RHS vs SHS for Gate Frames

RHS and SHS look like the same thing in a different shape. On a gate they do different jobs. Pick the wrong one, or turn the right one the wrong way, and the gate flexes, racks or sags years before it should.

This guide is written for aluminium gates, which is most of what we draw. The shapes work the same way in any metal. Aluminium just makes the choice matter more, because it bends more for the same size of tube.

What is the difference between RHS and SHS?

RHS is rectangular hollow section, like 65x35 or 100x50. SHS is square hollow section, like 50x50 or 100x100. Both come in a range of wall thicknesses. In aluminium, 3 mm wall is the everyday choice for gate frames.

Most aluminium box section on the rack in Australia is 6060 alloy in a T5 temper, sold in 6.5 m lengths. Check with your supplier, because a harder alloy or a different length changes your cut list.

Why does a deeper tube stiffen a gate so much?

How much a tube bends depends on its depth in the direction of the load. Stiffness grows with the cube of that depth. Wall thickness helps too, but only in a straight line. Double the wall and you roughly double the stiffness. Make the tube 40% deeper and you can more than double it.

A 65x35x3 RHS and a 50x50x3 SHS use the same amount of aluminium, about 1.5 kg a metre, so they cost about the same. Stand the RHS on edge and it is about 45% stiffer than the SHS. Lay the same RHS flat and it is about half as stiff.

So when a stile is flexing, a deeper section is usually the cheap fix. A thicker wall on the same size costs more and does less.

What sizes of aluminium RHS and SHS suit gate work?

These are the sizes that come up again and again on gate jobs. The numbers are worked out for square-cornered 3 mm tube. Real extrusions have rounded corners, so they come in a little lower.

Look at the RHS rows. Every one has a strong number and a weak number. Which one you get depends only on which way round you weld it in.

Does an aluminium gate need a bigger tube?

Usually, yes. Aluminium is about a third as stiff as a galvanised tube of the same size. Put the same load on both and the aluminium one bends about three times as far. It is also about a third of the weight, which is why aluminium gates are so easy on posts and hinges.

The fix is depth, not wall. Going from a 50x50 to a 65x35 on edge buys back far more than going from 2 mm to 3 mm wall. On wide leaves, step up to 75x50 or 100x50 RHS for the rails and add a diagonal brace.

Which way round should RHS go in a gate?

Stand the deep side in line with the load. On a swing gate, the leaf wants to drop at the latch end, so the top and bottom rails bend up and down. Those rails want their deep side vertical. On a sliding gate, the long top and bottom rails carry the whole leaf between wheels or rollers, so the same rule applies.

Stiles are different. Seen from the front, a stile set with its narrow face showing looks slim, which is often what the customer wants. On a pedestrian gate that is fine. On a wide or heavy leaf, the hinge stile is pulled at the top hinge and pushed at the bottom one, across the width of the gate. Set its deep side across the gate, even if that shows a wider face, or use a bigger SHS.

Whatever you choose, write the orientation on the drawing. "65x35 RHS, 65 face vertical" takes ten seconds to write and stops a rail going in flat.

When is SHS the better choice?

Reach for SHS when the load can come from any direction:

• Posts. A gate post takes the leaf's weight one way and wind from another. Equal stiffness both ways is what you want.
• Short members. Jambs, joiner blocks and short braces do not span far enough to bend much, so the shape does not matter.
• Matching the look. A square frame on square posts reads as one piece. Plenty of customers want that.
• Knocks. Gate stops and bollards get hit from odd angles.

Pick RHS for the members that span: the rails of a wide leaf, the top rail of a sliding gate and a tall stile. Mixing the two is normal. Most good gates have SHS posts and an RHS or SHS frame chosen member by member.

What frame size should you use for each gate width?

Starting points for aluminium gates with batten or slat infill. Solid infill, high wind or a heavy lock set needs a step up. Check your supplier's span tables for anything out of the ordinary.

For posts, the gate post size guide goes deeper. For building the frame itself, see how to build a metal gate frame.

What changes when you weld aluminium RHS and SHS?

Welding softens aluminium next to the weld. That strip, the heat-affected zone, can lose a third or more of its strength. On a gate it means two things.

• Keep welds near the ends of members where you can. A weld in the middle of a long rail puts the soft spot where the bending is worst.
• Do not size a hinge stile right at its limit. Welded hinges sit in the soft zone, so give the stile some margin, or bolt the hinges on.

Mitred corners on box section are the standard gate joint. Tack all four corners, check the diagonals, then weld in a pattern that does not pull the frame out of square. The guide on welding an aluminium gate walks through it.

Common questions