To be honest, having your dog pee on a non-porous surface that you wash down is disgusting. It's going to sit there and stink and start to damage the roof materials until you can wash it down. And where do you think it's going to go when you wash it down?

If you don't have some kind of sizable garden or grass area with about a foot soil (dirt) or other porous surface on the roof on which the dog can pee and the urine can filter through, don't let your dog pee on the roof. There are some artificial grass pads made for this purpose, but I'm not sure how well they work. See: https://www.pinterest.com/pin/402368547937842566/

I saw on an episode of Shark Tank where they were pitching a fresh patch of grass that got delivered to your home on a monthly schedule I believe. It was promoted for balconies, patios, etc for when your dog cannot get outdoors. I know they have similar products (maybe more affordable?) on Amazon too. Maybe look into something like that.

Thanks for the suggestion Lyndy, it's a great idea! It's such a small space though (1.5x3m) that we really want to maximize this for our own use and not just a toilet for our dog. He prefers not to pee in the yard and he gets taken out a few times a day so a finish that is washable would be our preference.

It's times like this I wish we bought a bigger house!! :)

Well, the choices are either porous/absorbent, like - to some extent - concrete, gravel, wood chips, sand, or impervious, like glazed, sealed tiles with a waterproof underlayment, or better yet, a solid welded watertight metal pan. Actually, if this is an upstairs terrace, it's critically important to make sure you have water tight catchment and drainage for this urine load to avoid serious damage to whatever is below. Animal urine is uniquely corrosive, pungent, and damaging as I'm sure most of us have observed over the years, and there are few materials that can change this ultimate reality. To suggest that this can be managed with no odor or offensiveness on a small terrace is asking for a material.

Even a completely impervious decking would still result in dilute urine being washed over the sides and onto the building below. Over the months and years, you would see odor, staining, and corrosion developing.

The size of the dog and its trainability make a big difference. I hope this is a Chihuahua or maybe very small terrier or poodle, and that it can be trained as to where to relieve itself. In that case, I'd build a large litter pan, solid and impervious, and fill it with gravel and maybe some odor-counteracting substances like charcoal. The pan should include a plumbed drainline, preferably heading straight into your sewer line, or worst case, draining into an unlined gravel pit (maybe 4' x 4' x 4') in the ground. The drainline must be guarded against clogging. That way, when it rains, the overflow from the litter tray does not found your terrace or corrode the building structure. And teach your dog to use this pan exclusively. And remove this whole thing if you ever want to sell. Good luck!

Depending on the size of the dog there are 'potty patches' which are essentially a tray for urine catching with a section of artificial turf over it. The artificial turf is washable and while it's not ideal the tray should hold the dog pee until you can drain it somewhere suitable (where that would be I can't say. :P) My dog is a chihuahua miniature pinscher mix and she's trained to use pee pads on her tray if she has to potty and for whatever reason I am not there/able to take her out for a walk to do her business.