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Discussion Starter · #1 ·
Wood Arch Greenhouses.....

I designed a wood arch truss for building a 16 Ft wide greenhouse. Today I built the first truss to see if the design and wood would work.

After reading some time about wood species I was still baffled as what was the best for a greenhouse and would bend well for an 8ft radius.

Why I have no idea I found myself using Cypress and paid $3.80 a liner foot for 2 - 2x12 clear rough sawn. This is the first time I have every used Cypress and it was not that hard to use however I had to soak them in the pool overnight to get them to bend enough to build the truss.

I still broke a few bending them however I think it was the pieces with the grain going off the side of the board and not straight as the others were.

I have enclosed a picture of the first truss. I am hoping others can tell me a better wood species to use that cost way less than Cypress and may bend well cold without having to soak them or build a steam box.

The radius is 8ft with a the top of the arch. The rips are 2 ply 3/4 x 2" with a 2x4 block and then 2 ply's more of 3/4 x 2".....

I have had no success in contacting the wood dealers about the properties of the wood species they use for bending.

I was going to try Western Red Cedar next but have heard Hemlock may work. I have no problem soaking them in water a day before like I did with the Cypress however it seemed like the Cypress was at it's max for cold bending (wet) at the 8 ft radius inward to the last ply that is about an 7'-6 1/4" radius.

Does anyone have any suggestions as to the best wood for constructing this type truss and something way cheaper than Cypress.​
 

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Project Superintendent
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Cypress is very brashy, definitely not a good bending wood.

Sorry I don't have a good alternative that would stand up to the weather, cedars are brashy as well. Maybe locust or some other straight grained hardwood?
 

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Maker of Fine Sawdust
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Go to a sawmill and find something green, not dried.

White Oak is a good bending wood


http://www.woodweb.com/knowledge_base/Rx_For_Bending_Wood.html

The U.S. Forest Service has evaluated 25 hardwood species for relative bending quality. In their testing, the best 17 woods were:

Hackberry (Best)
White Oak
Red oak
Chestnut oak
Magnolia
Pecan
Black walnut
Hickory
Beech
Elm
Willow
Birch
Ash
Sweetgum
Soft maple
Yellow-poplar
Hard maple (Worst)
 

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Discussion Starter · #4 ·
I think any good wood would work if I steamed it however I have never steamed before and I would have to build a 16ft steam box. I suppose I am looking for a process more than species because I want to build many.

So steaming is an option however I don't know where to find the resources in building the box, the timing of the steaming and how to glue wet steamed wood.........

As a carpenter I am never afraid to try new things but process is everything and this is pushing my knowledge to its limit as I enter into a new territory..... steaming and gluing wet wood..... I'm at a loss.

The steam boxes seem simple but most of the steam boxes people build are small for smaller projects. Right now in the current picture I have only bolted the truss at each block with one through bolt and no glue. I did not put glue because the wood was wet during bending in the template I marked on the OSB. It seemed to hold shape real well when it dried however the wood still seems under some pressure especially without glue. I was able to hang by it and it seems real strong but would feel way better with glue.

If anyone has some ideas I would welcome them......
 

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Project Superintendent
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Wow, I didn't know hackberry was good for anything other than dripping sap on my truck every spring and fall.:rolleyes:
 

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I would ask a few questions on a wooden boat forum, as bending wood is common. I made a steam box years ago to steam red oak shoe molding. Made it out if a PVC pipe and my wood stove.
 

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Project Superintendent
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Hackberry is great stuff. Tough as hell to split / break Mostly used for pallet wood.
It's pretty good fire wood. Burns hot even when it's green.
I got a new tree ID book a while back that claims it has commercial value for fine furniture. I call BS on that one!:no:
 

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If you post this on a wooden boat forum, they're going to ask you why you designed the frame like that. Around here, greenhouses and temporary structures are made like Gothic arches. It's the two strips of wood with blocking between, coming to a point at the top, then plywood gussets at the top to hold the ends in place. Fewer cuts, less wood, no steaming. You can make it out of pine.

If you want to glue wet wood, Gorilla Glue will do it, but it's something of a poor glue joint. The stuff will tend to foam out of the joint / spread the joint.
 

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Project Superintendent
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I googled hackberry furniture, and there's hackberry furniture for sale, so it didn't lie to you.

