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Condensation issues low pitch roof

We have a low pitch roof, 14 degrees, on our single storey extension done about 2 years ago,
We’ve had nothing but issues with it and it was completely re roofed in the summer. The new roofer added 3 extra rows of tiles as the original lap wasn’t enough. He also added vents in the eaves, and vents underneath the flashing at the top.
It’s been absolutely fine until about 4 weeks ago when it turned cold.
Since then we’ve had condensation coming through the ceiling in one particular place. Its definitely condensation as it appears when it’s cold, then dries out when it gets slightly warmer.
The roof doesn’t get any sun this time of year so it’s a very cold roof.
The new roofer still hasn’t been back to have a look….
Is there anyway we can put more vents in? The old roofer used klober universal tile vents but I know they’re not suitable for such a low pitch. I’ve been looking at ubbink vents for 5-25 degrees, but not sure if they’re suitable for our roof. It’s been tiled using redland regent.

Thanks for any advice!

Hi yes it’s the Redlands regent tile they’ve used with a 100mm lap. I’m not sure which membrane they used though.
We had just assumed it was condensation, it gets so much worse in cold dry weather. Even though it hadn’t rained for about 10 days the patch was getting bigger. But as soon as it warmed up and started to rain the patch dries out.
I’ll have a look at a flat roof, but this roof joins another with a hip joint. This other roof has a much steeper pitch. Would a flat roof covering be suitable for a much steeper pitch too? Thanks

Ok thanks. When the roofer comes back I’ll have to ask him about changing it to a flat roof system.

Sorry could I just ask, if we went for a flat roof covering would we also need to replace the r velux? We had them changed for the lower pitch ones, would they need to be changed again for the flat roof ones?

Hi Hugh,
Thanks for your response. We’ve been away for a few days and come back to wet patches on the ceiling again!
The roofer used Roofshield felt when he re roofed it, we have soffit vents all the way round, with more vents under the flashing at the top and more in the eaves. The builder used 100mm insulation between the joists and 50mm across.
It’s really odd, it went cold after it rained a few weeks ago, got wet patches, then dried out and now come back. But it’s not rained at all during that time.
It’s a lean to roof, a kitchen extension.
We also have recessed spotlights in the ceiling which I’m starting to wonder if that is the issue, especially one of them nearest all our kitchen appliances isn’t flush at all to the ceiling. And obviously they cut away the insulation in those areas to fit the lights….
We’re now considering actually removing them and putting the insulation between the joists and using the 50mm insulated plasterboard and having different lights to see if that solves the issue….
Thanks again for your advice.

4 Answers from MyBuilder Roofers

Best Answer

Hello Jane,

In my professional option a roof under 15 degrees is a flat roof.

All the major tile manufacturers Redlands, Marley & Sandtoft would agree with this also, Redlands have put a new tile on the market called regent though.

Which can go down to 12.5 degrees, however I believe it’s a very conditional tile which won’t always work on every 12.5 degree pitch, due to different levels of wind exposure etc.

Klober have developed a low pitched breathable membrane system also but again this is very complex system, that personally I think is too complex & expensive to work for proper long term use if you google and educated yourself see what i mean.

Do you know what tile you have on your roof?

I haven’t seen your roof but I would assume & comfortably bet it’s the wrong tile for the pitch of roof and the roof hasn’t been done in the klober low pitched breathable membrane system to properly protect it, this system is design so you can use the wrong tiles for a lower pitch like plain tiles/slate which are more nicer looking then true low pitched tiles.

Your issue is not condensation the roof is leaking but very slowly, due to the pitch being so shallow the water can’t run off fast enough.

Water will back up behide the tiles then seeps through the breathable membrane, where the batten is nailed to the rafters.

This will be worse in cold, wet windy months then summer months.

The longer this goes on it will start to breakdown the breathable membrane, and come in multiple places.

The solution:

You either use the Redlands regent tile and hope your wind exposure isn’t to bad, theirs also tiles that go down to 10 degrees but personally I would steer clear as this tiles are manufactured by less well know company’s with not a long proven track history of success.

Or better solution take the tiles, batten & breathable membrane up and put down a flat roof covering, like 3 layer high performance torch bitumen membrane, GRP fibreglass, cold applied liquid rubber etc.

The pitch and structure stays the same just the covering is different.

This would involve putting 18mm decking over the roof rafters.

