Flat roofing

Cod83 · 14-11-2018 8:14pm #1

Looking for advice. We are putting on an extension to our house which will require a flat roof. We have heard they can be troublesome and expensive so we want to do our homework. One builder has quoted and said he would use a Desmapol waterproof product on it. Any reviews and advice appreciated.

pawdee · 14-11-2018 8:19pm

You could consider a rubber roof. Depending on the size it can come in one roll and is just stuck to a timber deck with adhesive. Fibreglass is another option. If the roof doesn't have to be totally flat you could consider a standing seam metal roof. I'm no expert by any means but I'm thinking of doing a flat roof extension myself and have only started looking into all of this myself. I'd be very interested in any advice you get.

Rows Grower · 14-11-2018 8:25pm

Torch on felt done properly will last a lifetime.

rob007 · 29-03-2021 6:31pm

Hi Paudee, how did you get on in the end??

am_zarathustra · 04-04-2021 2:45pm

Anyone used fibreglass? Very interested in using it and the finish seems lovely compared to other roofs of it's type.

2018na · 04-04-2021 2:53pm

am_zarathustra wrote: »

Anyone used fibreglass? Very interested in using it and the finish seems lovely compared to other roofs of it's type.

It’s a very good waterproofing material that will solve any roofing balcony or wet room problem. Timber structure for roof needs to be built correctly or you get massive condensation underneath. I have been using it for 10 years now

am_zarathustra · 04-04-2021 3:24pm

2018na wrote: »

It’s a very good waterproofing material that will solve any roofing balcony or wet room problem. Timber structure for roof needs to be built correctly or you get massive condensation underneath. I have been using it for 10 years now

I've heard good things and I have to say the finishing trims are really nice, very clean lines. Seems to be more resilient than other forms of flatroofing too. What would your thoughts be on insulating underneath and skylights? I'd love to get a decent skylight in the middle of the flat roof.

JustJoe7240 · 04-04-2021 3:40pm

2018na wrote: »

It’s a very good waterproofing material that will solve any roofing balcony or wet room problem. Timber structure for roof needs to be built correctly or you get massive condensation underneath. I have been using it for 10 years now

When you say built correctly, what would a common problem in which they're not built correctly be?

Gumbo · 04-04-2021 3:48pm

JustJoe7240 wrote: »

When you say built correctly, what would a common problem in which they're not built correctly be?

The incorrect grade of OSB.
Laying the OSB while Damp or wet.

Thespoofer · 04-04-2021 3:51pm

JustJoe7240 wrote: »

When you say built correctly, what would a common problem in which they're not built correctly be?

Insulate & ventilate.

Rows Grower · 04-04-2021 3:53pm

am_zarathustra wrote: »

I've heard good things and I have to say the finishing trims are really nice, very clean lines. Seems to be more resilient than other forms of flatroofing too. What would your thoughts be on insulating underneath and skylights? I'd love to get a decent skylight in the middle of the flat roof.

I've seen it done in a few house extensions, Velux are one company that make them.

2018na · 04-04-2021 5:40pm

am_zarathustra wrote: »

I've heard good things and I have to say the finishing trims are really nice, very clean lines. Seems to be more resilient than other forms of flatroofing too. What would your thoughts be on insulating underneath and skylights? I'd love to get a decent skylight in the middle of the flat roof.

What I do is put a 25mm sheet of polyiso on top of joists. Then use 8 by 2 Osb t and g on top. And fix through the insulation. We always use skylights from velux for flat roofs. The fiberglass Matt goes right up the upstand and then a whole glass cover goes over the finish roof. The ones we have used have been non opening but others can be got. Larger areas require expansion joints but this would not apply in most domestic settings. Keeping everything dry pre application is very important. Most roofers use a one Matt 650 system which is said to last 10 years. I use a 450 Matt twice which is a 20 year system. I personally believe both systems will last much longer.

am_zarathustra · 04-04-2021 5:53pm

2018na wrote: »

What I do is put a 25mm sheet of polyiso on top of joists. Then use 8 by 2 Osb t and g on top. And fix through the insulation. We always use skylights from velux for flat roofs. The fiberglass Matt goes right up the upstand and then a whole glass cover goes over the finish roof. The ones we have used have been non opening but others can be got. Larger areas require expansion joints but this would not apply in most domestic settings. Keeping everything dry pre application is very important. Most roofers use a one Matt 650 system which is said to last 10 years. I use a 450 Matt twice which is a 20 year system. I personally believe both systems will last much longer.

Brilliant, thanks for that. I'd been leaning towards it anyway, the price is excellent for the longevity you get and it's a relatively quick install, all things considered. I've had a look and the long recangular velux look very well with it and the seal looks so clean on them. Thanks again for the info, no substitute for experience!

italodisco · 11-04-2021 1:43am

EPDM Rubber Roof, built as a 'warm' roof, not the traditional 'cold' way.

Joists, then OSB, then Tyvek Vapour Barrier, them a minimum 120mm Kingspan PIR Insulation, another layer of OSB...... Finished off with epdm rubber membrane.

No condensation worries in the roof area, mold between joists. No leaks with epdm, no cold spots.

2018na · 11-04-2021 7:01am

italodisco wrote: »

EPDM Rubber Roof, built as a 'warm' roof, not the traditional 'cold' way.

Joists, then OSB, then Tyvek Vapour Barrier, them a minimum 120mm Kingspan PIR Insulation, another layer of OSB...... Finished off with epdm rubber membrane.

No condensation worries in the roof area, mold between joists. No leaks with epdm, no cold spots.

That would certainly have excellent insulation quality for sure. I would wonder how to get the outside fascia down to a feasible size or are you happy for it to be massive. The internal timber structure would still need ventilation am I correct in saying ?