Who woud have thought...
Well, there is furniture made from recycled pallet wood as well, but that's not what I call "fine" furniture. :no:
 

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Box Builder
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Definitely post this on a boat building forum. Building a steam box isn't a huge deal. You could try buying CVG Fir decking and bending that. It would need to be steamed still. Ash is also a good wood for bending. Both aren't the best rot resistant species, but ash is often used for boat interiors. You could also try gluing up thinner pieces to get your thickness. I know you don't want to, but it will be easier.
 

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If you have never steam bent wood before DO NOT try it on a fixed cost price. Lots of trial and error.

The box isn't a big deal but the amount of steam you need to fill that box and keep it at 212 is everything. You don't glue wet steamed wood, it gets put on a drying rack for at least 24 hrs (for something that size) and really should be there no less than 48 and depending on the wood allow for at least 5-10% spring back. At least 1hr per inch of thickness in the box at full steam. Pull it out too early, start over. Take more than roughly 15 seconds to bend, start over. Too much spring back, start over.

Eucalyptus steams okay and does fine outside but not cheap. Always use green wood for steaming.

Bend it in lamination style using epoxy with dowels every few layers and life will be easier.
 

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Discussion Starter · #20 ·
I think it all good advice. I was thinking on bending 3 half inch strips in lieu of 2 three quarter strips. That would take just a bit more ripping however may bend easier without steaming and stronger because elf the three ply.

I know building a steam box is not that difficult however for production I would prefer cold bending options before steaming if possible.

And yes, the design comes from the Gothic arch of a boat hull so I suppose I may find some good advice there. Was not thinking about that.......

I don't know how important the rot and insect resistance wood will play a factor if the trusses are polyurethaned however no wood will last if the owner does not take care of it and at least recoat it in a few years. It's not the kind of greenhouse you can let sit uncovered for a few years then try and cover it again later. It must be kept up and maintained. Like any wood product it must be taken care of.

The Gothic arch design with plywood gussets is common and will be used for the smaller and wider models. I was just trying to create a design that's all one piece in lieu of two rafters to a ridge with gussets plates. My idea was to make it simp,e to,install and look beautiful. This is just the prototype to work out all the kinks.

The Gothic arch 2 rafter design is less in wood and less bending because the arch stops at the point unlike the one I created which only does this on the top ply's and continues the full arch radius below. That said it is more wood, cause the design to need more ply's to hold it together but leaves more opportunity for design features and allows for a simple DYI install for the average person to put together.

That was my goal coupled with keeping the package under or close to 9 feet when angle set in a trailer for delivery.

All around the 2 arch one ply top and bottom cord strut design is much more simpler and easy to deliver, manufacture and build vs the one I designed. Like most creative people...... We complicate things however its very hard to duplicate by the average homeowner where the Gothic arch simpler 2 rafter design isn't. That said it still takes skill to design and built it to a quality that could be sold retail.

Cost is a factor in my design but there is a balance to beauty, simplicity of build and the production factor of duplicating the build over and over. This is the balance I am trying to find. This is the only model I am able to meet that criteria. All other models will not fit on the road for delivery or are too small to create in one piece, although I may try to create a smaller model than this 16 ft one in one piece. I just like the idea of one piece truss in stead of the two rafter with a ridge design. To a builder or contractor it's not a problem but for a homeowner it just seems simpler to stand them up, put on the bracing and cover it over. Not to mention I can prefab the end walls with the door and end wall framing in place with the 2 rafter model the installer will have to do this and if its the homeowner it could break the deal for the DYI guy or gal.

Buy, stand, brace and cover..... Trying to keep it simple. I am suggesting a standard foundation of 2 to 3 high 6x6 treated timbers mounted to footings with a 4x4 on top to bolt on the trusses. Of course it could also be mounted to a masonry foundation or even a post in a footing for each truss with a 4x4 on top for mounting to ground level though this puts the trusses at dirt level or close that would not be a good idea.

Just tossing around a few ideas because I love the knowledge and experience on the form. It has been so helpful in the past. This has been a passion of mine for some time building these greenhouses and this fall and winter is when I'm going to get things going. I just need to work out all the kinks. Who knows..... Maybe the 16 ft one piece truss won't sell however I am going to try and work it all out because I just love the look and simplicity of the build of a delivered kit package I could develop with this design. In the end it just might not be cost affective.
 
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