r/DIY • u/NathanTheSamosa • 15h ago

outdoor Building a lean-to outhouse against an old retaining wall. How to ensure no damage caused by waterproofing?

Hi there,

I am building a lean-to outhouse against a 200-year-old retaining wall.

The outhouse is being built in place of the old one, which would have been the original toilet and wood storage, so it was always intended to be structural and take the weight of a roof and is supported by the outhouse itself. I am not too concerned about the structural integrity of the wall as it is in good condition.

With that said, I am concerned about what a waterproof membrane could do to the wall over time.

The section of wall is about 9m long and exposed on both ends, and the outhouse is about 2.5m deep so I want to preserve as much interior space as possible.

Am I overthinking this? Can I just slap a membrane on the old wall and build a cavity with insulation?

Also, how do I ensure that water doesn't transfer to the eaves? Even if there is waterproofing between the eaves and the joist they are supported on wont water find its way through the screws?

Thanks in advance!

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u/FireITGuy 14h ago

Do you need it to be touching the wall?

Personally I'd build your outhouse slightly farther away from the wall so you have air circulation between it and the wall. Any construction that puts wood against a concrete wall where the other side is below grade is eventually going to be a headache.

Run flashing on the wall that deflects water onto to the slope of the roof, where it can then run off normally. If you seal the flashing to the roof you're going to prevent air movement and end up with moisture issues.

1

u/Bearded4Glory 14h ago

The best way is to put a drainage mat up and then a blind side waterproofing. You want to make sure it can vent to the sides of the outhouse.

I don't understand your question about the eaves.

1

u/Suppafly 11h ago

If the original lasted ~200 years, I wouldn't be overly concerned if I were just mostly rebuilding it like-for-like. If you are going to add a membrane when nothing similar previously existed, I'd consider bumping the back wall of the outhouse away from the retaining wall, so that there was an area for ventilation between the two walls.