Biker1 · 11-04-2021 2:13pm

2018na wrote: »

That would certainly have excellent insulation quality for sure. I would wonder how to get the outside fascia down to a feasible size or are you happy for it to be massive. The internal timber structure would still need ventilation am I correct in saying ?

Warm deck flat roof so no requirement for ventilation between timbers.

2018na · 11-04-2021 2:35pm

Biker1 wrote: »

Warm deck flat roof so no requirement for ventilation between timbers.

Does all internal structural timber not need ventilation

italodisco · 13-04-2021 1:23am

2018na wrote: »

Does all internal structural timber not need ventilation

No.

In this instance (warm roof)the roof is air tight, assuming you've fitted the PIR snug and not left any space, sealed up with aluminium tape etc.

A cold roof leaves space between the osb and Insulation, roughly 60mm if done right, so there's a space for condensation to hang about.
Mushroom vents or soffit vents are an option.

I've installed both along with hybrid roofs, I'm currently building my own outdoor room for my gym and study and I'm going with the warm roof approach as I mentioned in my last post.

Yes it will cost a bit more but the way I see it, spend a few quid extra and have less problems down the line.

2018na · 13-04-2021 6:54am

italodisco wrote: »

No.

In this instance (warm roof)the roof is air tight, assuming you've fitted the PIR snug and not left any space, sealed up with aluminium tape etc.

A cold roof leaves space between the osb and Insulation, roughly 60mm if done right, so there's a space for condensation to hang about.
Mushroom vents or soffit vents are an option.

I've installed both along with hybrid roofs, I'm currently building my own outdoor room for my gym and study and I'm going with the warm roof approach as I mentioned in my last post.

Yes it will cost a bit more but the way I see it, spend a few quid extra and have less problems down the line.

And will airtiteness prevent dry rot

sydthebeat · 13-04-2021 8:14am

italodisco wrote: »

No.

In this instance (warm roof)the roof is air tight, assuming you've fitted the PIR snug and not left any space, sealed up with aluminium tape etc.

A cold roof leaves space between the osb and Insulation, roughly 60mm if done right, so there's a space for condensation to hang about.
Mushroom vents or soffit vents are an option.

I've installed both along with hybrid roofs, I'm currently building my own outdoor room for my gym and study and I'm going with the warm roof approach as I mentioned in my last post.

Yes it will cost a bit more but the way I see it, spend a few quid extra and have less problems down the line.

Its not "air tightness" that removes the ventilation requirement in a warm roof construction, its the fact the dew point occurs outside the structural timbers, when you use enough insulation outside of the structural timber work.

2018na · 13-04-2021 9:01am

sydthebeat wrote: »

Its not "air tightness" that removes the ventilation requirement in a warm roof construction, its the fact the dew point occurs outside the structural timbers, when you use enough insulation outside of the structural timber work.

Does timber not need air flow regardless. Have you seen any results of people putting down unvented suspended timber floors for instance in the last 20 years which is relatively recent. You can put your finger through the timber in places in some of those constructions. Dew point has nothing to do with this. This is all timber that has been sealed up and has no air flow

sydthebeat · 13-04-2021 9:14am

2018na wrote: »

Does timber not need air flow regardless. Have you seen any results of people putting down unvented suspended timber floors for instance in the last 20 years which is relatively recent. You can put your finger through the timber in places in some of those constructions. Dew point has nothing to do with this. This is all timber that has been sealed up and has no air flow

dew point has everything to do with this.

if you dont understand that then you dont understand the physics of whats happening.... and why we have the need to have ventilation in the first place

2018na · 13-04-2021 9:50am

sydthebeat wrote: »

dew point has everything to do with this.

if you dont understand that then you dont understand the physics of whats happening.... and why we have the need to have ventilation in the first place

Condensation is not the cause of every type of wood rot or fungus growth. Would I be right in saying this?

sydthebeat · 13-04-2021 9:55am

2018na wrote: »

Condensation is not the cause of every type of wood rot or fungus growth. Would I be right in saying this?

nope

all fungal wood rot needs moisture as an active ingredient to happen.

2018na · 13-04-2021 10:30am

sydthebeat wrote: »

nope

all fungal wood rot needs moisture as an active ingredient to happen.

Ok so the key is to eliminate moisture in all forms from both sides of the structure. Difficult enough details to achieve in the real world of construction with recessed lighting and so on but I do get what you’re saying ðŸ‘ðŸ‘

PMBC · 25-05-2021 1:55pm

italodisco wrote: »

EPDM Rubber Roof, built as a 'warm' roof, not the traditional 'cold' way.

Joists, then OSB, then Tyvek Vapour Barrier, them a minimum 120mm Kingspan PIR Insulation, another layer of OSB...... Finished off with epdm rubber membrane.

Does the 120mm PIR, taken together with the rest of that construction, give the required U value?
Do you use an OSB or M Ply deck? I prefer the latter, just in case and seal the board joints with clear silicone mastic - just in case! Overkill?

monseiur · 26-05-2021 2:22pm

Not wishing to hijack the tread, but would like your opinions on the following flat roof on big extension. Build up is as follows Trocal membrane on 80mm PIR on vapour barrier on galvanised medal deck on 175mm galv. purlins
For ceiling 3'' x 3'' timber will be bolted to underneath of purlins running opposite direction spaced at 16''

sydthebeat · 26-05-2021 2:45pm

i really hope youll have more than 80mm PIR insulation in the construction.

monseiur · 26-05-2021 2:57pm

sydthebeat wrote: »

i really hope youll have more than 80mm PIR insulation in the construction.

Oops my bad - it's two 80mm boards so total 160mm

Flat roofing

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