recover a concrete flat roof

Milkman

Hi all looking for some advice.

We have a concrete flat roof 21m sq over part of the house that needs to be recovered.

Have had 4 quotes, all suggesting torch on, €900-3700
There is a small amount of pooling which none of the quotes have addressed.
Can't say I've been filled with any confidence by these 4.

I rang two fibreglass companies and sent them a few photos. They will lay OSB, fibre glass with colour and nonslip topcoat €1200-2400. Very impressed by both of them.

There is an overlooking window which we will eventually replace to create a fire escape. So there might be occasional footfall in the future.

So may two questions:
1. Torch On or Fibreglass?
2. Should we insulate under the OSB if we go fibreglass.

M.

Calahonda52

whats the problem with roof?
When u say part of the house pls explain more:
do you have a massive thermal bridge from the roof?

Milkman

Hiya calahonda,
Tnx for the reply.
We have a 1970's semi-d, to the right was a single story garage which was converted into a bedroom before we bought the house. The roof of this is concrete. I'm not sure what u mean by thermal bridge, and I'm sure not explaining this very well.

Would an image make it easier?

Tnx M.

Calahonda52

No need for image, I know the design well.

You don't say why the roof needs fixing:(
I presume the roof is insulated inside.
However the roof is sitting on half or maybe all the wall of the house and it will transmit cold into that room.
If the room is dry lined then it won't show.
I think maybe you think about insulation g the roof from the outside

Milkman

Ahh yes, sorry didn't give all the info.

There is a leak, which I think is just because there is no flashing, especially if wind/rain from the north.
There is old, degrading torch on felt, and some pooling.

Bedroom below is cold. It would be nice to think that there is internal ceiling insulation above the plasterboard, but I'm not confident about that.

And yes, we are thinking that it makes sence to externally Insulate(it only adds €380 for 100mm) if we go fibreglass.

One Grp company has quoted €2k to: insulate 100mm, OSB, level rain runoff to back of house, fibreglass, nonslip and colour topcoat, and new gutter/down pipe.

Another has quoted same work €2.5k
It seems very good value.

What are your thoughts ?

Calahonda52

Potential disaster!

have a read of this
https://www.nsai.ie/S-R-54-2014-Code-of-Practice.aspx. its 13Mb so not for the one:)
Fibre glass is this case is beyond useless.
Also you need to ventilate the roof because whatever you do the slab won't be 100% dry so you may get in hot sun what is called inward solar vapour drive.
http://www.greenbuildingadvisor.com/blogs/dept/musings/when-sunshine-drives-moisture-walls

Milkman

That's a great doc, thanks for the link.
I had a good look last night and the solution we are looking at is on pg80, figure 37.
There in no mention of venting in the diagram, so I'm not sure how to incorporate it or if it's needed. The roof space will also get very little direct sunlight.
As u can tell I have v. limited knowledge here, and I really appreciate your feedback.

There are a couple of other houses in the estate that have a similar solution, I've had a good look but see no obvious vents.

If it was your roof, what would u do?

Calahonda52

Okay, the key assumption with that is that the existing roof covering is 100% in terms of being water proof.
My concern is that, as its not clear what the issue is, that the roof might be wet and you would just trap the water.

If there is light direct strong sunlight then grand, keep the colour light.

You asked: if my roof I would remove the slab
You did ask

Whats the plan with the fibre glass: a set of wooden "rafters" to hold up the USB?

Milkman

Yes the plan is to have timber strips that the OSB sits on. These will be tapered to slope slightly to the back, so all rain is sent to the back of the house.

I genuinely don't think the roof is wet, I'm sure the 'leak' would be sorted if there was flashing fitted. It is minimal and only occasionally if there's a north wind and rain

Calahonda52

If its only a flashing issue then why redo the whole roof: I would stick a bit of suitable tape over the gap in the flashing: what part of the world re u in...so as I can point u to some good sources for proper tape

Right so we now have furring strips to help the water flow away, which begs the question: how will they achieve 100mm insulation constant...
I would suggest 100 XPS in two layers between 100m deep timbers and make sure it is done right, no gaps and joints, offset and taped.
Then put on the furring strips and the osb.

What u don't want is a 4 by 2 cut in half as a furring strip with 100 fg at one end and it compressed to death at the other, compressed fg is useless

Milkman

Sorry only getting back to u now, life got in the way.

So, yep, why do all the work? Good question!
We had budgeted to do it this year. We need the flashing sorted, we want to insulate and figured do it all in one go and re-surface so its done for the next 20yrs.
It's a 70s house that we are slowly renovating, one job a year, next year Windows, then bathroom etc

So if I'm right in understanding your advice, we need to make sure that:
1. The insulation is not compressed
2. The insulation layer is taped and sealed in a horizontal timber frame.
3. Then the tapered timber furrows.
4. Then OSB.

I really appreciate both your advice and the time u have taken to help with this.
M.

Calahonda52

No problem ,whats important though is to understand why all these steps are been taken so that you can bat knowledgeably when the builder says......
You are missing the one to try make sure the roof is dry under the existing felt.
In an ideal world the edges should get at least a bit of insulation, ideally not less than half whats on roof, does the roof over hang your neighbour?
Is it flush with front of house or forward or set back?

Milkman

I think the slab is dry, except maybe a small point where there is the leak on a wet windy day due to no flashing. I think this leak runs down the north facing side wall, and not through the slab.

There is a small overhang to the front and back.
I've attached an image which I hope helps..
M

Calahonda52

So the plot thickens!
Are u doing all the top.
of so whats the plan for the underside at the door?

There is also the importnat issue of your neighbours wall and the prospect of the lads cutting a new line for lead flashing into their wall

Poor Uncle Tom

Just wondering why you're only considering torch-on or fibreglass?

Milkman

Oh I hope a can of worms isn't about to open, we are planning to do the whole top of the roof.

Honestly we were not going to do anything to the under side of the roof outside the door, maybe a lick of paint.

Poor Uncle Tom what other covering do u suggest? The misses also likes the clean look of fibreglass.

As for the neighbours, we have a good relationship with them.

Calahonda52

Milkman wrote: »

As for the neighbours, we have a good relationship with them.

All very well until some Joe is up there with a consaw cutting a groove in the wall with all the dust and herself next store has the nice Ann Summers lingerie out on the line:D

You had better ask the builder exactly what he will be doing and then tell them.
He may only used good quality flashing tape..

Milkman

Now there's an image I don't need ;-)

I have talked about our various roof plans with the neighbours, but you are right, I will get specifics about the detailing.

Thanks again for your help, I will put re-approach all the quotes and ask a few more specific questions.
M.