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Waterproofing roofing plywood.

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490808
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Joined: Mon Jul 19, 2021 1:10 pm

Waterproofing roofing plywood.

#1

Post by 490808 »

I'm stripping back the felt roof over our back door that I did 15 years ago.

It wasn't leaking but was a mess and the felt was never really burnt on right evidenced by the fact it comes up leaving a fairly clean deck.

Now the plan is to put down a membrane then two layers of 25mm insulation then a waterproof felt or fiberglass finish (might use a single 50mm layer of insulation two 25's is for overlap).

Image

I'm stripping back and "4" in the diagram above is in decent condition but sticky with tar and the grain has started to open up. What I'd like to do is waterproof the surface before putting the barrier membrane "3" down. Its unnecessary I know but would free me up for a few days/couple of weeks while I decide how to finish it all off. I've some very awkward detail where the roof meets the main house and could be messing with that for quite a while.

What I was thinking was treating the roof side of the plywood with Cromapol Acrylic Roof Coating has anyone got any experience of using it just concerned I might end up with a rough surface when I want a smooth one got the insulation to against. Other suggestions welcome.
kadman
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Re: Waterproofing roofing plywood.

#2

Post by kadman »

Personally I have no experience of the coating in question, so cant offer any help there.

Have you checked out any reviews on the proposed product you intend using.

The fact that you got 15 years out of a flat roof with no leaks means you did it well the first time around.

If you put up a picture of the messy detail, when you get a chance, then you might get a solution here :)
kadman
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Re: Waterproofing roofing plywood.

#3

Post by kadman »

Interesting test on its abilities :)
Maybe you have seen this

490808
Posts: 1487
Joined: Mon Jul 19, 2021 1:10 pm

Re: Waterproofing roofing plywood.

#4

Post by 490808 »

Thanks, I remember watching that ages ago you can't mistake the guys accent.

My question is more on how well that stuff works directly on wood?
kadman
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Re: Waterproofing roofing plywood.

#5

Post by kadman »

Maybe test it on a similar piece of material your self in different scenarios.
Plywood, painted and raw, and smoother materials like melamine faced boards ect.

I would imagine that a rougher surface like plywood would give any material better adhesion than
a smooth plastic like surface. But a test would tell all.
490808
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Re: Waterproofing roofing plywood.

#6

Post by 490808 »

Just to follow up here I changed my mind as I was doing the job and decided to do a glass fiber resin roof. That didn't really change my requirement for waterproofing the original roof layer 4 in the diagram but because it had had tar on it in the past I gave the whole lot a coat of black bitumen paint which I know will last then put a dpc on top when it was dry.

If anyone is interested I might start another thread on how the GFR roof goes. So far despite much over thinking its going really well but need some good weather to get it finished and will need to sand it all if I have to wait another day.
kadman
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Re: Waterproofing roofing plywood.

#7

Post by kadman »

Of course we are interested, what do you think we are here for :lol:
CelticRambler
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Re: Waterproofing roofing plywood.

#8

Post by CelticRambler »

Wot 'e sed. :lol:
490808
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Joined: Mon Jul 19, 2021 1:10 pm

Re: Waterproofing roofing plywood.

#9

Post by 490808 »

Well for the GFR roof I didn't have a bulls notion. However my reason for going that route was that there is an old thatch under a tin roof abutting the flat roof and I didn't want to burn the house down.

First mistake I might have made but it seems to have been a non issue was I used plywood for the deck. Several youtubers say not to use it but if you do to sand the surface first. I also used much thicker sheets than I really needed - I used 18mm ply and could have used 12mm OSB.

I also wanted to buck the GFR system by waterproofing at least the edge of the deck near the main roof before doing anything else. The reason being that its very difficult to do a good job of flashing in that area and it will be the first area of failure long term. So what I did after sanding the plywood deck was apply a coat of resin with 10% Acetone added. The idea is that the Acetone improves the penetration into the wood.

