This tube is the stock a greenhouse frame is built from, made to an ASTM A53 grade and hot dip galvanized after forming so the zinc covers the outside, the bore and the cut ends. It arrives round and straight, ready to be cut, bent and bolted on site, and the same stock is used for hoops, purlins, braces and ground posts. A frame built from one grade of tube is easier to repair years later, because a damaged member can be replaced with a section from the same supply rather than a piece that only looks similar.
A greenhouse frame is only as good as the tube it is welded and bolted from. The grade sets the strength the hoop can carry over a span, the galvanizing sets how long it lasts in a wet house, and the straightness sets how easily the hoops line up when they go up. Buying tube on grade rather than on appearance is what keeps a frame true in year five.
A specified grade is a promise about the steel itself: how it was made, what it will carry and how it behaves when it is bent. On a greenhouse that matters at the hoop, because the arch carries the film and the wind load across the whole bay, and a tube that is under grade will flatten at the shoulder long before a correctly graded one does.
The standard also makes the frame repairable. If every member comes from one grade, a section that is bent by machinery or by a falling limb can be replaced with a piece that matches, and the arch geometry does not have to be adjusted to suit a tube that behaves differently. Houses built from this stock are described on the film greenhouse range page on this site.
Round tube is used for the members that have to bend, which in a greenhouse means the hoops and the braces that follow the roof line. A round section bends evenly in every direction, so a hoop formed from it keeps a smooth curve rather than forming a flat spot on one face, and it has no corner to collect water where the film runs off.
Hot dip galvanizing is done after the tube is formed, so the zinc reaches the bore and the cut ends rather than sitting on the outside only. In a greenhouse the members sit in a humid, warm, occasionally condensing environment, which is exactly where a coating matters, and the inside of a tube is where an uncoated one starts to fail out of sight. A frame on the same stock is described in the structure greenhouse.
The tube is supplied to be worked on site, and it can be cut, bent and welded or bolted depending on how the house goes together. Where a member is bolted rather than welded, the coating survives the assembly and the joint can be taken apart again, which is the arrangement most farm frames use for the members that are likely to be hit.
Two things matter when it is worked. Cut ends expose bare steel and need to be treated before the member goes up in a wet house, and heat from welding burns the coating back along the tube, so welded joints are the ones to watch at the first season service. On a bolted frame neither applies, which is one reason it is the usual choice for a house that has to be repaired rather than replaced.
The table below is the standard stock. Size, section and length are set to the frame drawing.
| Type | Round |
| Standard | ASTM |
| Technique | Hot rolled |
| Surface treatment | Galvanized |
| Alloy or not | Non-alloy |
| Section | Round, square, rectangular and hydraulic |
| Processing service | Bending, welding, cutting and OEM |
| Special use | Commercial greenhouse |
| Application scenarios | Construction, greenhouses and fencing |
| Delivery time | 7 to 15 days |
Tube is delivered in bundles, which means the unloading and the stacking are part of the job rather than an afterthought. Bundles are lifted with slings rather than forks wherever possible, because a fork that slides under a bundle drags the coating off the bottom layer, and the coating is the whole reason the tube was bought galvanized.
On site the stock should be kept off the ground on bearers and covered, so water does not sit in the tube ends and the bundle does not sit in standing water through the wet season. The same handling discipline applies to the finished frame: a member that is nicked during assembly is touched up at that point, because that is where the rust starts. A house using this frame in a glass build is described in the steel frame glass house for vegetables.
This stock is used for greenhouse hoops, purlins, braces and ground posts, for farm fence frames, and for shelter and shade structures on a mixed operation. It suits growers building or extending a frame from a drawing, contractors who need one grade of tube across a whole house, and operations that want a frame they can repair with a section rather than replace as a set.
Standards for steel products and for the coating systems applied to them are published by NEN. Production and market reporting for the agricultural sector is published by Agriculture India. The specification that matters is the one that still suits the crop ten years from now.
Because the grade is what the tube will carry and how it behaves when it is bent. On a greenhouse the hoop carries the film and the wind load across the whole bay, and an under grade tube flattens at the shoulder before a correctly graded one does.
The bore and the cut ends. Galvanizing is done after the tube is formed, so the zinc reaches surfaces a brushed or sprayed coating cannot, and the inside is where a tube in a humid house starts to fail out of sight.
A galvanized steel frame is rated for 15 to 20 years under normal growing conditions, with the fixings and any cut ends checked as part of the season service.
We provide a 10 year structural warranty and a 5 year anti-corrosion warranty on the galvanized steel. Details are set out in the contract for each project.
Send the frame drawing or the sizes you need, the quantity and the delivery port. We reply with a quotation and the delivery schedule.
