I just went to a rafter calculator and just plugged in the run of 42" and pitch of 3/12 and it gives something you could work with, how that would apply to pipe would require some testing I guess unless you can fine a similar program for pipe.

Roof Pitch Calculator - Calculates Pitch, Rafter Length, Angle and Slope (mycarpentry.com)

 

I just went to a rafter calculator and just plugged in the run of 42" and pitch of 3/12 and it gives something you could work with, how that would apply to pipe would require some testing I guess unless you can fine a similar program for pipe.
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Roof Pitch Calculator - Calculates Pitch, Rafter Length, Angle and Slope (mycarpentry.com)

I thank you for this effort. I love when things are done in a logical, calculated manner. WIth that said, because I had not seen this before. I had to eyeball it. What I did was purchase a bender that is made for bending gothic style arch bows and so if I had a 10 inch 1 3/8 steel pipe, I would measure ten inches down from the top of the pipe and place that marker in the strapped part of the bender and then bend. I would measure 10 inches from there and do it again, to where I would do it five or six times. I am still unclear if I should do it five times or six times, but definitely a minimum of five bends. One of the reasons I am unclear is because where my greenhouse currently is, one side of the ground is a big higher than the other. Now the toe kick boards of the greenhouse are leveled, but the ground is not. I then screw a hole through the toe kick boards and steel pipe and strap it in tight with a 1/4 inch by 4 or five inch carriage bolt. The five inch carriage bolt is okay for the four corners, but for the rest I need something smaller than 5 inches but bigger than 2.

So far the first bows worked. So I am working with a combination of 10 1/2 feet steel pipe and 10 foot steel pipe. For the 10 1/2 foot steel pipe I am measuring 10.5 inches on the bends and again doing five or six bends. I forgot how I did the first bows that are successfully on, but I have a follow up question:

I have to install some pearlines and the gable peak you see below takes 1 3/8 inch steel pipe at the beveled or crimped end, but do they sell 1 3/8 steel pipe that is beveled at both ends that are also a minimum of four feet long? Or do I have to purchase steel pipe and cut it to size myself? If I have to do the latter. What size pipe would I need since I cannot fit an unbeveled 1 3/8 pipe as a pearline into the gable peak you see below. I would need something a size smaller. Would that be a 1 1/4 inch steel pipe or a one inch steel pipe and do they exist in 4 foot lengths or do I have to cut them?

 

Alloy Steel Pipe Bend |

Material: Chrome Moly Grades P1, P5, P9, P11, P22, P91 Pipe Bend Specification: A/SA234 WP1, WP5, WP9, WP1/ WP5/ WP9/ WP11/ WP22/ WP91, WP22, WP91 | A/SA182 F1, F5, F9, F11, F22, F91 Type: Seamless / Welded / Fabricated / Forged Range: ½”NB to 48″NB in Sch 10s, 40s, 80s, 160s, XXS. Dimensions...

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A 1-3/8 pipe would be uncommon but there are charts available for pipe bending. If you can find a bender for that pipe it may have instructions .
A alternate method for that angle could possibly be a acetylene torch if you know when and where to heat .

 

What I am using is actually common for a low budget 20 ft by 9 ft high tunnel. I am using Chain Link Fence 1 3/8 in. diameter 10 ft loong 17 gauge thick galvanized steel top rail post

 

Depending on where the bend had to be I have seen people diy a template on the ground with stakes of rebar and basically manhandle the pipe until it bends to the right angle. Otherwise harbor freight might have a rigid pipe bender or get creative with a bottle jack and plywood template.

 

Give all of your pipes to my nephew Tommy, tell him "do not bend these pipes"....your pipes will all come back bent

 

Play w/a 10' piece of 1/2" PVC pipe to get your height, etc. to get a baseline.
How to bend the 1 3/8"...got no clue.

Good luck...Don.

 

What kind of pipes? Where are they coming from?

Is this like black iron gas pipe, silver colored fence post, or EMT conduit or muffler pipe or what are we talking about here? Where did you come up with the size 1-3/8"? Is this outside dimension or inside dimension? Do you actually mean 1-1/4"?

If I had to guess, I'd say that you're really dealing with some type of thin-walled 1-1/4" pipe.

Either way, here's a link to one from Harbor Freight which might get the job done. 12 Ton Hydraulic Pipe Bender

 

What kind of pipes? Where are they coming from?

Is this like black iron gas pipe, silver colored fence post, or EMT conduit or muffler pipe or what are we talking about here? Where did you come up with the size 1-3/8"? Is this outside dimension or inside dimension? Do you actually mean 1-1/4"?

If I had to guess, I'd say that you're really dealing with some type of thin-walled 1-1/4" pipe.

Either way, here's a link to one from Harbor Freight which might get the job done. 12 Ton Hydraulic Pipe Bender

Its chain link fence 1-3/8 in. diameter, 10 foot long 17 gauge galvanized steel top rail post.

 

Most REAL muffler shops have equipment that can bend car exhaust pipes - seems they might be able to make your bends for a few bucks each...

 

I'd say taking it to a muffler shop is your best bet. If they can't do it, it's time for a new construction alternative.

 

Kudos for making a gothic arch, let's just get that out of the way. My fave!
They are a simple radius with the center point offset down the ground plane, and the distance determines the shape of it. Closer to the middle, it is more of a barrel. Farther out, it is more of a point.
The trick with bending a big radius is staying in-plane with all the small bends. One of my friends made a barn/shop/home with a big radius and I will go look at his tools and get back. His was much bigger and thick walled tubing so your little greenhouse will be easier.

 

Thank you. To provide more detail that may help you help me, The high tunnel is 9 feet wide and I want the peak of the tunnel to the ground to be no more than 7 feet tall give or take. I am also using that shelter logic angle thing to insert the tubes in there as well as the pearlines. I believe I bought them from Shelter Logic but for some reason I did not think to get end ones.

 

that is fairly pointy as arches go. I'll see if I can take a WAG at a radius for you.

 

That is about an 8' radius. I figured that out with some small bowls and graph paper, and scaled it up.
The radius is perpendicular to the ground plane to start. When it reaches 7' up on the graph it is about 4.5' over which is what you are seeking.
The foci are 3.5' away from your arch centerline.

I would make a plywood template and cut a couple of pieces of 3/4 ply and screw them together, and bend the pipe around that if you want it DIY it. The template does not need to be full size, just do segments or bites of the bend at a time. How you bend it is up to you.

 

Some visuals of what you are sharing in terms of unit of measurement like unfortunately I am unfamiliar with what you mean by foci. I am really a beginner in bending pipe for any kind of greenhouse. Also, I cannot completely visualize what you are suggesting with the plywood, but let me share this with you, Johnny Seeds has a hoop bender specifically for a gothic arch, but its made for 14 foot wide spacing between the bows, at the base obviously. Could I make that work for a 9 foot wide spacing?

Gothic Tunnel Hoop Bender – 14' - Quick Hoops™ | Johnny's Selected Seeds

Bends Gothic arch-shaped bows to shed snow more efficiently. Create 14'-wide, fixed or moveable, peaked structures with bows made from 2½ pieces of ...

www.johnnyseeds.com