Polyurethane liquid roofing resin vs GRP

[andyscotland]

I'll soon be ready to do the flat roof covering on my project. I'm going to use a wet-applied product, in particular because it's an L-shaped roof with several rooflights on site-built upstands and I like the idea of not having any seams / joints in the final finish.

 

I had been going to use GRP, but have come across advertising for a couple of polyurethane-based resins e.g. Ultraflex http://www.ultra-flex.com/ 

 

It looks like they're the same basic concept as GRP - applied over a chopped strand matting etc - but according to the marketing with a few benefits:

 

• Moisture curing so instantly rain resistant - attractive in Edinburgh where it's not uncommon to get rain even if the forecast says it'll be dry
• Usable as soon as you open the tin - no catalyst mixing etc
• Resultant membrane is highly flexible - I've read that one concern with GRP vs e.g. EDPM is that because it's more rigid you can get more issues with expansion/contraction and related noises. Although my roof area's not that big, 7m x 3m on the longest side of the L. But my project spans across an existing building and a new section with new rafters - potentially there'll be a little bit of shrinkage movement at first so the idea of a more flexible covering is attractive.

 

But apart from their own marketing I can't really find anyone online talking about / using polyurethane products - everyone seems to go for standard GRP. Does anyone on here have any experience with these?

 

The ultraflex one is BBA/NHBC certified and meets my fire resistance criteria, so presumably it should be OK. It'd just be nice to feel a bit more confident people actually used / recommended it - I'm always cautious about putting too much weight on what a manufacturer says!

[MJNewton]

The flat roof on our extension was covered with a PU liquid membrane - Desmopol. Whilst we initially had issues with the roof construction (fall and quality of finish) all those clouds have blown away (nearly; I struggle to let go of such anxieties) leaving me quite a fan of this type of coating. Of course, it is too early to comment on longevity but there are various 'features' to it that appeal:

 

- Single covering with no joins

- Very flexible and seemingly very tough

- Easy to re-cover should the need arise (damage or planned renewal/maintenance) just by putting another coat over the top (when it comes to doing this I'll definitely be DIYing it - my roofer clearly showed there wasn't much skill in it!)

- Minimal risk of failings around upstands etc unlike other coverings that depend on careful detailing

- Can be used with zero degree falls (good for my roof that doesn't fall quite as much as it should due to an error in furring calculations!)

 

The technical director of a roofing supplier (who supplies all types so I don't suspect bias) told me that, like seemingly many building technologies, here in the UK we seem to hold tightly on to 'traditional' methods and so new/novel techniques can struggle to catch on despite offering improvements and being used widely elsewhere (particularly on the continent).

 

Edited May 28, 2019 by MJNewton

[Conor]

I've only experience with GRP.

 

When it goes well, it's great. But tbh I had loads, and loads of issues. First sheets went down in cold weather... Resin too viscous and then ran out. Next time, it was quite sunny and warm.... Resin started going off too soon. Went lumpy and ended up having to bring a lot of it down. And last attempt was in the evening in perfect conditions. Went well.

 

And to cap it off, when I did the final part of the top coat, left it too late in the day. Got cool and a heavy dew descended before the resin fully dried, so left a white, dusty sheen to to. At least it was only a shed!

 

I'd use it again, as I know what to expect, as it's cost effective, and one on its bloody solid and I can't see it failing or leaking in my lifetime.

 

But I can see the advantage of being able to use a product and not having to worry about a light shower, or getting catalyst mix right.

 

You can see the light dusting on the RHS. Still, happy enough with it despite the multiple mess-ups!

Edited May 28, 2019 by Conor

[scottishjohn]

so to be fair all you grp issues were due to your inexperience of its use

grp is the way to go ,if its really hot use less hardener . don,t do it when  its frosty 

wait for a dry day .do not fit new sheeting your going to cover with grp  weeks before ,as it will get wet and that will cause adhesion problems ,or rot of boards if you seal it in 

If you must then it needs to be covered until your GRP day 

don,t try to do it when its  very cold --and damp 

most professional roofers will not grp a roof unless its new boarding as well ,because of above problems 

always under harden if anything --but try to be accurate with mix and mix it up very well .lumps in mix is hardener not mixed in and you have a hardener rich bit ,so it goes off quicker in that bit--do more smaller mixs than one BIG mix ,untill you got the hang of how far it will go and how quick you can use it  up 

after your first mix you will see how long it is before it starts to go off .

