Window leak help

tony3619

Hello,

So I understand that this may be hard to visualise or diagnose on the internet but I'm after some basic Info on a window leak before I hire someone to investigate and charge a fortune.

So I dont have visual water anywhere but the MDF cill has been swelling in one corner for the last 2 winters.

There was obviously a big gap underneath when the window was installed so they have tried to fill it with mortar /cement before sealing the cill. Overtime it looks like it has cracked along the brick line. My 2 theories are

1. Water is running off the sealant on the edge of one corner (one photo) and straight into this crack and puddling under the board inside allowing it soak up water.
2. It's entering under the gap between the cill and window frame (but I'd expect more water present for this)

It's a hard to reach area, 3 stories up and doesn't open so these limited photos are from me hanging out of the window underneath so dont judge the quality! 🫣

Any advice?

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grumpy_codger

My 2 pence…

1. Can it be condensation inside leaking down?
2. The fact that it's the cill that is wet doesn't prove that the leak is at the bottom. It can be at the top of the window and leak down around the side.

If it's a cavity wall, the leak can be even higher up if there is some problem with the cavity tray above the window.

twopenny

Looking at the windows from the outside there seems to be gaps in the frames. They have a lot of mossish stuff on them which seems to suggest water is collecting there as well as around the sill.

Looks like it all needs cleaning and sealing first to get it clear. Including under the sill. Then see what happens.

I have a similar problem with a bay window, corner of the sill inside collects mold and splits at the join. It is east facing so a cold spot. I'm working on it 🤔

tony3619

https://forums.moneysavingexpert.com/discussion/comment/81871555#Comment_81871555

To seal.under the frame would you remove the window or just fill in the gap?

twopenny

I would scrub the frame, toothbrush/washing up brush and get all the verdigris off . Dry right in the gaps. then fill any space there will window silicone.

Put a temporary filling in gaps around the sill when it's dried out. No point in sealing in the moisture.

Then note where the verdigris starts to grow a new. If it does.

Clean the inside off too and see where it starts again. Then you have some idea what needs doing if you have to get someone in.

tony3619

https://forums.moneysavingexpert.com/discussion/comment/81873956#Comment_81873956

Climbing up 3 stories on a ladder is going to be fun!

twopenny

Didn't see that obvs🤔

Depends on the height but a window cleaner on a mobile scaffold? I saw a couple of blokes fixing some wood soffit 3 storeys up with a platform on a crane based on the back of a flatbed lorry the other evening.

Amazing what's about now.

expertsupport

From what you're describing and the photos, I'd lean towards your first theory being the most likely culprit.

The cracking along that mortar/cement fill is a classic weak point, mortar isn't flexible, so when there's any movement (thermal expansion, settling, or just natural building flex), it cracks. Once you've got those hairline cracks, water absolutely will find its way in, especially on a north or west-facing elevation where you get wind-driven rain.

The corner sealant issue you mentioned is probably contributing, but I'd also look at what's happening above the window. Water doesn't always enter where you see the damage, it can track down from higher up (like from the brick course above, or even from further up the wall) and then find that crack as its exit route into the cavity behind your cill.

A few things worth checking before getting someone out:

Even though it's hard to access, try to see if there's any pointing issues, cracks in render, or gaps in the brickwork in the courses above this window. Water loves to travel behind things before showing itself.

If it's been there a few years, silicone sealant degrades from the sun and becomes less waterproof. It might be worth having someone rake out and reseal that entire perimeter properly, it's a relatively cheap fix if that's the main culprit.

Is there a weep hole or any drainage provision in the window system? Some installations are meant to let water drain out, but if that gets blocked or wasn't done properly, water can accumulate.

The fact it's only swelling in one corner suggests localised water ingress rather than general condensation, which backs up your theory about the crack.

You're right that you'd typically see more obvious water if it was coming through there, unless it's a very slow seepage. But given that it's only affecting one corner, the crack in the mortar fill is the more likely route.

When you do get someone in to look at it:

Make sure they're comfortable working at height (3 storeys up) and have proper access equipment like abseiling, cradle or cherry picker. You'll want someone who can properly assess the external seal, the pointing above, and ideally re-do that entire mortar joint with something more flexible (there are modern sealants designed for exactly this kind of application that won't crack like cement mortar does).

If they just want to slap some more sealant over the crack without addressing what's above it or replacing that failed mortar, I'd be wary, you'll be back to the same problem in another year or two.

Hope that helps narrow it down a bit before you get quotes!