Garage roof replacement

booksandbikes

jj_home_80 said:

Those upright sections (parapet-style walls) mean proper edge detailing and upstands will be key – mention that when getting quotes, as some roofers skimp on this and that’s where water ingress starts.

Is there anything specific I should mention (ie. ask about materials, sizes/lengths/methods used) or just point it out as a concern?

Albermarle

booksandbikes said:

Thank you so much for your replies. You have given me a lot to consider (and google!).

Tucosalamanca - you say you wouldn't go GRP or EPDM as introducing insulation won't be straightforward. Can you expand on that please? Insulation won't work with either of those, or potentialy cause rot from the underside? I will consider single-ply membrane as an option - it's not one I saw regularly mentioned on roofers sites.

I think that is for two reasons.
PVC roofing sheet is more expensive than EPDM, and I think you need some specialist equipment ( hot air welder) .
It tends to be used more on commercial buildings AFAIK. 

Tucosalamanca

booksandbikes said:

Thank you so much for your replies. You have given me a lot to consider (and google!).

Tucosalamanca - you say you wouldn't go GRP or EPDM as introducing insulation won't be straightforward. Can you expand on that please? Insulation won't work with either of those, or potentialy cause rot from the underside? I will consider single-ply membrane as an option - it's not one I saw regularly mentioned on roofers sites.

Single-ply is generally only available to approved installers. It isn't widely available off-the-shelf and it requires a degree of technical skill (which is why so many 'roofers' don't offer it).

You can't fibreglass directly onto insulation, you would need install an OSB deck over it. This deck would be outside of the thermal envelope. Over time, moisture can build up and cause the OSB to go rotten.
It also adds an unnecessary cost, as two decks rather than one are required.

I've taken this image from LABC warranty. Site-applied GRP roof systems onto timber substrates
Note the importance of condensation risk analysis and airflow.


The method below is far better because everything is contained within the thermal envelope and the membrane is applied directly to the purpose made insulation (tissue or mineral faced, not foil as it's unsuitable)

In my opinion, single-ply or torch-on felt are the best materials for this task.
You could use a fleece-backed EPDM (not standard EPDM). The membrane is more expensive but installation times are significantly quicker. Detailing will be formed with Russ tape (which I'm really not a fan of).
Single-ply and torch-on use the application of heat to create a seamless finish, whereas EPDM relies on adhesives (it's basically just a big bicycle inner tube).
Installers of single-ply and torch-on tend to be time-served and often approved contractors.
EPDM can be installed with limited experience (even DIY level), to become an accredited installer is only a four hour course. It's why it's so popular with 'roofers'.

Grenage

It's also worth mentioning that insulation beyond reducing condensation (which would be minimal amounts) isn't worth it unless it a heated space.

Improving airflow is a better option for reducing dampness.

booksandbikes

We've just had the first roofer around to take a look. He will draw up a quote in a few days, and has said that he would suggest felt. He said he would add additional joists but on the whole leave the existing ones there (except the rotten one at the back). Surprisingly he mentioned that insulation wouldn't be needed and the current condensation is from the cement asbestos. He would put OSB down, coat it and then lay the felt (he was talking about 2 layer or 3 layer). And if insulation is needed at a later date it could be done from the inside. He was pretty certain that GRP wouldn't be the best choice and he says he doesn't work with EPDM. Any thoughts on that?

Albermarle

I would presume the OSB board and felt would add some degree of insulation, compared to the asbestos cement sheet, so probably condensation would be less of an issue in cold weather. Also OSB will be cheaper than insulation.
Also as said good ventilation in the garage will reduce condensation as well. 

booksandbikes

Second roofer has come around and suggested felt as well, so that is likely what we are going to go with. He was very detailed in what he would do and we got a good vibe off him. Just waiting for the quotes now...