if gel time is too long then make next one slightly more hardener ,and i mean slightly

1% is normal mix ,1.5% -2% is a quick setting mix for very very cold day   on a stinking hot day  0.5%--it iwl ljust effect gel time -- it will still harden completly with 0.5%well mixed up

seeing your resin bucket smoking,because too much hardener is not funny or good use of resin 

 lean on the roller --should be see through if you have done it enough ,and if surface is wet with resin -you used a bit much  --but not a problem + if you laying 2 layers don,t put as much on second time and roll it hard for see though finish 

I have used a special grp that is uv activated and will go off in any conditions ,but its expensive and only really for emergency roof repairs in winter .

it has a shelf life of 30 days and once you open it it will start going  off  in the daylight .

 

[Conor]

@scottishjohn spot on. Was using 1.5-2% and was just going off too fast. Used 1% for the top coat and was much better. 

 

I'd also suggest you have loads and loads of spare buckets, mixing paddles, gloves etc... It's very messy and brushes etc are one time use. And have extra tubs of resin and catalyst... Better to have too many and have to return to shop than not enough just when you get to your last sheet...

 

[Jeremy Harris]

There's no substitute for a bit of GRP experience when doing a roofing job, as it's probably one of the most potentially difficult GRP jobs to do, just because of the extreme sensitivity of polyester resin to slight changes in temperature.  I've seen experienced GRP boatbuilders struggle when doing roofing, because of the need to fine tune the amount of catalyst to add.  What's worse, is that you can run the risk of under-catalysing the resin and have the styrene evaporate off before the resin has cured, leaving a sticky mess.

 

I've done a couple of roofs with GRP and found that the secret is to mix the resin very quickly and quickly empty the whole mixing pot on to the roof, then roll it out.  If the resin sits in the pot it will warm up and start to cure too quickly, and spreading it out slows this down a lot, giving more working time.  This only works well if the roof isn't hot, though.  Not a good idea to try and do a roof in bright sunshine.  Equally not a good idea to try and do a roof if there is a trace of moisture anywhere, as moisture is a real enemy when it comes to GRP.

[SteamyTea]

Under catalysing is a cardinal sin in the composite industry.

You can get hardeners that have different 'speeds' so that you still get full polymer crosslinking.

If you have not already fitted the roofing sheets, you could coat them in GRP before fitting.  That way you can do it inside and only have a few joints to seal site.

I have never understood why it is done totally onsite, that is just making an easy job difficult.

[andyscotland]

Thanks, some really helpful advice here.

 

9 hours ago, MJNewton said:

The flat roof on our extension was covered with a PU liquid membrane - Desmopol. 

 

Looks like that specific product doesn't meet the fire rating I need but very good to know of someone who's used a PU membrane and been happy with it!

 

@Conor @scottishjohn @JSHarris thanks for the helpful tips.

 

7 hours ago, scottishjohn said:

wait for a dry day .do not fit new sheeting your going to cover with grp  weeks before ,as it will get wet and that will cause adhesion problems ,or rot of boards if you seal it in

 

It's a new OSB deck going down, it won't be there weeks before but I won't manage to get it done in a single day. It's an awkward shaped roof abutting the house with every side angled and rooflight upstands. So a lot of sheets and trims to measure and cut. Plus the insulation and VCL is held down by screwing down through the OSB, so I'll have to cut and fit all of that at the same time too.

 

I was hoping if I choose a fairly dry week and put a decent tarp over it at night/if the weather changes with space for some airflow between deck and tarp that would be ok?

 

5 hours ago, JSHarris said:

There's no substitute for a bit of GRP experience when doing a roofing job, 

 

Not had much experience yet, but I have a shed and a valley gutter (over a narrow void between us and the neighbour so wouldn't be the end of the work if the waterproofing wasn't perfect) that I'm planning to do first for practice.

 

5 hours ago, JSHarris said:

Just because of the extreme sensitivity of polyester resin to slight changes in temperature.

 

As I understand it, that's supposedly one of the benefits of the polyurethane resin - as it's pre-mixed and moisture cured it's not so sensitive to environmental conditions. They advertise an initial drying time of 2-8 hours. I guess as it's immediately rain resistant once the mat is covered they can manufacture on the basis there's not the same need to get it to cure rapidly so can produce a slower/less volatile chemical reaction.

