Pitched to flat roof ridge ventilation - how to do it?

[Timmyk86]

Hi all,

Really hoping you can help with a problem with the roof of my newly converted loft. I had a torrid time with the build (over budget, overran, had to do loads myself) but it has been generally finished and has been signed off by building control.

The loft conversion is a full width flat roof dormer to the rear of a victorian semi. The front is standard slate pitched. The flat roof is a cold construction, EPDM on the top. There is an overhanging soffit to the rear with a full length vent in it. The ridge is a dry ridge kit with the old ridge tiles laid over the top of it with no gaps between them. There is no VCL between the plasterboard and the insulation (yes, I know).

The main issue is that the ventilation of the flat roof is insufficient. It is sweating and there is already mould on the underside of the wood. I know this because I pulled down one of the downlights and looked up into the roof space. The electrician had bored holes all the way through the insulation into the roof space in order to fit the downlights. I had specifically asked him not to do this but he did any way. The builder returned to spray the mould to kill it and the holes in the insulation were filled. We also caulked in the lights to reduce any air leak around them.

I then looked into the roof space using an inspection camera through the full width soffit vent at the rear of the dormer and found more mould spots on the underside of the wood, not related to the (now plugged) light holes.

I figure that the issue is that whilst there is plenty of ventilation space at the rear through the soffit, there is no through-ventilation from the ridge. This is because although a dry ridge kit has been used (which should allow air transfer), the dry ridge kit is fully covered by ridge tiles. I was told by the builder that there can be ventilation from the front of the house between the fascia and the tiles to the back, but this either isn't happening or is insufficient.

I need to get air flowing in the ventilation space of the flat roof. So question is - are there any products I can use to ventilate the ridge between a pitched and a flat roof? Or is there a standard design for this kind of build?

Any help would be very much appreciated - this has been an absolutely awful experience for me and I'm just trying to get it over with. Pics attached of design/layout.

Thanks
T

 

[Notch7]

Ventilation on a flat to pitched abutment is never very successful.

unless there is through ventilation there is risk of condensation.

maybe flat roof air vents would help you - these can be fitted externally and easily sealed on an EPDM roof.

 

[robinbanks]

So question is - are there any products I can use to ventilate the ridge between a pitched and a flat roof?

https://www.rubber4roofs.co.uk/breather-vent for example

It's a shame the firrings run continuously to the ridge; there isn't any need for them to do so given that the ply over can withstand a gap of at least 400. Leaving a gap in them would have allowed for sideways ventilation. Hey ho, you are where you are.. The insulation looks to be continuous and taped joints? That should minimise house moisture getting into the space if the damage has been well repaired (I'd look to fill and patch over the hole with aluminium tape, rather than rely on caulking down lights in)

I had specifically asked him not to do this but he did any way.

Worst bit of any job, when the people you have work for you don't follow the specifications laid out

 

[Timmyk86]

Ventilation on a flat to pitched abutment is never very successful.

unless there is through ventilation there is risk of condensation.

maybe flat roof air vents would help you - these can be fitted externally and easily sealed on an EPDM roof.

This was actually very useful advice. Thank you. I was also thinking that mushroom vents are the most pain free way to go. I did a lot of googling and couldn't find an example of a ridge detail for my type of roof.

https://www.rubber4roofs.co.uk/breather-vent for example

It's a shame the firrings run continuously to the ridge; there isn't any need for them to do so given that the ply over can withstand a gap of at least 400. Leaving a gap in them would have allowed for sideways ventilation. Hey ho, you are where you are.. The insulation looks to be continuous and taped joints? That should minimise house moisture getting into the space if the damage has been well repaired (I'd look to fill and patch over the hole with aluminium tape, rather than rely on caulking down lights in)


Worst bit of any job, when the people you have work for you don't follow the specifications laid out

Thanks. Yeah it is annoying that there is no cross ventilation. Also annoying that there is no VCL. But neither as annoying as holes drilled straight through the insulation! I did consider no having any downlights at all for the exact reason that they compromise the insulation, but they were unfortunately necessary due to height restrictions.

At the moment, I've filled the holes with rock wool wrapped in black plastic ('sausages' I called them) - which was necessary to fit around the transformer for the lights. Then I cut a disc of celotex around 3cm thick to push up beneath the rock wool, which was reasonable well wedged into the existing celotex. Then I caulked the light in.

