I have a leaky porch. It leaks into the basement below it. Right now it has a 3/4 plywood subfloor topped with 5/4 deck boards. I have attempted to repair it, and I hired a carpenter to try to repair it, to no success.
I beleive I have learned from my mistakes the first time around, and I think the solution to my problem is a waterproof membrane made by Kerdi. I will use this to cover the whole surface of the porch, then adhere some sleepers using silicone, and then affix the deck boards to the sleepers.
So, is Kerdi membrane up to this task? If you can use it to waterpoof a shower surround, I would think it could stand up to porch traffic, but I am concerned about temperature making it brittle. Is that a valid concern? Are there any other concerns I should be aware of?
I’d suggest a membrane like they use on flat roofs.
basic article on the three types. The first group sounds like what you’d need.
I’d have concerns about Kerdi outdoors. It wasn’t designed for the extreme temperatures and UV rays.
On top of the membrane you could do a floating deck.
Your plan might work and might not even violate building codes but to me it just seems like some kind of half-hearted attempt.
I think you need a new carpenter.
What about it seems half-hearted?
I would love if I could get a carpenter out to fix it, but after the last guy (who was a friend) failed twice, I decided to try others, and I can’t get anyone to come out and do more than take a look and promise to call back. The call never arrives, however.
I did get an acquaintance who runs a construction company to quote me. He wanted 8K to redo the porch (it’s 8 x 8) or over 2K just to do a waterproof membrane similar to the roofing one above. My concern withthe roofing membrane is that I am not sure I have the know-how to install it properly. The Kerdi membrane at least has DIY instructions, rubber roof membrane seems to suggest a requirement to be installed by a pro.
UV rays are something I did nto consider, though, thanks for pointing that out.
To my way of thinking, this kind of waterproof membrane should not be required. Without actually looking at the structure in question, it’s hard to actually comment. I would need to look at how it was put together and see exactly where the water was entering. Either the design of the porch was inadequate from the beginning or something fairly specific failed. Too many other questions spring to mind. When was the house built? Under what specific conditions does it leak? What is the current condition of the gutters and downspouts? Is water allowed to pool next to the foundation?
Maybe I’m just watching too much Holmes on Homes. He’d completely rebuild it to the point that it would outlast the rest of the house…by centuries. Then he would hand you a bill with more digits that your phone number. I need to get it through my head that there is such a thing as ‘good enough’.
I dunno, I’m with you, AlphaTwit.
What kind of porch is this? Why is it leaking? How is water getting into the basement?
Most wood porches aren’t really meant to be water-tight, they let water through. It seems to me that something else is wrong for water to be getting into the basement, which should be water-tight.
You can spend a lot of money and time and effort fixing things - but if you’re fixing the wrong thing, it’s all wasted.
If I could tell exactly where the water was getting through, I would just patch those holes 
The porch is elevated, so it is a good 4 feet above ground level. During heavy rain, when water gets on the floor of the porch, the ceiling in the basement drips in several places.
The original porch was tongue and groove flooring. After it started to rot, I rebuilt it myself with treated deck boards, hoping to avoid the rotting issue. So far, no rotting 
Also the house was built in 1913, but I have only owned it for the last 8.
Again, without looking that the structure, I’m hard pressed to offer useful input. Your plan may work just fine. I certainly can’t prove that it won’t. It just seems ‘off’ to me in a completely useless and indescribable way.
Good luck to you.
The OP’s post reminded me of those houses with the small 2nd story walk outs. For such a tiny deck they can be a major PITA for leaks. That tiny flat roofed area tends to hold water unless it’s angled correctly to the scuppers. Leaks go right into the house.
Holmes on Homes fixed several of those walk out decks by correcting the pitch and then reroofing with a rubber membrane. Then they install a floating deck for people to walk on.
I agree without seeing the OP’s structure it’s hard to know the best fix.
Based on several comments, I am now looking more closely at a product called Laticrete, which is rated for outdoor use, and is highly flexible.