Concerned about new flat roof covering letting in water/damp

[Aldrige63]

Hi all, I wonder if someone can advise.

 

I'm having an outbuilding built for an office. It's a brick structure with flat roof and has timber support beams, flat layers chipboard/OSB on top of the beams, and then a layer of Vent 3 classic breather membrane over the top of the chipboard. 

 

My concern is the roof is being left exposed to the rain while the builder is off doing other jobs. I've read that the membrane is "water resistant" but also breathable and has "moisture vapour permeability". I can see the water droplets sitting on top of it but given it's breathable/permeable, I assume these gradually move through the membrane. With just OSB sitting directly underneath, is this an issue? 

 

I'm concerned the OSB boards are going to get soaked/damp/mouldy and lead to damp issues when the roof is later sealed off.  I could be worrying about nothing but I really want to avoid damp and mould as we've had ongoing issues with this before.

 

I know the simple solution would just be to have some waterproof cover sheet over the top of it but I've asked him to do this for various thighs and he never seems bothered about things getting wet and I asked him to cover the roof today as they have a waterproof roll of black plastic there (not sure if the name) and he hasn't bothered doing it.

[Aldrige63]

Have now also spotted this. Water dripping through the beams around where the skylight is due to go.

 

[Andehh]

It will be fine! We had several weeks with plywood sheets getting soaked during our full build, but it all dried out quickly once the roof was built up. Looked a horror story for some of them where they blackened etc.

 

Most houses end up utterly soaked through at some point during the build... But it all dries out. Any mould will die off with the removal of water once the roof is on.

 

Running a dehumidifier once the roof is on will accelerate it.

[Aldrige63]

1 hour ago, Andehh said:

It will be fine! We had several weeks with plywood sheets getting soaked during our full build, but it all dried out quickly once the roof was built up. Looked a horror story for some of them where they blackened etc.

 

Most houses end up utterly soaked through at some point during the build... But it all dries out. Any mould will die off with the removal of water one roof is on.

 

Running a dehumidifier once the roof is on will accelerate it.

 

Hmm. Thanks. I've just had so many issues with damp and mould before and it my experience just drying something doesn't get rid of mould. Once it's wet that's it, the grows and the remains there. But unbeknown to us it's hidden behind a ceiling but can still be impacting your health. I'm probably over the top about it but it's a flat roof so isn't had to just throw something over the top to keep it dry at the end of each day.

[Iceverge]

The roof will be fine. The membrane alone be ok for many months. 

 

Trying to eliminate mould by "keeping it out" is not likely to be a successful strategy. It's like trying to pick up every spec of dust with your fingers. 

 

A far more successful approach is to keep humidity and condensation under control. Appropriate ventilation and heating will do this.

 

What's your previous issue with mould and how did you try tackle it? 

[ADLIan]

That membrane look to be for pitched roofs. Doubt it would keep the water out on a flat roof hence your problem

[Iceverge]

Agreed. 

 

It is a missed oppurtunity to make it a warm roof with some insulation.

 

It will make condensation and therefore mould much more unlikely in future. 

[Aldrige63]

2 hours ago, Iceverge said:

Agreed. 

 

It is a missed oppurtunity to make it a warm roof with some insulation.

 

It will make condensation and therefore mould much more unlightly in future. 

 

I'm confused by this. It's a missed opportunity to make a warm roof which is a bad thing I assume? But also it will make condensation and therefore mould much more unlikely in future?

As in not having a warm roof would make it more unlikely that we'd get condensation and mould or having a a warm roof would make it more unlikely. Confused by your wording, sorry.

 

He may walk put insulation between the beams under the osb board later on but I'm not sure. 

[Kelvin]

Are you saying there’s a possibility the roof won’t be insulated at all? That’s a huge mistake if so. Warm  moisture laden air hits the back of the cold OSB and condenses which eventually leads to mould. It absolutely needs to be insulated and a warm roof design is typically the best way to insulate it and avoid condensation and mould. What flatroof waterproof membrane are you using? 

[Iceverge]

What @Kelvin said. 

 

Rafters. 

OSB 

membrane

Insulation

OSB

EDPM

 

would be an example

 

[Aldrige63]

On 29/10/2024 at 15:47, Kelvin said:

Are you saying there’s a possibility the roof won’t be insulated at all? That’s a huge mistake if so. Warm  moisture laden air hits the back of the cold OSB and condenses which eventually leads to mould. It absolutely needs to be insulated and a warm roof design is typically the best way to insulate it and avoid condensation and mould. What flatroof waterproof membrane are you using? 

 

On 29/10/2024 at 17:26, Iceverge said:

What @Kelvin said. 

 

Rafters. 

OSB 

membrane

Insulation

OSB

EDPM

 

would be an example

 

 

This is the currently from the inside so yes I assume insulation board will go in the gaps her beneath. On top it's just the breathable membrane currently directly/flat against the OSB. I don't know what waterproof membrane he's going to be using yet.

