View Full Version : Advice on How to "Beef Up" Greenhouse/Shed...

Rich Riddle
06-01-2015, 6:21 PM
I am building an 8' x 12' greenhouse on the hill after asking a lot of folks for advice last year on the creek. It's less than 120 square feet and an accessory building so no permit/codes are needed. They don't seem to care if we use it as a shed or greenhouse as long as it's not a living space thing. Anyway, the plans aren't written down, just in my mind. Threw this together today out of 2 x4 walls and a 2 x 12 double ridge beam with 2 x 10 rafters. The slots in the roof are for three skylights an those have additional rafters. Used double 2 x 6 collar ties in the center rafters about 4' apart and a single on each end as a collar tie. The neighbor's wife wants to know if I can "beef it up" so it doesn't blow away. Everything is on 16" centers. Hurricane clips were also used. In her defense, a neighbor's Harbor Freight greenhouse blew into another neighbor's driveway last year. Is this one build solid enough?

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Mike Cutler
06-01-2015, 6:37 PM

I don't know what she means by "beef it up", but it appears to be solid too me. The only thing you didn't detail was how it was attached to the slab, or ground.

I don't believe there is any comparison between your structure and a "kit" from HF.

Ryan Mooney
06-01-2015, 6:37 PM
I think you're neighbours wife needs to mind her own business unless she's helping stop trespassers from cutting down trees.

The only part that I can't tell if its horrendously overbuilt or not is how the whole assembly is tied to the base. I suspect as its built now it would have to become airborne as a unit so that's likely the weak spot :D

Will be interested to see how you do the skylight install...

Also are you doing anything for automatic ventilation?

Rich Riddle
06-01-2015, 7:00 PM
It sits atop a deck. The deck is made from 4" x 12" exterior and direct contact Parallam beams. The beams are supported by 12" tubes that go 5' deep and have rebar in them. On top of the parallam beams are 2 x 10 pressure treated decking frame. The frame is solid under the outline of the shed. That is, the bottom plate sits atop solid triple 2 x 10 joists. I will lag bolt the bottom plate into the decking. The decking is aluminum and quite strong. Everything is on 16" centers. Each pier is 6' apart from its neighboring pier going longways and 8' the short side. The total deck is 12' x 28' and was simply a replacement for an outdated and unsafe deck already in that location. The underside of the deck has hurricane ties. It seems unlikely this unit can go airborne like the neighbor's kit sailed.

Rich Riddle
06-01-2015, 7:03 PM
The skylights are solar powered units and will get installed later in the week when things dry out and I can lay the roof. The automatic ventilation is the fly in the ointment at the current time. Still figuring it out. It will have soffit and a continuous ridge vent though.

Ryan Mooney
06-01-2015, 7:42 PM
It sits atop a deck. <snip> It seems unlikely this unit can go airborne like the neighbor's kit sailed.

Highly :D

I have limited experience with automatic venting other than to say that everyone I know who's had a greenhouse noted that they tended to get way to hot way faster than you could have expected (myself included, although I moved before I could fix it).

The soffit/ridge might suffice, I don't know but it would be interesting to find out. If not you might be able to put in automatic vents in the front/back at the upper end (above where the door is more or less, or the same place in the back). That would avoid having to have the skylights be moveable (they're hard enough to do imho without adding that and will likely be heavier as well). I see you've framed that in already but that's quickly fixable.

My friend who just installed one was somewhat impressed at how well the wax filled cylinder setup worked and how heavy of a window it lifted - that's a sample of one user so ymmv - http://www.amazon.com/Univent-Automatic-Vent-Opener-Standard/dp/B0036EJ9HW is what he got.

Neil Gaskin
06-01-2015, 8:28 PM
Your biggest concern is how the structure is tied to together, which sounds well done, and how its tied to he concrete. If there are decent anchor bolts or straps bedded in the concrete there isnt much more you can do. For what its worth, it appears well done and stout. I wouldnt worry about it, nothing going to stop a tornado from tossing it around, but you cant really expect that.

Looks nice

Greg R Bradley
06-01-2015, 9:08 PM
The bigger HF greenhouse is a complete piece of junk. It can be made to work OK with a huge amount of work. One of my employees is pretty happy with his - but it is completely shielded from wind from 3 side and isn't in an area that gets high winds anyway. Even then, the clear plastic needs to be silicone sealed in place.

From what I can see yours if FAR superior. I can't see how well the walls are tied to the ground in the pictures. Are you going to seal the inside so the humidity can't get to the wood?

Rich Riddle
06-01-2015, 10:33 PM
From what I can see yours if FAR superior. I can't see how well the walls are tied to the ground in the pictures. Are you going to seal the inside so the humidity can't get to the wood?
I am actually considering applying exterior siding to the interior of the greenhouse. With proper painting that should protect the interior wood. With such small area not covered by doors or windows, it wouldn't be difficult. Still pondering what to put on the ceiling though. Perhaps boxcar siding or tongue and groove cedar.

Karl Andersson
06-02-2015, 8:36 AM
Just from eyeballing the structure, I'd be a bit concerned with the strength of the main walls in resisting a shear load from the ends of the shed - unless the window frames you're installing have structural properties that would add to the wall's strength, it seems there isn't much to resist that kind of loading. Normally, you'd use inset diagonal bracing (unsheathed wall) or structural sheathing, but the windows prevent that. As it is, your exterior sheathing is providing this structural strength, and it PROBABLY would be fine for normal weather - but if the area is prone to high winds, I would definitely screw on (or use ringshank nails) some decent plywood structural sheathing or siding on the inside as well so the framing is stabilized all around and those strips of wall between the windows become structural columns. The roof structure you've built has a lot of mass compared to the rest of the structure, so you need to stabilize it against wind movement - otherwise it could cause annoying problems like sticking or cracked windows, mislaigned doors, siding gaps, etc.. Just plain high winds won't place it in your neighbor's driveway; you've seen to that.

Prashun Patel
06-02-2015, 8:48 AM
Was the HF greenhouse similarly situated? You're protected on a couple sides by trees. For the sake of the relationship with the neighbor I'd ask her what she has in mind for more 'beef'.

In my town, even if a permit is not required, the inspector can be convinced to come out to advise. I've done that. I offer to pay, and they're usually just tickled that someone would ACTUALLY try to do it right, that they've come out for free. I'd bet they'd at least review your pictures over a cup of coffee in their office.

Looks beautiful.

julian abram
06-02-2015, 10:46 AM
"Beef it up", my gosh double 2x12 ridge, 2x10 rafters, it's way overbuilt for a greenhouse the way it is. Don't know what else you could do outside of bricking the exterior! ( I'm kidding)