I started just mixing up 200ml of resin (+ 20ml Acetone + 4ml hardener) so I could waterproof and prime the edge where I think water might get in before nailing down a fillet trim. iirc I then made up 250ml of the same mix to finish coating the rest of the roof, think I did two mixes. That stage would normally be completely unnecessary but worked out really well for me because of the weather. Rain was forecast and when it came the deck was all taped up and waterproof so I didn't have to spend a load of time covering it up.

Anyone else would have used a roller but I did the whole initial base coat with a 3 inch brush and it went on really well and smoothly. The deck is only 6 square meters so might stick with the brush method for the glass fiber mat. 6 square meters thats two 8x4 sheets side by side.

With the current weather I'm resigned to doing this is stages so I'll have to sand the base coat down as I doubt I will be able to get back to it within 48 hours because of the weather. With an electric sander thats no more than an hours work so no big deal for a DIY job but I can see why a professional wouldn't want the hassle.

As soon as the base cost was dry I spent a couple of hours putting the trim on. Way to fussy and much overthinking later (existing roof gutter (non) issue) it all fitted really well and was nailed down.

Last thing I did with minutes to spare before it rained (last resin went down about 15 minutes beforehand) was tape all the edges of the trim. Again used a brush and mixed small amounts resin (250ml at a time). I spent extra and bought a roll of 75mm glass fibre tape rather than cutting stips off the main blanket roll. I suspect the tape made it much easier.

So thats where I am now. If you want part numbers and quantities I'll through them up when I've finished and know exactly how much I've use.

This how I'm doing it and video 3 is where I am - at the end of the video. Obviously find video 1 of the series if you want all the steps which include the part numbers of the trim.

kadman
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Re: Waterproofing roofing plywood.

#10

Post by kadman »

I know its outside work, but when you are sanding that stuff, wear a mask....well done.
490808
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Re: Waterproofing roofing plywood.

#11

Post by 490808 »

I've a vacuum cleaner running all the time and picking up the dust, but the sanding will get real before I do the laminate coat so have a decent cartridge mask for particles (as opposed to solvents).
490808
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Re: Waterproofing roofing plywood.

#12

Post by 490808 »

After a day and most of a night of serious rain everything is holding up. Don't forget I've not even done the main lamination layer yet.

I'm out now tidying up some pin holes which are made very obvious by the facing on the cheap chinese plywood.

What has happened is water has got into a few pin holes and soaked up by the blotting paper like surface facing of the plywood. Thats not a big deal as it is mainly rings around the screw fixings. What I have just done is sand down these areas and recoat them. I was hoping to do the main lamination today (or should the by lamentation?) but the weather is against me. Tomorrow is looking good.
kadman
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Re: Waterproofing roofing plywood.

#13

Post by kadman »

Simple enough process, but you want the weather with you , instead of all this rain.
490808
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Re: Waterproofing roofing plywood.

#14

Post by 490808 »

No go again yesterday with the weather. So now 48 hours later I've sanded the roof. Cleaned it again. Cut out all my chopped strand mat CSM to fit and marked its position on the roof. Started out with a 55cm wide strip at the top of roof. Made up a liter of and a half of resin and started off. All went well but I was about half a liter short so made that up quickly and first bit done.

You can guess what happened next. About the worst thing other than me sticking myself to the deck - it rained.

Being a glass half empty pessimist I had planned for that and with the roof already primed and all the materials under cover no harm done.

Half an hour later I wiped the roof down with a cloth then with a bit of acetone and started on a full meter wide strip. Lucky I worked out in time that I couldn't reach across with a hand roller so managed to find an old broom I could steal the handle off to shove in the end of the roller handle and make it longer.

I did some more reading last night on catalysts and decided as it was warm enough to reduce the catalyst to 1%. Previously I'd been using 2%. 1% gave me a bit longer time to work but I don't reckon I could do more than about 3 square meters at a time. A larger roller would help, I am currently working with a 15cm one.

