how to fix polycarbonate panels in a greenhouse roof

[AnTTun]

@ Suzanne & Tony:

https://guttashop.hr/aluline-h-profil

Webpage is in Croatian but I guess if you search 'Gutta polycarb' you'd find something in English. Not sure if their products are available in the UK, but at least you'll get an idea 'what does the job'.

Good luck.

[topsy]

Hello all,

I fully believed that a number of you would come across with some suggestions for our problem for which I thank you all.

I think the simplest wil be the Dow Corning glue which we h ave used in constructing fish tanks in the past but that was glass to glass, aluminium to glass will be a alittle different and will have to wait for dry weather, I suppose we will get some one day!

Again many thanks for all your suggestions.

Suzanne & Tony

[habanerocat]

Hello all,

I fully believed that a number of you would come across with some suggestions for our problem for which I thank you all.

I think the simplest wil be the Dow Corning glue which we h ave used in constructing fish tanks in the past but that was glass to glass, aluminium to glass will be a alittle different and will have to wait for dry weather, I suppose we will get some one day!

Again many thanks for all your suggestions.

Suzanne & Tony

Fish tanks use high modulus sealants. You need a low modulus sealant because of the difference in expansion and contraction between the polycarbonate and aluminium.
Have a look at the links I posted.

[iann]

A sheet of plywood over the top, screwed into the greenhouse frame, should do it

[SimonT]

I had a big section of cellular polycarbonate roof try to blow away during a winter gale.
To put this back on the wooden frame I used screws with big washers through the polycarbonate to pin the sheet back in place.
I was a bit worried it would split the sheets but actually it seems to work well - and you can buy similar fixings online for polycarbonate (typically with rubber washers). My roof has metal slots that the sheets are mounted into, but as others have mentioned the polycarbonate tends to flex and then come out of mountings, I found the extra screws held the polycarbonate to the frame but also stopped a lot of the flexing. I used sealant as well and you have to check it has the right chemistry and modulus as habanerocat mentioned. Some sealants react badly with polycarbonate and so you can end up with bigger problems if you use the wrong type. Strangely crows seem to really, really like to pull out the sealant- not sure what can be done about this problem!

[iann]

I've had magpies attack the sealant around my house windows. Not every year, but consistently over a long period when they get in the habit. Must be filling

[ralphrmartin]

A sheet of plywood over the top, screwed into the greenhouse frame, should do it

If you can get transparent plywood, why bother with the polycarbonate at all?! I must be missing something here...

Given that the polycarbonate is so flexible and has a large area, trying to secure it by the edges may not be a good strategy. I favour the solution of exterior rigid thin diagonal struts (wood / aluminium) across each face to prevent them popping out.

[SimonT]

I guess what to choose depends upon the overall design.
In my case the wind has to get under the panels from outside or blow panels in from outside to cause issues.
The pins just fix the panels into the internal supports and stop this.
This was how it was built originally- I just made stronger fixings when some of the old ones rusted.

[Keith H]

Replacing glass with polycarbonate is in itself a problem as older aluminium framed greenhouses used the glass to aid rigidity in the frames, as is seen when building the greenhouse which becomes much more stable when fully glazed.

I vaguely remember a discussion on here where additional aluminium cross bars were added to the inside of the greenhouse to which the polycarbonate was screwed.

I guess the OP has searched the forum with key word polycarbonate and not come across it.

[Acid John]

Simon, I don't think the wind blows the panels out unless you have the door and vents wide open but more like the wind passing over the greenhouse causing lower pressure like a aeroplane upper wing surface so sucking the poly-sheets out of the frame.