 

I'm also wondering if the moisture-cure might give a little more tolerance for any slight dampness in the sheathing...

 

1 hour ago, SteamyTea said:

If you have not already fitted the roofing sheets, you could coat them in GRP before fitting.  That way you can do it inside and only have a few joints to seal site.

 

Would that work? Feels like it'd conflict with the idea that a big benefit of GRP is the total absence of joints. I'd have to go back over all the fixings too, I guess - could be quite a lot to redo by the time it's all on the roof and screwed down. I'm using tongue and groove, 2400x600, so there's quite a lot of board joints...

Edited May 28, 2019 by andyscotland

[SteamyTea]

5 hours ago, andyscotland said:

Would that work? Feels like it'd conflict with the idea that a big benefit of GRP is the total absence of joints. I'd have to go back over all the fixings too, I guess - could be quite a lot to redo by the time it's all on the roof and screwed down. I'm using tongue and groove, 2400x600, so there's quite a lot of board joints...

It would work.  A self-etching resin would make the bond just as strong.

 

Most hand layup composites are done in multiple stages to get the desired thickness to avoiding temperature buildup, shrinkage and unevenness. 

[scottishjohn]

9 hours ago, SteamyTea said:

I have never understood why it is done totally onsite, that is just making an easy job difficult.

i think its so the T&G fit together right --any resin in joint could make it harder to fit .

maybe if you running out of time to fit grp  same day -then just coat the boarding once fixed + sheet it up before you leave for the day ?,but will end costing more in resin  as it will not help wet out the cloth 

 

 

 

[SteamyTea]

I think this is the difference between a DIY bodge and a professional service.

But I may be biased as I worked in the composite industry for 25 years or so.

I still itch.

[Jeremy Harris]

10 hours ago, SteamyTea said:

Under catalysing is a cardinal sin in the composite industry.

You can get hardeners that have different 'speeds' so that you still get full polymer crosslinking.

If you have not already fitted the roofing sheets, you could coat them in GRP before fitting.  That way you can do it inside and only have a few joints to seal site.

I have never understood why it is done totally onsite, that is just making an easy job difficult.

 

 

I agree, but it's been completely normal practice for as long as I can remember to vary the catalyst ratio to compensate for temperature variations.  IIRC, the minimum is around 1%, the maximum around 3%, with a decision being made on the ratio to use based on the temperature and the volume of the mix (higher pot volume = higher initial temperature rise = lower catalyst ratio needed).

 

As long as there's enough catalyst to start cross linking before the styrene evaporates then that's generally OK.  I well remember laying up the deck of a fairly large (~40ft) boat indoors in winter and deciding to erect a plastic sheet tent over it to try and reduce the risk of the styrene evaporating before the cure was well-established, as being such a large single lay-up we'd used a low catalyst ratio to extend the pot life.

[SteamyTea]

The 'newer' low styrene emission resins that came out about 30 years ago were a revelation.  Could abuse them something rotten and seemingly get away with it.

My old boss came to work for me on a large project when I was working on site for a couple of months.  To save a few quid for the customer he started to use an old, but cheaper polyester resin.

The parts lasted a few days in service.

Now if anyone wants to buy an osmosis free, high temperature resin system, I still have it for sale.  Not cheap, but cheaper than the alternatives.

I am still amazed that no one has cottoned on to what I did, especial the boat builders.  It did cost me a contract to make wind turbine blades for one of the few large turbine manufacturers back in the early 90's.

They are not about anymore.

[andyscotland]

4 hours ago, scottishjohn said:

i think its so the T&G fit together right --any resin in joint could make it harder to fit .

maybe if you running out of time to fit grp  same day -then just coat the boarding once fixed + sheet it up before you leave for the day ?,but will end costing more in resin  as it will not help wet out the cloth 

 

 

 

 

You mean just do a coat of the resin (without matting etc) as the boards go down just to temporarily waterproof them? And then apply a full coat of resin & matting (as if going directly onto the wood) once the OSB layer is complete? That's an interesting idea. I could possibly tolerate the cost of a bit of extra resin if it reduced the risk of moisture problems. Would the final resin coat bond OK to the initial one if it was a day or two later - would I have to sand or otherwise prepare it once it had cured?

