Advice for 12x24 greenhouse design

[corbin]

Hi! I'm new to timber framing and designing my first project. I've been reading a lot (Sobon's books, Beemer's and both of Chappell's). This will be a greenhouse, and my design requirements are based on the glazing and how I'll attach it. So, I'd like 8' (or less) side wall heights and not too high of a ceiling to keep heating costs low. Let's not discuss if the wood will rot, but I'm using Western white fir and jeffery pine. I'm working in SketchUp and I'm happy to share the design files.

Goals: aesthetic design, and practically: ideally I'd like the top plate to be where I attach the top of my vertical glazing and the bottom of my roof glazing. Based on that, here are my concepts.

Timbers in the images below are 7x7 except for the top plate and tie beam which I did as 7x8. Purlins are not shown.



I like the top arch design; it is similar to a guy's design on YouTube who is doing a "Spring Fed Timber Framed Greenhouse". The joinery is similar to what is seen here on Timber Frame HQ: https://timberframehq.com/20x20-king-post-with-shed-roof-plan/ -- I'm just not sure if it is right. What do you think?



This version I used a rafter foot to post joint, as seen in "A Timber Framer's Workshop". This seems to require less wood being removed from the post, but may not be as strong. Thoughts on this version?

Some other ideas:



I like this design, but the English tying joint will require larger posts or a jowled post, and seems a little too complex for my first frame. People on Facebook recommended a continuous top plate, which also makes things more difficult.



I think this version is the easiest to do for joinery. Its just a modification of the standard shed seen in most books. I just don't like the look as much, and I have to make the side walls higher than 8' to get an 80" height door, unless I push down the top plate another 6"...but I read that having those joints so close isn't as good a thing to do. Thoughts on that?

Thanks all for any advice! I originally posted on Facebook but heard about this forum.

Corbin

[Jim_Rogers]

As I am a traditionalist in timber frame design, I like the easy version.

But your, king post version is also good and shouldn't require a larger post, as the plate is not a through plate at the center post. This interrupted plate could be connected to the center post by using a spline joint, as well as housing on each side of the post. This type of connection would take the least amount of wood out of the center post. Then there is only one tenon on the top of the post going into the tie beam/bottom cord timber. This is why I say you don't need a larger "jowl" post. As drawn it is not that bad. Your interrupted plate at the end bents can be done with a lowered through tenon and a housing. Which would leave you enough post to do a top tenon into the tie beam/bottom cord timber.

I don't like the TFHQ version you provided a link to, because (at least the 3d version) doesn't show any braces in the long side walls.

The rafter foot to top of post connection shown in version A, is not the best at all, in my opinion. I have personally cut that joint, in my very first timber frame class it took, and I don't like.

Your second version "B" shows three braces connecting the center post at the same elevation. This is not the best design, as it takes away too much wood at that location. And the interrupted plate tenon is too short going into the side of the end post. With a short tenon like that you don't have enough wood "relish" beyond the peg hole, if pegged. There are rules about the distances from the peg hole to the end of the tenon that his joint does not comply will. The center post to the interrupted plate could be solved again with a spline joint and housing.

Just my thoughts on your designs.

Jim Rogers 

[corbin]

Thanks Jim! I really appreciate you taking a look at it and giving me feedback. I've also read a lot of your older posts and appreciate all the great info you have provided over the years.

I have heard of the spline joint for the interrupted top plate; that might work out well for my "king post" version; it would definitely be a bit more complex for my first frame. 

I'm going to take a longer look at the "easy frame" version -- sometimes simplest might be the way to go.

Corbin

[thecfarm]

Green house? Going to be used all winter? Reason I am asking, I built one out of 2x4's, I only heat it in the spring. Rest of time I had the doors, windows open to let the air through. Never rotted at all.
I suppose you know you will need fans to move the air around.
Warm water is better for the plants too. I just had a big tub, probably held about 500 gallons and I pumped the water from the tub.

[corbin]

Green house? Going to be used all winter? Reason I am asking, I built one out of 2x4's, I only heat it in the spring. Rest of time I had the doors, windows open to let the air through. Never rotted at all.
I suppose you know you will need fans to move the air around.
Warm water is better for the plants too. I just had a big tub, probably held about 500 gallons and I pumped the water from the tub.

Yup! Our plan is to use it year round and heat it in the winter. I posted this on Facebook a while ago and a lot of people had concerns about rotting; I'm not too worried about that, and I might put a layer of epoxy on the wood to ensure it is a little more moisture tight. 
I'll definitely have a few automated fans, and probably a small tub/pond to hold water -- they are also a great thermal mass for holding heat.
For the design, I think I'm settling on the "simple frame" version, and just dropping one wall to 8'.  After I settle on the frame design I'll add in the accessories (glazing, openable windows, vents, doors)
Corbin

[Don P]

Since you probably can't make wood moisture tight, I think I'd make it shed liquid water but breathable to vapor trying to dry out from within. Film forming finishes unless perfect tend to create a composting bag.

You are in a high snow load area I think. This is small but restraining the spreading force from the roof is something to consider.