Moisture in ceiling - advice please!

[Ellsa100]

I have a question about alternating moisture levels in plasterboard.

Our flat roof is 1 year old and the paintwork in the plasterboard ceiling, right by the parapet, has been peeling off and which appears to be due to a water leak from the roof. There does not appear to be any other water damage in the room. The room itself is about 7m x 6m, of which 3m x 6m has a flat roof.

A roofer recently examined it and thought the problem was most likely due to damage to the fibre glass roof, which he then repaired. He noted the coping stones on the parapet were a little too small. He thought it would be best to monitor the moisture levels after repairing the fibre glass roof. If the dampness persisted, then I could look to replace the coping stones, the DPC underneath the coping stones etc.

I have been monitoring the situation by measuring the moisture level in the ceiling at 4 different points at 7am and 7pm every day. I've also been running a dehumidifier in the room overnight. The dehumidifier has been collecting lots of water, although I do not know to what extent this is from the ceiling or the general atmosphere.

The moisture levels seems to see saw - it tends to be lower in the morning after running the dehumidifier, however in the evening, the moisture levels rise (and this is irrespective of whether there has been any rain). This is confusing - why would the moisture levels INCREASE when there has been no rain during the day?

See data/graph of the moisture levels below, with trend lines.

Thoughts?

 

[Wayners]

We run two dehumidifiers.
Normally for 4 hours in the evening and when drying clothes. I empty them every other day.
It's amazing how much water there is in the air.
I wonder if you got little to no insulation in the ceiling void?
Also wooden floors with damp ground under?

 

[foxhole]

Parapets are very prone to causing problems.

 

[JohnD]

What sort of instrument are you using to measure moisture?

Tape a piece of clingfilm tightly to the ceiling. If moisture forms on the room side, it is condensation. If it forms behind the film, it is coming from the roof.

 

[Ellsa100]

thank you for all the responses and sorry for taking so long to reply...I totally forgot and didn't see any notifications about replies to my thread.

In answer to some of the questions/comments:

1. I have Dryzone Moisture Meter Detector from Amazon - see pic below. I put it in masonry/building mode as I am sticking the pins in plasterboard.

2. I haven't tried the clingfilm test - I will give that a shot.

3. I don't think there is damp from underground. We have block and beam floors with screeded floors and tiles. The water damage is in a very specific location underneath the parapet, so I am certain there has been a leak from either the fibre glass roof or the parapet, although I am trying to figure out if this is fixed.

4. I have stopped running the dehumidifier. To be fair, it hasn't rained that much for some time but the moisture levels seem to be fairly consistent...

5. Interestingly, overnight I ran the dehumidifier in a different room which does not, as far as I know, have damp issues. I closed the doors. It filled the container with water by the morning, so it shows it is extracting a load of water from the air! Makes me wonder how effective it was at extracting water from the ceiling when I had it in the kitchen underneath the water damaged ceiling...maybe most of the water it was extracting was from the air!

 

[Ellsa100]

I think the coping stones on the parapet are simply too small - almost no overhang on one side; maybe 30mm on the other. I think I need to get them all replaced etc. Who knows how sufficient the DPC is underneath - the builders I used were proper cowboys.

Given the issues with parapet roofing, would it make sense to go for:
1. (flat) porcelain coping stones; or
2. once weathered concrete coping stones?

I understand porcelain is almost totally non-porous, unlike concrete. Appreciate the mortar joins remain a weakness with porcelain tiles, but to me, porcelain tiles seem an obvious no brainer?

I may still wait for some very wet weather and monitor the ceiling before deciding whether or not to go for replacing the coping stones on the parapet.

Thanks!

 

[foxhole]

“maybe most of the water it was extracting was from the air!”
Dehumidifier only extracts moisture from air .

Do you have cement fillet at tile to parapet or lead, the former leaks easily .

 

[Ellsa100]

Very fair point about dehumidifier extracting moisture from air ! I think what I meant is that the dehumidifier was having an effect on the moisture levels on the ceiling - by sucking moisture from the air by the damp ceiling - or was just affecting the general moisture in the room.

I suspect it is cement filler - it is mortar like - so I guess that means it isn't lead. Can you put lead between coping stones?

Any views on concrete coping stones vs porcelain tile coping stones?

 

[opps]

Moisture meters for plaster are incredibly unreliable. Great for wood, rubbish for plaster. I used to use one and then realised that the salts in plaster that has previously become wet will will indicate damp even when the plaster is perfectly dry. As per @JohnD, tape cling film over a section.