Is it a leak or condensation?

[Sockbogies]

We moved into our 1980s detached house in January this year. Paid for home buyers survey which was fine. Refitted the bathroom in August. Noticed some weird watermarks underneath the coving (it's on the outside wall above the window) that appeared around September time. We've had the roof checked over, no obvious problems with the tiles or flashing. Paid to get the gutters cleared just in case it was them causing the problem. Had guests over this weekend and the marks have got significantly worse. There isn't a fan in the bathroom but we do always open a window during/after showering. Have absolutely no idea what is wrong... any ideas?! Our house is rendered, so can't see brickwork.

If it’s condensation you will likely see water droplets on the coving after showering, do you?

These kind of water marks can be very hard to find the cause. Is your render in good condition?

Was the bathroom recently painted when you purchased? If so it could be they were hiding these marks. A homebuyers survey wouldn’t pick this up as it’s only what is visual. A full structural survey is the more in-depth one.

You can get a damp specialist in as they are usually good at working out causes even if it’s not ‘damp’

If there isn't an effective fan, then you are bound to get condensation, so start with that.

If the water appears after the shower has been used, on the ceiling, in cold weather, but not on days when there has been heavy rain but the shower has not been used, that also points to condensation.

A window is not a very good way to ventilate a bathroom.

Could be the extractor fan extracting into the loft or the ducting collection condensation and leaking it above the ceiling

Looks like condensation. Is the trickle vent open on the window.

Thanks all. Been up in the loft today and no visible signs of damp up there. There aren't any trickle vents in the window, and they are double glazed (newly installed in January, but flush fit so apparently no room for trickle vents). I'll try and check the render more closely to see if there are any cracks. Appreciate your suggestions!

Unfortunately heavy use of the shower has always coincided with heavy rain! And the leak has become visible all it's done is rain!!

Have your guttering checked. Water is coming in from outside.

Agree with @Bluerose91.
It's highly unlikely to be condensation as this would firm around your window reveals and over time mould would form although of course it can't be ruled out 100%.

If you have no water outlets or appliances in the room above its more likely to be an issue with pointing externally.

I recommend getting this checked out initially before instructing a damp expert to investigate.

Good luck!

The guttering has been checking and it's all good. I'll check the render outside (not easy to get to as the porch sticks out below making ladder access somewhat tricky). Thank you all for replying, it's really appreciated. I keep obsessively checking ceilings each time I walk in a room now, checking for more leaks! Last roof also leaked but was built by Persimmon.... so I deserved that!

Bear in mind that in an unventilated bathroom, the steam from a hot shower will rise to the ceiling and form a cloud (water vapour is lighter than air)
In a modern house the loft insulation will be very thick over most of the ceiling, so ceiling mostly warm, but if there are eaves will be pulled back to allow airflow.

Anyway, a bathroom needs an effective extractor fan, even if it also has a roof leak.

The water looks as if it is escaping under the coving. Could the coving conceal a water pipe? Do all rooms have coving?

As the source is not visible I can't see where to tape on a piece of clear plastic as an indicator.

You can purchase a small vapour collector for under £2. You can see how much vapour is being produced in a normal day. We have a window with a trickle and still need one.

Unusually for this time of year that doesn't look like condensation, or at least only condensation.

I would check the render externally and see if there are any signs of cracking. A photo of this section of wall would help.

As it's a 1980's house I'd say it looks more like a cavity issue, either the lintel, insulation or cavity tray.

Merry Christmas and thank you for your replies! This is a photo of the window and render. There is a small chip/crack but I'm not sure how water could travel up to it? The insulation is classic 1980s, a thin layer of what looks like polystyrene.

Incidentally I have been very guilty of throwing the windows open after having a shower, thinking I was being good letting the steam escape. I've stopped that now and trying to keep a consistent temperature. Still looks like a lot of water damage for condensation but clearly I'm no expert

Where is this "thin layer of polystyrene "

Is it like wallpaper?

A house of that age will usually have cavity walls, often with CWI

The stuff like wall paper is seldom seen outside run-down rental properties, it is fairly ineffective and usually a hopeless attempt to reduce condensation on the walls of badly-ventilated damp rooms.

It's ok to throw the windows open, but the effectiveness will depend on wind direction. It might blow the steam into the house.

If you leave them open for long, the room will get cold. Some people have an aversion to ventilation and will not open windows, even if they say they do.

The insulation is actually in the wall cavity, we only saw it when the windows were taken out.

I have seen polystryrene and other insulation boards in various thickness from 25mm to 150mm, they are sometimes used for house insulation. If you were using foamed insulation board in a house's cavity walls it would usually be between 50mm and 100mm (which is why the 2mm thick wallpaper gives negligible benefit). I don't know what is in your cavities. is it a brick and block house? Or timber-frame?

Maybe the stuff you saw was some kind of cavity closer or vapour membrane. Do you recall what it looked like?

I wonder if the problem is really the felt/membrane under the tiles doesn't run all the way to the gutter and is actually discharging into the cavity or onto the inner leaf of the wall.
Could easily have some damage at a low spot and if enough rain blows through the tiles it could cause that.
You would need a roofer to remove a few tiles at the eaves to confirm though.

Agree with @johnd2, it's a bit high for a cavity issue and more likely to be the roof.

There probably won't be and eaves tray and the felt may have a hole above the window. Water will then enter the cavity, hitting the lintel on the way down.

You need a roofer to look at that area as a first step.

Thought I'd give an update on this! The roof was fully checked out, no problems. Loft space also checked, again all clear. Looking at what we should have checked first we decided to get a ventilation fan installed... and that has solved it! Who knew condensation could look so much like a genuine water leak! We'd been so careful opening the window and keeping fresh air flowing through, but as previous posters had suggested, this was making the hot/cold air situation much worse. Thanks to all who replied, in future we'll go for the simple suggestions first!!

Glad you solved it and thanks for updating us all!

We have just upgraded our fan much better but remember for it to work the air it sucks out must be replaced so keep a window slightly open in the bath room when the fans on it will make it perform so much better. Keep the grill inlet in the bathroom clean too as its surprising how much fluff will stick to it and this can reduce the performance of the fan

No, shut the window and let ut draw air in through the gap under the bathroom door.

Water vapour is lighter than air so will rise towards the ceiling and the extractor. The replacement air under the door will displace it. The stratification will help.