Effective, DIY friendly, ways to build a GH using twin(multi)wall polycarbonate

Hi guys and gals.

Moment is approaching when i'll have to design and build my lean-to greenhouse.
Twinwall seems good enough for what i need (a 2 zone extension) and price is decent.
It's only problem is the "fixing system".
You want cheap and easy ?
Screw it down.
Disadvantages are it's not really good.
The screws are tightened over a large polycarbonate washer that is not watertight.
Worse is, because the thermal expansion is large, holes are large to accommodate movement.
That means water and dirt will get in.
It's not a matter of IF, it's just a matter of when.

You want effective system ?
Use the gasket system.
It basically has 2 parts, 1 underneath the joining edge, having it's own gasket and another one on top, again, with it's own gasket.
It works in keeping moisture out and there are no holes for dirt to get in.
Excellent, right ?
NO.
It's like very expensive.
And it's only good if you have long, narrow boards, like 2ft narrow.
If they are wider, there's nothing holding the board in the middle.
You might wake up one morning to find your glazing blown away by wind since it's hold only by the edges.

So, what to do with 20ft/7ft twinwall boards (like almost all i have access to)?
Are there any large gasketed effective washers out there at a decent price ?
I could not find them (not where i live).
Are they really effective ? A hole is a hole after all ...

What if i'm not willing to drill holes in them ?
How do i fix them to the underlying structure ?
If i cut them to 2ft wide it's no problem, i can use the split gasketed system, but by the time i realize the cost ... screw it (pun intended).

SO, what solutions could you envision ?

hmm.... there are screws for roofing that have rubber-washers attached. they should be cheaper. but probably they re not wide enough.
you could put (clear) silicon under the washers of the system you mentioned.

a good solution would be to build a timberframe. with the posts nearly 7ft apart. join the two sheets above these posts. leave a small gap between them and screw a board above the gap. screws go through the gap, not through the panels. use caulking under the board and on the post.

so you would frame these boards like you would frame a painting/picture.

that would involve lots of wood-cutting, but would make a stable and tight structure.

Tobias, you're describing the second system, without special factory Alu profiles.

What if the twinwall is 7 ft wide ?
The only thing holding it down will be the strips that fix it on the sides, and that by overlapping it 1" at the most.
What's the problem you might say, as the main forces are gravity based, basically keeping the boards down ?
Well, wind gusts may determine greenhouse pressurizing which makes the whole thing swell and that makes the boards go up with nothing holding them down but those narrow strips on the sides.
THAT can be a problem.

Regarding screws for metal roofs, totally unappropriate.
I've used them and the rubber is too hard, which is normal since it is supposed to fix a sheet of metal to some wood member.

Regarding caulking, i've lost my faith in it as a long term solution a long time ago.

Here's a video showing a nice installation.

It does not say what the issues are if you put 7ft wide, thin boards, in this configuration.
Thick twinwall or multiwall has less issues but also is much more pricey and my budget is not endless.

if you worry about the wind pressure, then you could place additional posts/beams. with wood you ll have many options. you can put these additional posts/beams behind the boards to support them. you might screw the boards to these. or put them on the inside + the outside.

you can build it somewhat similar to that system in the video. the problem then is that the water needs to run off freely. standing water against outside beams will destroy the wood quickly.


maybe you could test things by building other stuff with this kind of boards and see how it works out. maybe a cupboards like mini.greenhouse for raising plant-starts. or a small roof to protect tools or bicycles.


the greenhouse you re building sounds huge. how big are you planning to go?

good luck and best wishes


EDIT to add: or you could make a solid timberframe and cob the boards in. like they do with fixed windows in cob-building. that should create a tight seal, but would not solve the problem with the support just being on the sides.

Couldn't you just attach a thin piece of wood across the middle of the sheet of twin wall and attach it to the boards on the sides?  It wouldn't take much to keep the twin wall from bowing.

The size should be 11/6m, that's 36/19 ft for the other side of the pond.
I call it decent size.

All the underneath structure can do is protect against up to down forces like heavy snow.
If the inside gets a little pressurized it's not a problem.
The problem arises when this pressure fluctuates and boards go up/down.
At some point they'll go up and away.

If doing it by screws, you're set but have to contend with infiltration.
If not, then some piece of material like a beam of sorts has to be on the outside.
Problem is that outside is subject to the weather, as you observed.
Wood is not very suited for this unless a good species could be found (pretty hard) or treated with yakisugi.
Even so, i don't plan on using much wood  because of this, at most just the purlins since you can easily screw things to them.
Otherwise, probably steel pipe square/rectangular, paint/primer protected.

you could make the outside "middle-beams" and the "boards" to cover the seams of the green-house-sheets from metal. the interior posts and beams could be made from would. or metal, too ... but that would be more complicated to build.

or you could build your lean-to like they do with some earthships and cob/mortar the sheets into a solid kind of structure. just thinking. clear sheets set into columns of brick/concrete with a steel construction for the roof.

I did some calculations and it will be more cost effective to buy the outside aluminum gasketed plates to cover the vertical seams than to roll my own.
On the other hand, it's cheaper to roll my own "beams" on the outside to keep them boards flying away.
Only issue here is that it has to be horizontal, fixed at each 7ft.
If i put it vertical, i can only fix it at the ends, that's 21ft, will have a lot of deflection, not very effective, might just as well use a wire.
The problem with horizontal is that dirt, water, snow, etc. Will accumulate.
Or i could use some rubber spacers 1/2" in height, every 10" or so, evenly spaced.
I've seen this done on the inside, where these spacers are placed between the sheets an purlins, basically creating an air space, so condensation water does not gather at the purlins but just flows down the sheets until the end.
It's a nice antidripping system easily aplicable to diy building.

Later edit:
Here's a file describing the anti drip system with the spacers.
Something like it could be  done in reverse but the thing must be devised carefully such that water can flow freely downhill and dirt does not accumulate.
Also, another factor to consider is that sometimes snow will accumulate on top and there are moments you would want to take it down.
The hold down system should not impede this.