As for materials I'll put this down here so I can find it again if I ever need to remember. 2.4 square meter of 600gram CSM took almost exactly 4 liters of resin.

There are lots of little things that no one tells you about working with glass fiber so I'll try and include as many things I discover as I can.

First total revelation was that resin doesn't stick to plastic mixing buckets so when the resin is dry you can just pull it off so you can reuse the bucket. Rollers are throw away, I've not found a way to clean them so each fresh mix uses a new roller. Brushes if the resin hasn't cured do clean up with Acetone but you need to use so much that its just as easy to throw them away as well. I'll try and add up all the consumables I used at the end so anyone else can see what you need. A 10 square meter kit for doing a roof sounds OK at around €270 ex VAT but by the time you have added in all the trim and consumables you can more than double that and add VAT on.

Just finishing a cup of tea before I do the last strip. I've a bit of sanding to do first as I have some blobs of spilt resin to clean up in the next section that had started to cure before I spotted it. Probably don't really need to bother but I will anyway.
kadman
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Re: Waterproofing roofing plywood.

#15

Post by kadman »

Sounds like no nasty headaches which is a blessing. AS trying to repair a glass fibre job that ran a bit off on a tangent,
can be worse than the job itself.
490808
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Joined: Mon Jul 19, 2021 1:10 pm

Re: Waterproofing roofing plywood.

#16

Post by 490808 »

I itched like mad the first time I used the resin (could have been the Acetone or the Catalyst) so was a bit worried however that didn't persist.

It was probably my nerves which can set me off itching whenever I tackle something new or that I'm not sure of.

Loads of learning. If I was quicker and more organised I could make the job easier by trimming edges before the resin has cured but a multitool takes care of that work and I can just get the main deck right without rushing too much.
kadman
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Re: Waterproofing roofing plywood.

#17

Post by kadman »

Itching is definitely related to the stranded fibre. It was a common complaint of all attic insulation guys who fitted without
gloves...and masks :shock:
490808
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Re: Waterproofing roofing plywood.

#18

Post by 490808 »

But I wasn't handling the glass fiber when I started itching.

Just waiting now to see if another shower is going to go over. Working between showers seems OK provided it doesn't rain on the uncured resin. From what I can see of a pot of resin that got water in it the water stops the process from curing fully.
490808
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Re: Waterproofing roofing plywood.

#19

Post by 490808 »

Main lamination finished but now I have half an hour of worry :roll:

I needed two 20ml syringes of catalyst to 4l of resin and I can only remember putting in one?
490808
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Re: Waterproofing roofing plywood.

#20

Post by 490808 »

Final (blog?) post on this today. Didn't get as much as I'd hoped done due to some massive showers this afternoon. I did get all the lamination done and the whole thing is well waterproofed. I've also done a good bit of tidying up of the lamination and cleaned up all the edges.

Tomorrow if I get a long enough dry spell I'll do a plain resin coat around the edges just to seal them where I've done some heavy sanding. Also two mistakes to seal over. One where something got trapped under the laminate and another were I didn't get the laminate stuck to a raised section of trim. I'll also finish off the trim against the house with a length of 75mm glass fiber tape. I didn't run the CSM quite far enough up or at least thats my thinking.

None of that should take to long so if the weather stays good long enough I can get the top coat on.
490808
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Re: Waterproofing roofing plywood.

#21

Post by 490808 »

Finished today.

Sanded down everything and wiped over with Acetone.

Patched up some bits I wasn't happy with and added a couple bits of tape joins I didn't like the look of. Probably totally unnecessary but had the materials so used them.

Then did the top coat. The messiest part of the job? Not 100% happy with the topcoat should have done it in smaller batches but the roof is well covered and I went back and did a couple of spots that seemed a bit thin before it all dried.

Finished now bar putting the guttering back on the house. All looks well will see if I can throw up at least one picture but might not as it would be easy to identify out house from it.
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