required to be constructed as a waterproof enclosed butynol deck. We could not span the new deck over the existing roof."

A cursory Google of "enclosed waterproof decking" dug up a lot of articles about water ingress issues they historically have caused, particularly with apartments.

Is this still a problem with today's materials and design code? Enclosed decks on buildings are all over the world so I would have assumed this is a solved problem by now but...

Any insights much appreciated.

[jimO]

butynol should be waterproof if done properly, will people be walking on it?

[hawkeye]

We have just had one done over an extension. They put ply I think then black tar type stuff then a membrane that went up at the edges. This whole roof was complicated as it sloped in 2 directions to one point which has a large storm water drain in it plus another storm water drain at the upper end. Over the top we have decking sitting up on adjustable feet.

There were complications installing it. First the architect had to redesign it as he didn't allow enough slope from one end to the other, and with the right slope it was too thick to go under the doors above. Then the new design meant the amount of wood around the edges made it difficult to get the required bolts in for the posts for the balustrade. The electricians had to wrap the electrical wires in foil for some reason then the builders put polystyrene around for insulation which is apparently a no no as it can react with the plastic around the wires.

All in all quite a drama but it is lovely now! I could post a photo of it but I don't seem to be able to.

[jimO]

We have just had one done over an extension. They put ply I think then black tar type stuff then a membrane that went up at the edges. This whole roof was complicated as it sloped in 2 directions to one point which has a large storm water drain in it plus another storm water drain at the upper end. Over the top we have decking sitting up on adjustable feet.

There were complications installing it. First the architect had to redesign it as he didn't allow enough slope from one end to the other, and with the right slope it was too thick to go under the doors above. Then the new design meant the amount of wood around the edges made it difficult to get the required bolts in for the posts for the balustrade. The electricians had to wrap the electrical wires in foil for some reason then the builders put polystyrene around for insulation which is apparently a no no as it can react with the plastic around the wires.

All in all quite a drama but it is lovely now! I could post a photo of it but I don't seem to be able to.

unusual for a architect to make a mistake.....said no tradesman ever

[Nick G]

Thanks guys.

It will be a big deck and will be a usable outdoor space as well as the entrance. I'll put some outdoor furniture and probably a BBQ on it actually.

Roof its going over is a flat slope away from the downstairs floor. She's old but no other dramas.

Here's the floorplan. I haven't heard whether is is cost feasible building out over that extension instead and going to a 5br / 2ba layout. My vague recollection is it would be very difficult and expensive due to age of house.

[Davo36]

Sorry Nick, I don't understand. Is that plant the upstairs flat or downstairs?

What are the areas with gray hashed lines?

[Nick G]

Upstairs plan. Downstairs has a wider footprint. The grey lines are roof for downstairs. Current roof runs the whole left side of the house and access up top is through an internal stair that we'll remove. Footprint for downstairs is basically that rectangle, including the roof/deck area as displayed. Cheers

[Davo36]

So the deck will be over the roof? Or replace part of the roof?

[Nick G]

I haven't seen a side sketch yet. I originally suggested having a deck over the roof, however that won't fit. I guess that part of the roof will be replaced by this as any roofed space underneath it will be a nightmare to maintain. Concern is it's 120 years old.

[Davo36]

Well as long as it's designed correctly then it should be fine. Butynol is actually very good.

You just don't want to have too many joins/corners I reckon.

[Nick G]

That's good to hear. I want to go for the "best possible solution" rather than cheap.

Next chat to have is about cost of that vs extending the house. I suspect it will be a very short chat due to height plane rules. It's in Wellington and places aren't exactly spread out.

[jimO]

Well as long as it's designed correctly then it should be fine. Butynol is actually very good.

You just don't want to have too many joins/corners I reckon.

Butynol is great, its even better if you stay off it, it can be tiled over to protect it but that also has some problems, you also want the laps in the direction of the fall and for some reason butynol installers run it across the fall creating little dams at every join

[Nick G]

It's a deck/main entrance and would get a lot of foot traffic.

[Davo36]

Butynol is great, its even better if you stay off it, it can be tiled over to protect it but that also has some problems, you also want the laps in the direction of the fall and for some reason butynol installers run it across the fall creating little dams at every join

We have a deck at home where they've done that.

You can put wood decking over butynol too.

[Nick G]

Target market will be senior students or young professionals. So wood decking over top seems like the way forward. It's on the second floor and an old building.