Bending Plywood to form ribs of a greenhouse

[Landsrud]

During the spring I'm going to start on a new project that I could do with some advice.

I'm going to build a greenhouse! Yikes. Anyway, due to the nature of the place I live (Norway), and at the moment we have 1m of snow in the garden the type of construction needs to take this into consideration. I'm also looking at making it as simply as possible as well as interesting.

While trawling through the internet looking for inspiration I came across a design. It looks in profile to look like a classic hull of an upturned boat. https://norhage.no/produkt/drivhus/drivhus-tre-300/

To make the ribs that would hold the polycarbonate sheeting the design looks to have two plywood strips on the outside sandwiching spacers. This would form the shape.

My question is: I wish to make the bow of the rib out of plywood strips, bent to the radius. My idea would be to make a template /gig and then bend the plywood strips and add the spacers. How thick do you think the plywood would need to be. and how realistic is it that I could bend the strips without breaking them (obviously the curves on the ribs would need to be within the plywood tolerance). Would it be better to go for 10mm ply and bend it or best to get 4mm ply and laminate each rib? I should be able to make a steamer to bend the plywood, but how realistic is it to be able to steam 10mm strips of plywood?

Thanks in advance

 

[Sgian Dubh]

First off, there's absolutely no reason to steam plywood to create curves: in fact, it's a bad idea. All you need is a former, or a pair of formers and a means to clamp the layers of ply together. You certainly need a male former, i.e., one that is convex in shape around which you bend the ply. If the curve is gentle enough, all you need are plenty of clamps and two or three extra pieces of plywood that are unglued and placed on the outer layer of your lamination so that when clamping pressure is applied the load is spread and you don't make dents in your laminated form. Adding a sheet of plastic between these 'clamping' pieces and the actual laminated form prevents them sticking to the lamination.

Depending on the size, tightness, and complexity of the bend, your most likely adhesive choices for this would be either a slow set epoxy resin (possibly with colloidal thickeners, or similar) which would give you up to an hour (temperature dependent) to spread the adhesive and apply pressure, to a faster setting (~15 - 20 minutes) urea formaldehyde adhesive. Both the previously identified adhesives do not suffer creep. Your least likely adhesive choice would be an emulsion such as PVA or aliphatic resin: both suffer from creep, i.e., laminated bends tend to straighten as the adhesive remains slightly elastic even when fully cured. You also only get about 5 - 10 minutes to spread the adhesive and walk away from the job having completed applying all the requisite pressure with you clamps. You can slow down the cure of both emulsion types with the addition of water, but then you're in a compromise between losing overall strength and limited additional working time, i.e., up to maybe 15 - 20 minutes if you're lucky.

As to the material, obviously plywood bends much easier one way than the other because of the odd number of layers. Generally, I'd suggest running the outer layered long grain of the plywood following the arc of the bend if you can because you get more strength with a thinner laminated structure, in this case because for each plywood layer of the lamination you get one more long grain layer following the arc than short grain layers that are set transverse across the arc.

As to plywood thickness choice, this depends on the arcs radius. If the plywood will bend easily around your proposed arc at its smallest radius, it's thin enough. If it's a real struggle, it's probably too thick to manage, especially if you're working on your own. Also, you experience significantly less springback if you laminate, for example, six pieces of 3 mm plywood to make up 18 mm thick than if you laminate two pieces of 9 mm plywood, albeit, there's more glue to mix and spread on the mating faces.

Whether what you're planning to make is suitable for the task, I can't really say because I didn't fully comprehend what you're trying to make, but I'm guessing it's something along the lines of engineered wooden I beams or joists. Slainte.

 

[sunnybob]

Plywood seems to be a very unsuitable material for an external building in Norway.
Especially one that will be damp on the inside and cold on the outside.

Have you seen anyone else use this material any where near you?

 

[Landsrud]

Thanks for your detailed reply. Very insightful

 

[Landsrud]

The plywood ribs would be the internal frame, with polycarbonate panels on the outside.

Yes, this type of construction has been used. It would be used as a greenhouse in the spring and summer.

 

[sunnybob]

The inside will have the moist moisture. make sure you paint the plywood very well.

 

[Inspector]

Does it have to be made with plywood? Reason for asking is depending on the size of the greenhouse you can make it without the complication of laminating the plywood to make it longer.

To make the forms you can just hammer stakes into the ground in the arc you want and form the wood over them. That's how my buddy and I made a temporary plastic covered shelter to put his boat in while he repaired the fibreglass hull of his Folkboat. We didn't use the spacer block type of construction you are contemplating. We just bent 3 or 4 layers of wood and nailed them together with a lot of nails and no glue. It was only supposed to be up for a year but like all boat projects it was in it for a decade. It isn't a terribly complex thing to make.

If you are going to use the plywood then use an exterior grade. 3 layers of 5 or 6mm (whatever you can get in that approximate thickness range) should be lots.

I searched building wood arch green houses and building gothic arch greenhouses and found lots on the subject. Doesn't appear to be much using plywood but if you can get it cheaper than solid then use it. The two links to show how it can be done but the one has lousy sound.

https://www.youtube.com/watch?v=ifXOVvb6-q4

https://www.youtube.com/watch?v=tNR0wL6-lB8

Have fun with the build and enjoy the veggies.
Pete

 

[AndyT]

Agricultural polytunnels use a similar system of hoops to hold a clear layer. Plastic pipe is used where you are thinking of using plywood, which is completely rot resistant no matter how wet it gets. Maybe worth considering.

 

[worn thumbs]

Is there any particular reason why you have chosen plywood?Any softwood combined with an inner and outer laminate of hardwood is likely to be stiffer.This site has a lot to say on the topic of bow-sheds https://www.by-the-sea.com/stimsonmarine/bowroof.html .

 

[Inspector]

Agricultural polytunnels use a similar system of hoops to hold a clear layer. Plastic pipe is used where you are thinking of using plywood, which is completely rot resistant no matter how wet it gets. Maybe worth considering.

When I lived on the west coast where the temperatures are more like England there were some of those PVC type green houses on farms. They tended to be smaller and not long lived. The metal framed ones were more common with polyethylene stretched over. They could take the wind better and were longer lived. Here on the prairies with our very cold winters I haven't seen any PVC framed ones, probably because plastic gets brittle in the cold and they don't survive.

Pete