Hello. This has probably been discussed before, but since I can't find it, I'll ask anyway. 
We are building an extension on the second floor that ends with a balcony, about 120 cm long and 300 cm wide. Under this, we will have a room. What waterproofing and slope should be used to ensure it is watertight between the balcony and the indoor room? We also have supports (studs) in the outer corner of the balcony that hold up an overhanging roof above the balcony. How do you ensure it's watertight against these? We don't want water to run down into any seam along the supports. - Mårten
We are building an extension on the second floor that ends with a balcony, about 120 cm long and 300 cm wide. Under this, we will have a room. What waterproofing and slope should be used to ensure it is watertight between the balcony and the indoor room? We also have supports (studs) in the outer corner of the balcony that hold up an overhanging roof above the balcony. How do you ensure it's watertight against these? We don't want water to run down into any seam along the supports. - Mårten
I placed hard plastic sheets under the deck boards to prevent water from entering the framework. It is important to have a good slope and proper overlap so water doesn't go the wrong way. You can also glue all the seams to make it really secure. You achieve the slope by cutting a 90 beam diagonally from edge to edge, then reassemble it with the plastic sheet in between. To avoid cracking the plastic, pre-drill and screw the package onto the beams. This method results in the deck being 90mm higher, so it only works if you've planned the construction from the start.
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There is a ready-made solution if you are a bit lazy..
TIl Tak seems convenient but a bit expensive
http://www.til-tak.no/
//per
TIl Tak seems convenient but a bit expensive
http://www.til-tak.no/
//per
Thanks for the interesting answers. Does hard plastic work in the long run? You don't want the roof to start leaking after 15 years. But maybe there are good materials nowadays. The tilt roof was interesting, and the lengths fit our extension, I'll look into this a bit more. Then it's just a matter of sealing around the supports...
If I understand you correctly, you are planning to build an extension under the balcony? If so, you need to demolish the entire balcony. Build the extension with about a 5-degree slope, install banded roofing, and then build the balcony on top. If you are going to have a sunroom, it works well to place sheets under the decking to avoid getting wet.