Membrane over membrane?

[Ed Davies]

My roof membrane has been exposed for most of a year. Even with the amount of UV here in the NE of the island it's not surprising that it's not very waterproof now. It's currently laid over sarking boards with 12x38 counter- battens running up and down the roof. Once the Velux windows are in I'll get 38x50 battens then a profile steel roof added.

 

I'm considering various options like rip it off and start with a new membrane and new counter-battens but that would be tedious and also tend to damage the roof underneath (particularly if the current ring nails are pulled out).

 

Therefore, another option I'm thinking of is just putting another layer of membrane over the existing membrane and counter-battens but under the battens. Probably with “stub” counter-battens between the battens to hold the new membrane down. The new membrane would need stapling down to keep it flat to the roof between the counter-battens. Since the old membrane is letting in so much water it's obvious it's not very liquid, let alone vapour, impermeable so there doesn't seem to be any risk of producing too much vapour resistance.

 

Any thoughts on why this might be a silly idea?

[CC45]

I'd take it off and do it right.

[Crofter]

I had my membranes exposed for longer than most people (not quite as long yours though) and whilst the roof remained perfectly watertight and undamaged, the wall membrane at the SW corner suffered some rips and tears. After advice on here I put a patch of new membrane over the top, as there didn't seem to be any downsides to having multiple layers.

[Cpd]

Out of interest what is the make / type of membrane on the roof at the moment? I have used roof shield and am not impressed at all...... the first time I used it frost got into it when it was wet and it was never the same again, second time / another roof after torrential rain it just became obvious that it is just not a good product. I know it’s wildly used throughout Scotland but if your going to redo it make sure to use the best you can get. Ask around on here for what’s tried and tested after being exposed to the elements.  

[Ed Davies]

It's Marley Eternit. It's dark red. Well, it was, it's now faded quite a bit.

 

I can't help feeling that the shiny membranes are probably better for long exposure [¹]. On the gables I've used Protect TF200 Thermo which I've seen left exposed for quite a while on kit houses around here without apparent problems. Unfortunately, that's not approved for roofs for some reason (not least because I'll have most of a roll left over by the time I've finished the gables ? ).

 

The first builders I approached to do the sarking, membrane and counter-battens recommended a particular membrane whose name I've now, unfortunately, forgotten (Redland something?) but it was heavier weight than the Eternit and shiny. When the first lot were too busy to quote I got the another lot in and they suggested this as the Marley equivalent but I'm not entirely convinced now. E.g., the other stuff said cover within 3-months whereas this says ASAP on the BBA cert.

 

[¹] Not to mention helping a bit with insulation when the shiny side is facing a cavity.

[Crofter]

I put a lot of research into my membrane choice- everything is there on the BBA certs fr tear strength etc.

Ended up using Cromar Pro Vent 3 on the roof, it seemed good value for money given the specs, and it lasted very well exposed for many months on a fairly exposed site.

[PeterW]

+1 to Cromar. We found it was who made the local merchants own brand and was certainly cheaper than the main brands but very good quality. 

[Thedreamer]

Is this the sort of job that is done despite the risk of damage? 

 

I.e you wouldn't lay blocks or pour concrete in bad conditions, so why do some jobs like this, not have the same risk assessment applied?

 

 

 

 

[Cpd]

7 hours ago, Thedreamer said:

Is this the sort of job that is done despite the risk of damage? 

I am not sure if this is isolated to a specific membrane (roofsheild) or if it effects all breather membranes. If it’s across the whole range then it’s something that needs to be  much more widely known about so that people are at least aware of the potential problems.  

[Ed Davies]

As I see it, there are two issues:

 

a) if any breather membranes are at risk when they're laid from a combination of wetting and freezing then that indeed needs to be widely known and taken into account in planning a build but, much more importantly…

 

b) the point of breather membranes is that they survive if they get wet (due to leaks in the roof material over them or condensation on its bottom) and the tops of roofs are very liable to freezing so you have to wonder if there's a possibility that these membranes will fail in the long run even if they're laid in ideal conditions and covered quickly.

 

That's assuming the problem is actually freezing when wet. The plural of anecdote is not data, etc.

[Jeremy Harris]

Interestingly, when clearing up our site before doing the landscaping, I found an offcut of our roofing membrane.  It had delaminated quite badly, with the film layer partially separated from the Tyvek-like fabric layer.  It seems likely that this may have been frost damage, as the bit of membrane was underneath a pallet, so out of the direct sun, but probably wet for a fair bit of the time.

[ProDave]

We used Protec VP400 plus for our roofimg membrane, on the advice of the builder that it would survive an extended period before the tiles went on.

 

This seems to have been good advice.  I have lined the external walls of the sun room  with that, and 3 (or coming up to 4) years later it is still like that, clad in the still exposed VP400 which still seems in good condition. It definitely will have got wet and definitely will have been well below freezing.

[Ed Davies]

I did my gable ends in Protec TF200 partly because I'd seen it on other houses around here left exposed for a long time without apparent harm, at least as seen from a distance. So the VP400's a definite candidate for the roof.

 

What membrane was yours, @JSHarris?

[Jeremy Harris]

6 minutes ago, Ed Davies said:

What membrane was yours, @JSHarris?

 

Ampatop Aero Plus, the stuff that has built-in self- adhesive tape at both edges.

[Mr Punter]

I am surprised at how easily a lot of these membranes burn, especially bearing in mind they are in well ventilated concealed cavities.

[Jeremy Harris]

8 minutes ago, Mr Punter said:

I am surprised at how easily a lot of these membranes burn, especially bearing in mind they are in well ventilated concealed cavities.

 

 

Not sure if they are more flammable than the stuff they are replacing, though, bitumen coated felt.  I've never tested it, but I suspect that the old bitumen coated stuff probably burns pretty well, and may well drip burning drops of bitumen as it does so.

[ADLIan]

Important to follow manufacturer instructions and BBA certs here. Some membranes can only be left exposed for a matter of weeks, maximum I've noted recently is 4 months. All state however that the roof should be tiled asap and the membrane not left exposed for any longer than necessary. In the case of Roofshield the manufacturer specifically mentions the damaging effect of wetting and subsequent freeze/thaw cycling (they reference, along with others,  BBA information Bulletin No. 2 Permeable Roof Tile Underlay — Guide to Good Site Practice)

 

As regards fire the membrane is protected from fire internally by the plasterboard ceiling and externally by the slate/tile finish so will never be involved in the development stage of a fire. They are also very thin so the heat energy will always be limited and peak very quickly. As per JSHarris they cannot be any worse than the old bitumen based products!

[Cpd]

16 minutes ago, ADLIan said:

In the case of Roofshield the manufacturer specifically mentions the damaging effect of wetting and subsequent freeze/thaw cycling (they reference, along with others

.............that will teach me not to read the instructions......... it’s crazzy that this is used so widespread around Scotland right through the winter months...... when I originally asked in the builders merchant about the best stuff to use..... well I’m glad we are all now aware of the perils of certain roofing membranes another win for build hub all in all. Luv this forum.