Part Five: Modifying the Frame for Strength

Making Modifications to Improve the Frame Strength

At this point, our greenhouse frame has been constructed as per the manual. It really feels fairly sturdy at this point, but then again, there are still no panels in it, and we’re not standing in a windstorm. ;-)

Now’s the time to make a few additions to stop the frame from flexing. I’ve read many of the changes made by folks putting up this greenhouse. Methods vary, but they seem to fall into these general categories:

1. Prevent the two side walls from pulling away from each other, by adding horizontal braces that go from side to side, at the tops of the walls.

2. Stiffen the back wall, by bolting one or more pieces of solid material all the way across the wall. Some creative folks have tied this solid bracing material into the design of sturdy benches.

3. Stiffen the front wall, bolting a piece of solid material over the doorway.

4. Prevent the C-shaped steel base from flexing. This can be done by bolting small plates of some type to the top and bottom lip of the base at regular intervals, or by covering the inside of the base entirely with wood that's also attached to the top and bottom lip of the base.

5. Strengthen the corner posts, by adding reinforcements at the bottom of the posts.

Of course, everyone decides which modifications they think are the most critical, and then finds their own ways to complete them, using the tools and materials they prefer. You should also consider the placement of your own greenhouse, and how exposed it will be to winds. (Mine will sit in a very windy spot.)

This is where some reading on the internet is time well spent. It's really helpful to look at photos of other HF 10 x 12 greenhouses shared by posters on the GardenWeb forum. They’re each worth considering for their good ideas.

We decided the first four modifications above were the most important for us.

If aluminum comes in contact with steel, a corrosive reaction occurs that causes the steel to rust. So, regular steel angle iron isn’t a good choice for bracing. Aluminum angle stock is one solution, but it can be hard to find in long lengths, and fairly expensive. Another solution is to use EMT (electrical metal tubing) since it’s galvanized, fairly easy to bend and cut, easy to find and cheap.

We started with the side-to-side wall braces. People usually use either ½” EMT or ¾” EMT for these, and they usually add two or three braces. We decided on three braces made from ¾” EMT.

TIP: Stop and check for squareness again. The more braces you add to the structure, the harder it is to correct if it's out of square. If it's out of square, the panels may not fit into the frame correctly later. We measured diagonally from corner to corner inside, and we used a level on each corner post. Better safe now than sorry later!

Making Our Wall-to-Wall Braces

A good basic way to attach EMT braces is to flatten the ends for several inches, bend the ends 90°, drill for bolts, and attach them to the greenhouse wall studs. There are many good photos of this attachment method on the Garden Web greenhouse forum.

We tried something different, using 2" plated L brackets and bolts. First, L brackets were attached to the top of the stud.

Tip: We didn’t have T bolts on hand to add to the stud tracks, so we used 3/16” carriage bolts, clipping off part of the flange on the head with bolt cutters. This left a rather jagged head that slipped into the track, and grabbed the inside of the track nicely when turned.



Here’s an L bracket attached to the central stud (had to add a few washers behind the L bracket, because of the connection plate at that stud.)














We held up each length of EMT to make sure the length was right before cutting. We wanted a snug fit to keep the side walls parallel and stable during wind. Our braces turned out to be a bit over 116", but (of course) you should measure your own structure carefully to determine the length you need.

Each ¾” EMT brace was drilled all the way through on one end, to accept two bolts.


The other end of the EMT brace was also drilled for two bolts, and we cut a slot in one side of the brace, using an abrasive saw (or chop saw, used for cutting steel.)









One end of the brace goes over the L bracket, and is secured it with bolts. Slipping the bolts through can be a little fiddly because you're working around the nuts already in place (tight quarters.) We found it best to install the bolts first on both ends, then go back and add the nuts and washers last.













The slot allows the other end of the brace to slip sideways onto the L bracket, and it’s also secured with two bolts.

















Three EMT braces now connect the three central studs of each wall.
















The three wall-to-wall braces should prevent the two walls from moving in and out during strong winds. They’re strictly for side-to-side bracing purposes, and we won’t be adding any extra weight to them (hanging plants, for example.)




Adding a Brace to the Back Wall

To stiffen the back wall, we used another piece of ¾” EMT, running corner to corner, snugged up against the bottom of the central Part 31 braces. Once again, we drilled holes in the EMT and used our modified carriage bolts in the tracks.










We butted the round EMT brace right into the inside corner, as shown here…















…and we secured it with one sheet metal screw from the outside of the greenhouse, as shown here. We used a sheet metal screw because the head needs to be flat enough to not obstruct the polycarbonate panel, which will cover this area entirely.















So, here’s the back wall with the EMT brace in place, right under the center Part 31 braces. This single EMT brace makes a big improvement in the stiffness of the back wall, and it would be easy to another later if we feel it’s necessary.









Adding a Brace to the Front Wall

The frame over the door also benefits from stiffening; it prevents the front wall from flexing, and it also helps the sliding doors to work more smoothly.

We could have figured out a way to use EMT or aluminum angle here, but we got creative and used a 1” by 2” piece of oak instead. We’re planning on building sliding screen doors for the greenhouse later, and the wood will allow us to attach the track for the screen doors more easily.

A piece long enough to go corner to corner would have been better, but we used what we had, which was 8’ long. Even so, adding this piece made a big difference in the front wall strength.

There was a row of bolt nuts where we wanted to affix the wood, so we drilled out the wood on the back to sit over the nuts. Then it fit flush against the aluminum trim on the inside of the greenhouse...



…and we secured it at close intervals with wood screws and washers on the outside of the greenhouse.









Adding Lightweight Braces to the Gable Ends

In addition, we decided to add a lightweight horizontal brace to the upper part of each gable end of the greenhouse.

The addition of this brace will allow us to add screws to the polycarbonate panels that insert here, and will help keep them in place during strong winds (as explained in the next section, Adding the Panels.)

We used 6’ lengths of aluminum angle, ½” by ½” by 1/16” thick. That's pretty thin, but we were surprised how even this small brace helped stiffen everything up. Once again, they were secured to the wall studs using modified carriage bolts. At the ends of the pieces, we drilled holes in the frame and added sheet metal screws.



Insulating and Stiffening the Steel Base

The last modification we made was to prevent the c-shaped steel base from flexing when wind pushes on the greenhouse walls. It also gave us a chance to add some insulation.

We used ¾” thick expanded polystyrene (styrofoam) insulation, cut into strips about 4 ¼” wide. We also stuffed some foam sill insulation in there first, to remove as many air pockets as possible.



To stiffen the base (and protect the insulation) we attached boards made from 1” x 6” red cedar fence pickets. Before attaching them, I stained the boards with a redwood stain I was also using on the benches.

Since the boards were actually 5 ½” tall, we notched out areas around the nuts and bolts. This wasn’t fun, but the soft cedar cut pretty easily with a box knife. In this photo you can still see the top and bottom lips of the steel base, which is where we'll put the screws, through the board and into the steel.










We drilled pilot holes through the board into the top and bottom lip of the steel base, and secured the boards with sheet metal screws, about every 18” around the greenhouse perimeter.

Now the base is insulated, and stiffened by the attached board...and it looks a little dressier, too.














Voila, the floor's done!

We took a break at this point to do some work while we could still enjoy the open breeze in the greenhouse frame.

We stubbed in the electric and water lines, and concreted in the supports for our benches to come. (More about our benches in the Greenhouse Enhancements section.) We also added concrete pavers in the walkways, and gravel under the bench areas.

So, now our greenhouse looks like this. Click here to go to Part Six: Adding the Panels.