I could add aluminium tape underneath that disc to try and get it completely airtight before I caulk the light in. Is that what you mean? Or would you do something different to what I have?

I also think I'll dress that terrible looking ridge with lead just to make sure it's watertight - I don't like the look of the cement up there.

Pic for a view up the insulation hole

Thanks again for the responses and your time
T

 

[robinbanks]

I did consider no having any downlights

I've got about 100 down lights in my house, and wall lights too. For the most part I don't bother with the downlights, preferring the diffuse spread of the wall lights. Task areas like the kitchen island benefit fro the direct light but I feel almost like down lights are a bit cliché, and getting sick of the sight of them. I'll try harder in the next place I build I

Is that what you mean? Or would you do something different to what I have?

I think what you've got is pretty good, tbh. I can't see it being responsible for warm, moist house air reaching the deck underside

I also think I'll dress that terrible looking ridge with lead just to make sure it's watertight - I don't like the look of the cement up there.

Might be better to rip it off and fit new dry ridge tiles; bizarre that they've cemented wet ridge tiles on top of a dry system. It kinda makes the whole job look a bit tatty given that everything around it is new. I personally don't like the look of huge splurges of lead on a roof so I try to minimise it

Perhaps the ridge is too wide for standard tiles now; let us know

 

[Timmyk86]

I've got about 100 down lights in my house, and wall lights too. For the most part I don't bother with the downlights, preferring the diffuse spread of the wall lights. Task areas like the kitchen island benefit fro the direct light but I feel almost like down lights are a bit cliché, and getting sick of the sight of them. I'll try harder in the next place I build I

I think what you've got is pretty good, tbh. I can't see it being responsible for warm, moist house air reaching the deck underside

Might be better to rip it off and fit new dry ridge tiles; bizarre that they've cemented wet ridge tiles on top of a dry system. It kinda makes the whole job look a bit tatty given that everything around it is new. I personally don't like the look of huge splurges of lead on a roof so I try to minimise it

Perhaps the ridge is too wide for standard tiles now; let us know

Yeah I almost never turn them on! I probably should have left them out but it seems weird to just not have any ceiling lights at all and pendant lights definitely wouldn't have worked.

Thanks - I might add the aluminium tape as well just for good measure.

Re ridge tiles - what could I use instead of what they've done already? I think the problem was the change between the flat and pitched roof and getting a dry ridge tile with the right angle to fit that. Hence them just chopping the originals in half and cementing them on the back. I agree though that it looks so tatty. What I meant by using lead flashing was to use it to cover from the peak of the ridge tiles back over the cement and down to the EPDM. That way the cement would be covered and it would always be 100% waterproof. Not a good idea?

Thanks!
T

 

[John D v2.0]

Agree with the mushroom vents, you need equivalent of 25mm continuous ventilation across both opposing sides of a flat roof. A dry ridge would not provide that.
That ridge detail is not right really, with no height to play with maybe a lead flashing bonded to the flat roof might be better? I'm not a roofer though but I'm doing our flat roof which has moisture issues. I'm having to raise the ridge 150mm at the back to deal with a very similar situation.

 

[Timmyk86]

Agree with the mushroom vents, you need equivalent of 25mm continuous ventilation across both opposing sides of a flat roof. A dry ridge would not provide that.
That ridge detail is not right really, with no height to play with maybe a lead flashing bonded to the flat roof might be better? I'm not a roofer though but I'm doing our flat roof which has moisture issues. I'm having to raise the ridge 150mm at the back to deal with a very similar situation.

Thanks. Yes, definitely going to get mushroom vents installed. I've been advised to place them directly over the joists so each one ventilates two gaps. Seems sensible.

Agreed that the ridge is wrong. It just looks so unprofessional with all that cement on top and I can't convince myself that it's 100% watertight. Options are:
1) Dress it with lead flashing from the EDPM to just over the peak of the tile so that the cement is covered and it is watertight
2) Rip it all off and put down new ridge tiles (maybe 135 degree hip tiles that fit the flat to pitched detail
3) Simply fit lead flashing from the EPDM to the slates with no ridge tiles

Not sure what to go for to be honest. Would be grateful for some other opinions!

Thanks
Tim

 

[robinbanks]

My opinions on your options:

1) large splurges of lead on roofs look gash
2) will give the most normal look
3) will make the roof look unfinished