Mullion window erosion issues

[Figleaf3000]

Hello all, first time poster here although I've been a member for a while. Maybe someone could help with this. We have issues with a mullion window on the top floor of our 17th C house. Pics attached. There used to be plaster covering the inner central bar, which has now eroded off. It looks like some sort of water ingress although not that obvious from where it is coming from/how it is getting in. And also who would be the right kind of professional to sort this issue out. Has anyone had similar issues and could shed any pearls of wisdom I would be much obliged. Thank you!

 

[a twig]

Slight aside but the design of the stone around the window is somewhat less than ideal, normally the cill / sill would protrude out of the wall and have a rain drip cut into the underside so rain water running down off the window is guided out and away from the wall below.

That aside I’d be looking at the verge of the roof, from the “toothy” silhouette I’d suspect mortar is missing allowing water in which is tracking down the wall

 

[Kearn]

Could be a few options there - very little roof overhang, nothing shedding water above the window itself, so wherever pointing / stone has eroded or failed is an entry point for rain. What orientation is this elevation and does it get battered by the weather?

Water will happily track down through stonework joints until it finds an exit point…

Internal plaster ‘eroding itself off’ would be quite a worry…! Although you’re better off without it attached to the stone mullion.

 

[Stevers]

You can see the stains on the sandstone where water is running down from the gaps in the capping or verge, including onto the window.

 

[Figleaf3000]

Slight aside but the design of the stone around the window is somewhat less than ideal, normally the cill / sill would protrude out of the wall and have a rain drip cut into the underside so rain water running down off the window is guided out and away from the wall below.

That aside I’d be looking at the verge of the roof, from the “toothy” silhouette I’d suspect mortar is missing allowing water in which is tracking down the wall

Thanks very much - yes the roof is an issue!

 

[Figleaf3000]

Slight aside but the design of the stone around the window is somewhat less than ideal, normally the cill / sill would protrude out of the wall and have a rain drip cut into the underside so rain water running down off the window is guided out and away from the wall below.

That aside I’d be looking at the verge of the roof, from the “toothy” silhouette I’d suspect mortar is missing allowing water in which is tracking down the wall

Could be a few options there - very little roof overhang, nothing shedding water above the window itself, so wherever pointing / stone has eroded or failed is an entry point for rain. What orientation is this elevation and does it get battered by the weather?

Water will happily track down through stonework joints until it finds an exit point…

Internal plaster ‘eroding itself off’ would be quite a worry…! Although you’re better off without it attached to the stone mullion.

Hi Kearn, thanks so much for your reply. The orientation is west-facing, so it does take a bit of a battering...