Another dreaded how big should my beam be...

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Forum topic by plantguy posted 10-25-2015 06:02 PM 1022 views 0 times favorited 6 replies Add to Favorites Watch
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5 posts in 1179 days

10-25-2015 06:02 PM

Topic tags/keywords: glulam wood beams greenhouse design load wood treatment

Ive been trolling your forum and scouring the web for a while now trying to find some answers … I hope someone can help….and possibly tell me something i “want” to hear! ;) lol

Im building a

Im in the process of building one of these greenhouse designs above ( 14’ x 20’ or similar design) and need some advice on two different elements. The beam and the arch.

Project background- this is a 20×14 greenhouse, of which i already have the foundation and block work done. Across the bottom and top will be windows spanning the length ( either recycled house windows or framed polycarb) to maximize ventilation.

The beam- Im really hoping to be able to clearspan the 20’ being that there isnt too much of a load on it.

I ended up getting a good deal on some 18ft cedar 6×6 and thought i could bold a couple together to make a 6×12in beam. However after more thought im not sure this is an option without a center post.

Now im thinking of a triple 2×12 or possibly making a gluelam (perhaps fir or cedar 1×4s 5 thick?) and sealed.

the Arch -12-14ft span at a 1’ 6” radius this will be holding a polycarb film and assuming a minimal snow load due to the temp of the greenhouse.

Ive seen gothic style greenhouse build with trusses of sort ( two 1.5 inch rips off a 2x with blocks in between. Which i dont think would work for my application and would rather them solid.

Codes in my area are sort of lax but would rather be on the safe than sorry side of this equation.

What are the best or possible ways to make these beams and recemendation on wood species

Also failed to mention that im and artist ( with a background in custom framing) on a shoestring budget…. and live in bfe…. so delivery costs on 20ft anything are pricey.

I live in the midwest with an avg snowfall of 26in

thanks for taking the time to read this and the patients and time to help!!

6 replies so far

View waho6o9's profile


8539 posts in 2811 days

#1 posted 10-25-2015 06:13 PM

Advice from a structural engineer would be money well spent.

Good luck now.

View plantguy's profile


5 posts in 1179 days

#2 posted 10-25-2015 06:45 PM

Thanks for taking the time to reply mr waho!! :) Sadly… I probably wouldnt be on the internet looking to forum experts for advice if i could afford a structural engineer ;) They say that one cannot have too many friends… any structural engineers out there looking for a friend ;) haha

Any thoughts…advice…. comment etc are appreciated …thanks in advance! :)

View Pezking7p's profile


3230 posts in 1886 days

#3 posted 10-26-2015 01:40 AM

You need to first figure out what Windows you will use. This will determine how you frame that roof and what the load will be on the beams. You’ll also have pretty significant snow load at times. And it can get pretty windy in the Midwest, and you are basically designing a big wind break.

Long story short, I would not take the structural aspects of this build lightly. The best bet is to add a center column. A 10’ span is really not a big deal.

-- -Dan

View plantguy's profile


5 posts in 1179 days

#4 posted 10-26-2015 02:00 AM

Thanks for responding Dan!! :)

Across the front top and bottom will be windows … however nothing that is fixed glass ( would be something like an old window pane with a hinge / or more likely a framed window with flex greenhouse glazing in it…both of which would be inside the building hinging in pressing closed against a gasket of sorts ( will make it easier to control with a servo) But shouldnt be effected by slight building movements etc.

The roof joist (and walls) will either be ply or hardie board covered in a poly pondliner. Was thinking of 2×8 … it only spans 4-5 foot.

Im actually in a “hollar” in the mountains… not too much wind here.

Being that its a rather small greenhouse im hoping to figure out a way to span the distance just so i have more flexibility inside… and if i start to see any deflection i can always post it but hope to avoid that! lol

I was curious if a 3.5×14 glu lam would do the trick.

Although ive done some small lam work they have been mostly trim pieces …. and am trying to get an idea of how big my arches would have to be . Ive seen some small trussed gothic arches stand up to massive snows… but havent a clue really! lol

Thanks again for the input mr dan… its much appreciated!!

View Ocelot's profile


2123 posts in 2872 days

#5 posted 10-26-2015 03:02 AM

A few years ago, I found a free program called “Beam Boy” that you can download. It solves problems of this kind but is set up for steel only. You might see if it has been updated since I looked at it.


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5 posts in 1179 days

#6 posted 10-26-2015 03:14 AM

thanks paul.. ill check it out!

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