Leak/damp in cavity area around window

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Hi all. We are currently refurbishing 1960s house to eventually live in.

After stripping wallpaper from inside walls of this window structure, we noticed the plaster which covered the cavity, at the very bottom on the left and right (circled in red) was damp and black with moisture. We've knocked the plaster back around the window as the plan is to remove this window structure and have a flush UPVC window fitted and the window structure itself removed.

We are complete beginners but it looks like to me that the right hand (from the inside) cavity is pretty much bricked up with the exception of missing pointing? It still caused damp at the bottom before paper was removed and plaster knocked back.

On the left side (from the inside), the cavity looks much more open and looks mainly filled with old plaster. There is fired in cavity wall insulation behind further back which is still soft.

I was asked by the builder to drill a hole on the outside top of the window structure to check for lintel provision prior to it being removed by window fitters. I have done that, and there is both an inner concrete lintel and outer concrete lintel holding up two rows of bricks, currently hidden from the outside below that window structure at the top. I have inserted some polythene where I have drilled and covered with tarpaulin temporarily until the structure is due to be removed for window replacement.

Last week we went in one day and exactly where the damp plaster had previously been on the left (from the inside) circled below in red, there was water on the window sill, onto the sheet used to cover the window and the water had gone onto the floor.

We are concerned as to where this water is coming from and why it is running through the cavity onto the window sill.
I initially considered whether my drilled hole on the outside of the window may have caused this but then remembered that the previous plaster covering that area was brown to black in the first place in that exact area (circled) before I even drilled any holes in the outside.

Any idea where this water is coming from, what needs to be done and how the cavities should be filled or sealed prior to getting window fitters to install?

Also should anything be done above from the inside between the two concrete lintels to seal the cavity in some way prior to new UPVC windows?

Thank you.

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Water has leaked

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There should be some kind of cavity tray (google it) above the window. The outside pic shows what look like holes in the perps (vertical joints) between some of the bricks, these may be the weep holes to allow water out. If the cavity tray (bitumen possibly in that era house) has failed, my guess is water coming in through those holes, not helped by that exterior plinth, and coming straight down through the cavity.
Looking directly up into cavity above window may help confirm/deny this…
 
The outside pic shows what look like holes in the perps (vertical joints) between some of the bricks, these may be the weep holes to allow water out. If the cavity tray (bitumen possibly in that era house) has failed, my guess is water coming in through those holes, not helped by that exterior plinth, and coming straight down through the cavity.
Looking directly up into cavity above window may help confirm/deny this…
Yes I believe you are correct that those holes in between the bricks above the large lintel are indeed weep holes as the whole estate has the same.

I do wonder if there's even a gap under those though as that large concrete lintel which spans the whole width of the house is about a foot in depth, so could water actually get through it and drop further down towards the two smaller lintels above the window?
 
I’d try and see exactly where it’s coming in, you should be able to get a decent view up between the lintels. Give it a bit of a spray from outside and see what happens inside
 
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There should be some kind of cavity tray (google it) above the window. The outside pic shows what look like holes in the perps (vertical joints) between some of the bricks, these may be the weep holes to allow water out. If the cavity tray (bitumen possibly in that era house) has failed, my guess is water coming in through those holes, not helped by that exterior plinth, and coming straight down through the cavity.
Looking directly up into cavity above window may help confirm/deny this…
I have taken two photos from inside looking up into the cavity above the window.

IMG_20240126_141353.jpg

Looking up between the two window lintels. I believe what can be seen is the remains of some cavity wall insulation to one side, two courses of outer skin bricks and in the gap above those, what looks like a wooden battern. I would be fairly confident to say that directly above the wooden battern would be the large concrete lintel which spans the whole width of the house supporting the first floor (as this lintel is about a foot in depth and can be seen from outside in photos above).


IMG_20240126_141420.jpg
 
No sign of a cavity tray though, to direct water out. I’d get the hose out and see where it’s getting in. I don’t think the plinth would be effective as a tray, there should be something above it.
 
No sign of a cavity tray though, to direct water out. I’d get the hose out and see where it’s getting in. I don’t think the plinth would be effective as a tray, there should be something above it.
Thank you

Would a cavity tray be needed for two rows of bricks if there's a huge concrete lintel above the two rows which spans the whole cavity?
 

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