Steel Greenhouse Frames: Strengths and Weaknesses

Steel greenhouse frames are the most commonly used type of greenhouse frame.

Steel frames are used in greenhouses ranging from tiny hobby houses to huge multi-bay commercial greenhouse complexes.

Here's a look at some of the comparative advantages/disadvantages of steel frames:  

Appearance...

Steel greenhouse frames are not the most ornamental. Beauty is in the eye of the beholder, but most people would probably consider a wooden or even aluminum frame greenhouse to be more attractive.

That doesn’t mean that a steel framed greenhouse must be an eyesore. A steel frame can be nicely painted, of course – a good idea for reasons other than simple aesthetics.

Cost...

The cost of steel has been escalating rapidly in recent years. Even so, steel greenhouse frames are not necessarily the most expensive. An aluminum frame greenhouse of an equivalent design will probably cost more than steel.

And depending upon the type of wood used, a wooden frame greenhouse can be also more costly than an equivalent steel frame greenhouse.

Ease-of-Construction...

Of all the different materials commonly used in greenhouse frames, steel is probably the most difficult to work with due to the hardness of the material. Other materials used for greenhouse frames (including aluminum) are softer, making it easier to drill holes for bolts.

If you’re assembling a greenhouse from a kit, that will make little difference since most kits come with all bolt holes pre-drilled. But if you’re building from scratch, be prepared to expend a little more elbow grease and go through some extra drill bits if you’re building a steel frame.

Durability...

As long as rust is prevented, steel is as durable as (or more durable than) any other greenhouse framing material. But if rust gets a foothold in a steel frame, its durability will be quite limited.

And since even galvanized steel will eventually rust, it’s a good idea to paint the steel frame with a white paint. In addition to protecting the frame, the white paint will help to reflect a bit of additional light into the interior of the greenhouse.

And if you’re covering the greenhouse with plastic film, the white painted frame will be cooler where it’s in contact with the plastic. This will help prevent ‘hot spots’ where the metal and plastic are in contact, extending the life of the plastic.

Strength...

There’s no question that strength is the greatest advantage of building a steel framed greenhouse. Not just strength alone, but the favorable strength-to-bulk ratio offered by steel.

You could build an aluminum-framed greenhouse or a wood-framed greenhouse that would be just as strong as the same steel greenhouse. But the load-bearing members of the aluminum or wood house would have to be much thicker than the steel to achieve the equivalent strength.

And in a greenhouse, thin is in, because thicker structural members reduce the amount of light entering the greenhouse – a distinct disadvantage.

Great for Do-It-Yourselfers

Steel is the most commonly used material in large commercial houses. But it’s great for DIY greenhouse builders, too.

Steel, available in many forms for intended purposes other than greenhouse building, can be adapted to many shapes and sizes of greenhouses.

We built the frame of the greenhouse in the photo, for example, entirely out of galvanized steel pipe manufactured for use as the top rail of chain link fencing. We used a bender to bend the pieces to the proper curvature for the bow, and assembled each bow using the swaged ends of the pipes. 

Our steel-framed greenhouse packed to the gills with hydroponic lettuce

We used the same top-rail pipes for the greenhouse purlins, and secured the purlins to the bows using hose clamps (no drilling and bolting required!).

That greenhouse has been standing for years. It’s been through a couple of hurricanes (downgraded to tropical storms after making landfall), and at the time of this writing, we're using the greenhouse to grow hydroponic lettuce.

For such a ‘hard’ substance, steel offers a great deal of ‘flexibility’ for DIY greenhouse builders. 

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