 

Is this all ok how it's done so far?

 

Image attached. It's a skylight in the middle where the square is, just for reference.

 

[Iceverge]

You can Use this method but I would avoid it unless you

 

1. have an installer that is absolutely surgical about

2. installing an internal membrane and you have

3. continuous mechanical ventilation and 4. heating in the shed to control humidity. 

 

A warm roof is relatively fool proof. 

 

 

 

[Aldrige63]

42 minutes ago, Iceverge said:

You can Use this method but I would avoid it unless you

 

1. have an installer that is absolutely surgical about

2. installing an internal membrane and you have

3. continuous mechanical ventilation and 4. heating in the shed to control humidity. 

 

A warm roof is relatively fool proof. 

 

 

 

 

Thanks. Sounds like a warm roof would have been a better option. Would it be easy to revert to a warm roof from this point? Or does it require an entirely different structure?

 

The room will have heating and the internal membrane as you say. Continuous mechanical ventilation, that hasn't been planned but if you have an example of something suitable I could maybe look at that being installed.

Edited October 29 by Aldrige63

[Iceverge]

Pics from the outside would help to see, Also discuss it with your builder. 

 

 

In my hierarchy of building for longevity, comfort and health of the occupants.

 

I would put things in this order. 

 

1. Water management. Make rainwater and ground water can go about their business without trying to get into your building. 

2. Planned ventilation, preferably mechanical and continuous. 

3. Airtighess. Make sure drafts don't go where they shouldn't. 

4.Good windows and doors. 

5. Continuous insulation. 

 

For the ventilation get yourself a Greenwood CV2GIP, fit it and forget. Often available on eBay for £50 ish. 

 

[Iceverge]

9 minutes ago, Aldrige63 said:

So you feel that should suffice to keep things dry if it's a cold roof?

 

If there's an appropriate membrane installed below the joists with zero penetrations it should be fine. 

 

Something like intello plus. My preference would certainly be mineral wool between the joists rather than PIR. Then a countered battened 50mm service cavity with more mineral wool.

 

The fan can go out either but the wall is probably easier. 

[Aldrige63]

Thank you all for the sound advice. I was hoping to be able to clear/delete this thread / my posts not to disclose this all publicly at this point. Is that at all possible?

[Aldrige63]

I actually just spoke to the builder and he tells me it's going to be a warm roof. So there's another layer going on top of what is already there. That's reassuring.

 

So currently from bottom up there is timber frame. OSB flat on top of the timber frame. Breathable membrane flat on top of the OSB. And he tells me there will be insulation on top of this and......I'm not sure of the next bits but he said it's a warm roof?

[elite]

Looks like a warm roof from the exterior pic. 

 

Ventilation through the wall if possible to avoid penetrations to the roof

[Aldrige63]

5 hours ago, elite said:

Looks like a warm roof from the exterior pic. 

 

Ventilation through the wall if possible to avoid penetrations to the roof

 

So still have some sort of ventilation device even if it's a warm room?

[Aldrige63]

Is there any way for me to delete / remove this thread now?

[Kelvin]

21 minutes ago, Aldrige63 said:

Is there any way for me to delete / remove this thread now?

Why? 
 

Every question that gets asked on here is generally answered with people giving their time to do it. Consequently there’s useful information for anyone else with a similar question. Buildhub is publicly searchable so it’s a great resource for everyone. 

[Kelvin]

On 30/10/2024 at 20:56, Aldrige63 said:

 

So still have some sort of ventilation device even if it's a warm room?

Yes. These two things aren’t really connected. Every home needs ventilation and air circulation regardless of however else it’s built. If you want the room to be pleasant and free from mould it needs to be insulated, heated, and ventilated to let fresh air in and remove stale air. 

[Aldrige63]

19 minutes ago, Kelvin said:

Why? 
 

Every question that gets asked on here is generally answered with people giving their time to do it. Consequently there’s useful information for anyone else with a similar question. Buildhub is publicly searchable so it’s a great resource for everyone. 

 

That's a fair point. The issue is I'd rather then builder didn't see it to avoid difficulties in communication. If the photos were able to be removed to essentially stop the property from being identified, that it wouldn't be an issue for the thread to stay.

[AliG]

I have removed the house photo.

 

Really I don’t think the builder is likely to see this and even if they did I don’t think you said anything that they would be upset about. It is just a genuine question.

[Aldrige63]

25 minutes ago, AliG said:

I have removed the house photo.

 

Really I don’t think the builder is likely to see this and even if they did I don’t think you said anything that they would be upset about. It is just a genuine question.

 

Thank you. There's one other photo a few posts above showing the inside off the roof, which the workman would be able to tell is our home if they saw it. If that could be removed then happy for the thread to remain as hopefully of some use to others.