 

Still a bit more cautious about the idea of entirely coating the boards before they go up. As you say, unless I was very careful masking off the tongues and grooves I can imagine that'd be a bit of a nightmare to fit together. And can't quite get my head round the robustness and appearance of the end result once all the board joints and screw heads are covered. A little nervous about doing anything too far outside "standard" as well, in case it causes me problems when Building Control come to inspect.

 

In fact thinking about it - would it be better to break the roof into sections and do the complete buildup on each one at a time - pick an area, lay the insulation and OSB, fully GRP it before moving to the next area? Presumably would need to take care that each section was well joined to the ones either side. Feels like that would be a bit of a hybrid between the two ideas - fewer joints than doing the individual boards before they go up, and no screw heads to cover afterwards. Is that worth exploring?

 

 

Edited May 29, 2019 by andyscotland

[Conor]

5 minutes ago, andyscotland said:

 

You mean just do a coat of the resin (without matting etc) as the boards go down just to temporarily waterproof them? And then apply a full coat of resin & matting (as if going directly onto the wood) once the OSB layer is complete? That's an interesting idea. I could possibly tolerate the cost of a bit of extra resin if it reduced the risk of moisture problems. Would the final resin coat bond OK to the initial one if it was a day or two later - would I have to sand or otherwise prepare it once it had cured?

 

 

 

 

That's what I ended up doing when we realised there wouldn't be enough resin to do the roof in one go. Spread out what we had remaining in order to waterproof the roof. Then went back a second time to complete the job

 Just need to lightly rub down the base before putting the GRP down. Also need to take the join with the previous cured sheet.

[SteamyTea]

5 minutes ago, andyscotland said:

You mean just do a coat of the resin

Not how I would do it.

6 minutes ago, andyscotland said:

Would the final resin coat bond OK to the initial one if it was a day or two later

Depends on which type of resin you use.

6 minutes ago, andyscotland said:

As you say, unless I was very careful masking off the tongues and grooves I can imagine that'd be a bit of a nightmare to fit together.

You don't have to lay the mat up to each edge.  In fact I would not attempt to do that.  I would stagger the layers anyway, then same with any joining strips.  That way, there would be less of a thickness difference.

I would also apply the flow coat/top coat/glaze to 95% of the already laminated area.

This top coat is what really keeps the moisture out and is really the hardest to apply properly.

[jack]

23 hours ago, JSHarris said:

I've done a couple of roofs with GRP and found that the secret is to mix the resin very quickly and quickly empty the whole mixing pot on to the roof, then roll it out.  If the resin sits in the pot it will warm up and start to cure too quickly, and spreading it out slows this down a lot, giving more working time.  

 

I did some fiberglassing recently and this was exactly my experience. The first lot ended up heating in the bucket to the point where it went off extremely quickly (I only used 2% catalyst and it was only about 10 degrees). In fact, it got so hot that I actually burned my arm on the frame of the roller when I took it out of the bucket when it was partly cured.

 

In the end, I concluded that you needed to quickly get it out of the bucket and onto a coolish surface, or use small amounts each time. I also concluded that the ambient temp was more or less useless for calculating the amount of catalyst. The real temps that mattered were the starting temp of the resin and the temp of the surface to which it was being applied. Once I figured that out it mostly (mostly!) went okay.

[Onoff]

If anyone's interested:

 

[andyscotland]

10 hours ago, SteamyTea said:

I would stagger the layers anyway, then same with any joining strips.  That way, there would be less of a thickness difference.

I would also apply the flow coat/top coat/glaze to 95% of the already laminated area.

 

I'm probably being dense but I still can't quite picture exactly what the layers would be and how they'd meet/overlap.

 

Do you mean:

Resin, mat, resin to the centre of each board when on the ground leaving a border round all sides.

Then top coat 95% of that area, again on the ground

Then fit to the roof once that's all cured

Then resin, mat/bandage, resin along all the joins and untreated border areas and onto the roof trims

Then what - just topcoat the join areas / topcoat the whole thing / something else?

 

Just trying to get my head round it. If I leave a border on all the boards there'd still be a fair bit of wood that could get wet between putting the first board on the roof and being ready to do the final coat(s)?

 

[andyscotland]

5 hours ago, Onoff said:

If anyone's interested:

 

Thanks - Coventry is a bit far for me but I'd not come across the product before. Looks interesting. Have you used it / do you rate it?

[PeterW]

That Flexitec stuff has been around a while - even Wickes sell it