Dealing with damp caused by drilling through wall

[Fortheloveofscience]

Now that spring’s here (fingers crossed) I’m going to try and deal with a patch of damp on our living room wall. It’s at about shoulder height, 2x1ft and right next to a curtain tie-back drilled into the wall. Looking at the wall from the outside, you can see a bulge where this hole has been drilled.

It’s a Victorian cottage with thick solid walls, I reckon that somehow when drilling the previous owners went far too deep and have created a hole that is now drawing water. Obviously I start by removing the fitting and exposing the hole, but what can I use to fill a narrow 18” hole that will prevent water ingress?

go to the outside. Hit the bulge with a hammer. Does it crack and fall off, exposing a bister in the render that had trapped rainwater?

If so, scrape away all the loose material and re-render it. Depending on the age of the building it might be a lime, mud or cement material, and it might have been treated with an impervious paint that prevents it drying out.

It is highly improbable that anybody would drill a 2-foot hole for a curtain tieback.

Look above it for any source of excess water such as a dripping gutter, or a fault over or around the windowframe.

Yeah it does sound ridiculous, doesn’t it? I will hit the bulge with a hammer on my next day off. I’ve scoured the rest of the house for evidence of leaks, the gutters are fine and the window frame has been recently re-done and looks fine with no gaps or cracks either side. Loathe to get a damp specialist out because I don’t want them injecting crap in our walls.

The only other damp patch in the house is very obviously caused by a leak in a bit of roofing felt, but this one’s had me a bit baffled!

Update - I’ve identified a few issues:

1. Exterior has been covered with about 15mm of thick cement render, then painted with something oily.

1. Interior has been plastered with gypsum

1. French drain at the front has been filled with some cement/sand mix and a (broken) line of slate put down about 1cm from the exterior.

1. Pointing in corners of exterior wall has been (badly) fixed with cement.

Additionally, I’ve discovered a line of drill holes along the exterior wall to the side and the mortar that’s holding the bits of flint together is v v crumbly.

I’ve got someone from a building company that specialises in old buildings, lime plastering etc coming to have a look next week.