Your current roofers idea was to attempt to increase over lap to and introduce air flow to attempt to stop water ingress & dry out damp from ingress before it seeps in which could work 50% of the time until the really bad weather kicks in.

All solutions I’ve mentioned above aren’t cheap but the second option is guaranteed to work the first it’s down to wind exposure but in theory on a 14 degree pitch roof it should work if the wind exposure isn’t too bad.

Hope this helps,

BT Roofing Services Limited

Hello Jane,

If your roof joins onto a neighbouring roof/another steep roof my solution will still work it just more difficult due to the roof next to it requiring special attention to detail to make it work.

Yes a flat roof solution will work on a steeper roof also.

I would just like to add if it rains Monday, the roof will collect water especially if the membrane is tight to battens.

The water between the membrane & batten, will take time to steep through it’s a nail size hole with a nail in, it’s gonna slowly drip.

Think you put a screw or nail in a pipe, it will leak but slowly if you pull it out a water feature will happen, now think back to a membrane with a nail in it going into a wooden rafter?

The damp and water might not show until the following Monday, as it’s coming in so slow it takes time to show up on your ceiling.

Condensation in this example of your roof is very unlikely, especially given you have a breathable membrane & air vents.

Condensation will cause mould & the room/building will smell stuffy & damp.

It might be work ringing up Redlands technically team showing them a picture on of your roof & the area, and getting their option if they think your roof is too exposed to work with regents tiles.

Have you got any Velux windows as this could be another point or entry?

As these can’t go lower then 15 degrees if you have it might be cheaper & easier to use a correction kit to get the window to 15 degrees Velux supply these.

BT Roofing Services Limited

Okay so you have Veluxs also and you used the low pitch flashing system, so that eliminates a point of entry their.

Unfortunately if you go with a flat roof system you will need to change the Veluxs again.

Your need a special kerb trim basically, you can keep the window it self (glass and frame) but unfortunately your have to change the flashing system again.

Velux do something called an insulated kerb trim, that is what your need it designed to be used with pitched roof windows.


Answered 7th Dec 2021

I agree with everything said above,
It has the wrong tiles.
The wrong under felt.
And should be treated as a flat roof .
Maybe not what you wanted to hear but there is no cheap solution.
Moisture created externally by weather frost cold temperature can still create water on the roof even on dry days
Yes joining a steeper pitched roof to a flat roof should not create any issue if done correctly.
it your insulation that is causing the problem if there is no air gap between the felt & the insulation it will sweat, your heat is created by the lights,
sorry did not realize your roofer had not left an air gap
you can counter batten the joists if necessary to achieve the required air gap
Good luck Alex


Answered 24th Dec 2021

In my opinion your roof is done completely wrong ,and you haven’t covered all the angles.
You probably won’t like what I’m gonna say but there goes
Firstly strip the roof carefully keeping your tiles as I’ve used regents many times with no problems
Check your roof is insulated with at least 125mm of solid insulation such as kingspan etc
100mm between the joists and 25mm across the underside of your joists
You must have 50mm airgap between the insulation and top of joists.
Re felt it using klober permovent felt
You must have a suggestion of sag between the joists not tight as you have done
For extra protection use a eaves felt support tray around the perimeter going under your felt at the base dropping down into your gutter
Make sure you have a continuous vented soffit strip all the way along the soffit
Re tile the roof but everywhere you have a ridge or hip use a dry vent ridge hip system,if you abut to a wall like in a rear extension make sure you have 150 flashing on your tile and a 150mm upstanding flashing firmly secured to the brickwork.
Having a sag in your felt ensures any moisture runs down to the bottom between your joists
Having a eaves felt support ensures it doesn’t puddle and drains into the gutter
The vented soffit,ridge -hip and permovent felt will ensure that the roof breathes properly and keeps it watertight.
Having the correct insulation installed correctly ensures that the underside of your roof/ceiling is the correct ambient temperature and ensures that no heat loss which basically condenses the warm air into moisture occurs
This is a tried and tested method that I use on all my roofs working harmoniously with building control using modern practices that were devised learning from older and more redundant methods
If you do go down the flat roof grp etc method be very carefull how you vent it whether using warm or cold deck as you could end up in the same boat again if done wrong
Do it once do it right! You pay more but you get piece of mind
It is the only barrier between you and the elements after all
I wish you good luck many thanks Hugh


Answered 22nd Dec 2021

I would have used eternit slates if not it should be flat roofed ie fiberglass/3 layer bitumen torch on felt


Answered 4th Jan 2022

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