ᐅ How to Make a Plywood Balcony Surface Waterproof?

So far, everything is only planned, and nothing has been built yet.

For an extension of an exterior balcony, I am planning a thick plywood panel supported on posts, which will be made waterproof on the top and edges. Below it will be a terrace area that is meant to be covered by this structure.

If I build it myself, I want to prepare the roofing work (which should be done by a professional) as effectively as possible.

What is certain is that the edge of the base panel will be clad with titanium zinc sheet metal, and a plastic membrane will be applied on top. Water will simply flow over the edge and run down along the sheet metal, or be collected in a gutter (above the terrace).

The steel railing will be attached to the support beams underneath the plywood panel.

These beams can either extend all the way to the edge of the panel or stop 10 to 20 cm (4 to 8 inches) before it. To properly secure the railing, it would be best if they extend to the edge. Whether the roofer finds this practical for installing the sheet metal, I don’t know. The person I hoped would do this work has not responded (apparently quite busy).

Once it is built, it would be very impractical to change this afterward.

Whether the beams reach the plywood edge or are shorter is an open question. Based on experience, there will probably be additional details to consider one way or another ... but all this should be decided BEFORE the roofer starts.

I have an excellent roofing handbook, but I have not been able to find a solution to this specific issue in it. There are just too many possibilities.

I have been hoping for feedback from Mr. Fischer (the roofer) for several months.

The reason for this thread is that I am not sure how to proceed.

Although I don’t think this can be easily solved in a forum, sometimes it helps to talk things through….

The design you are working on is life-threatening. Please have a structural engineer or at least a professional carpenter create a plan for the balcony. Simply placing posts and a plywood board is not a proper balcony design. It starts with some kind of foundation, continues with horizontal and vertical beams, correct fastenings and waterproofing at the facade, and ensures the proper load-bearing capacity of the surface.

Hello Karsten,
the construction of the balcony itself is not the reason for my post, so I have not described it further. The design has been discussed with an architect and a carpenter and does not require any additional adjustments.

What concerns me now is the roofer’s part, and the takeaway from the post shared here is that I need to speak with a specialist in this area BEFORE the work begins must.

The issue seems to be that I had assumed “it will somehow work out,” the roofer who hasn’t responded is frustrating me, and at the same time I want a well-thought-out, sensible approach.

I need to set priorities.

As mentioned above: “talking about it helps.”

Thank you for reading and thinking along.

KarstenausNRW schrieb:

The plan you are making is life-threatening.

I suspect that, as often happens when there are no pictures, you are talking past each other, and @ateliersiegel does not want to extend the balcony with the slabs to make it walkable. Rather, the balcony itself is meant to remain as it is, but it partially covers a terrace below (which is now supposed to receive a small extension so that the terrace underneath is fully protected from rain).?

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https://www.instagram.com/11antgmxde/
https://www.linkedin.com/company/bauen-jetzt/

I am somewhat annoyed that the first assumption made about me is that I have "life-threatening" plans, even though the plans have never been mentioned.

11ant schrieb:

if pictures are missing -

True. I am a strong supporter of illustrated threads.

Here you can see the balcony, next to (in front of) which another one will be added—approximately 2.50 meters (8 feet) deep and 4.80 meters (16 feet) wide. One side will be attached to the left house wall; the other side will rest on a foundation (not visible under the plants on the right), where two wooden posts will be installed.

I will spare you and myself a more detailed description but can assure you that this will not be life-threatening, but rather enjoyable.

The visible terrazzo surface below is used a lot for work, so it is practical if it gets less wet down there.

11ant schrieb:

I suspect that, as often happens when pictures are missing, you are talking past each other.

Probably not... A railing is not necessarily installed on a roofed structure.

ateliersiegel schrieb:

I’m a bit annoyed that I was immediately accused of having “life-threatening” plans, even though the plans weren’t even mentioned.

Don’t be annoyed, be glad that you’re getting a reaction like mine.

You wrote:
- Balcony extension on the outside of the house
- Thick plywood board on supports
- Will there be more details that might be done one way or another?

I prefer to give strong warnings rather than have some poor work get done.

If you express yourself more clearly, for example "a balcony structure coordinated with a carpenter or structural engineer, including foundation," you can expect more helpful answers. And include a sketch or a photo.

Specifically regarding your question: There are various edge profiles or balcony angle profiles (e.g. by Schlüter) for your problem. The best option depends on how the rest of the balcony is planned—tiles, wood, stone slabs, etc. This will also determine how you can install the gutter.
From what I see, I don’t see an issue placing the support beams on the edge (either fully on the edge or perhaps with 1–2cm (0.4–0.8 inches) gap—depending on your choice).

P.S. By the way, my reaction was still mild; in other forums, it can be much harsher.