Window Profile Reinforcement: Steel, Aluminium and the Screw That Misses

Why reinforcement exists

A PVC window profile is a hollow section with a stiffness of about 2.5 gigapascals, which is low. Under wind load it bends, and the amount it bends is what decides whether the window is acceptable: too much deflection and the opening light binds, the seals leak, and the glass can be stressed.

Reinforcement is a steel or aluminium section inserted into one of the profile chambers to carry the bending. It is not there to make the window stronger in the sense of resisting breakage; it is there to control deflection. That distinction matters because it means the reinforcement is sized by stiffness, not by strength, and a surprisingly small steel section does a surprising amount of work.

The consequence is that reinforcement is a design calculation, not a default. A small window in a sheltered location does not need it. A large window on a tall building does, and the size of the section follows from the span and the wind load rather than from a rule of thumb.

Deflection against span

For a typical 70 mm multi-chamber profile under a design wind load, an unreinforced section deflects 2.5 mm at a 1.0 metre span, 8.5 at 1.5, 20 at 2.0, 39 at 2.5 and 68 at 3.0. The same profile with a 1.5 mm steel section deflects 0.8, 2.4, 5.5, 10.5 and 18.

Set an 8 mm serviceability limit, which is a typical value for a domestic window, and the unreinforced profile crosses it just past 1.5 metres while the reinforced one stays under it to nearly 3 metres. That is the whole argument in one line: the reinforcement does not make the profile a little stiffer, it changes the size of window that is possible.

The limit itself is worth thinking about, because it is not universal. A serviceability limit of span/300 is common for windows, which for a 2.4 metre span is 8 mm; for a 1.2 metre span it is 4 mm. A single absolute limit applied to all sizes is either over-conservative on small windows or unsafe on large ones, and a specification that quotes an absolute number should also quote the span it was written for.

Table 1 — Deflection under a design wind load, with and without reinforcement.

Steel or aluminium

Steel is the default because it is stiffer per unit cost and per unit thickness. A 1.5 mm steel section has a modulus of 210 gigapascals; an aluminium section of the same size has 70, so it needs to be about three times as deep to do the same job, which usually means it does not fit in the chamber.

Aluminium is used where corrosion is the governing concern, particularly in coastal installations and in profiles that will be exposed to cleaning chemicals. A galvanised steel section is adequate for most inland applications, and a stainless section is used where neither galvanised nor aluminium is acceptable.

Table 2 — Reinforcement materials and where each one is appropriate.

The thermal penalty is real and it is usually ignored. A steel section inside a PVC chamber is a thermal bridge that bypasses the profile's insulation, and it can raise the frame U-value by 0.3 to 0.5 W/m²K depending on the section size and position. That is a significant fraction of the 1.3 the profile achieves without it, which is why the reinforcement belongs in the room-side chamber where the temperature difference across it is smallest.

Where the reinforcement goes

A multi-chamber profile has three or four chambers and the reinforcement fits in one of them. The choice is between structural efficiency, thermal performance and the fixing detail, and they do not always agree.

• The largest chamber gives the most room for steel and therefore the stiffest result, and it is usually the outer or the central chamber.
• The room-side chamber is thermally better, because the temperature difference across the bridge is smaller and the condensation risk is lower.
• The chamber that the fixing screws pass through is the one that must contain the steel, or the screw will not reach it.
• The reinforcement must be continuous through the corner, or the corner becomes a hinge. Corner cleats or welded corner pieces carry the load across.
• Drainage paths must not be blocked by the steel section, or water will sit against it and corrode it.

The fourth point is where cheap fabrication fails. A reinforcement that stops 50 mm short of each corner leaves the corner as an unreinforced hinge, and the window deflects as a mechanism rather than as a beam. The steel has to be continuous, or the corner has to be reinforced by a cleat that is mechanically connected to both sections.

The screw that misses the steel

A screw driven into a PVC wall 2.5 mm thick pulls out at about 900 newtons. Into a 3.5 mm wall it is 1,400. Through a 1.5 mm steel section it is 3,200 and through 2.0 mm it is 4,100. The difference between a fixing that engages the steel and one that does not is a factor of three or four, and it is invisible after the screw is in.

This is the failure mode that generates most window warranty claims. The frame is reinforced correctly, the reinforcement is in the right chamber, and the installer drives the fixing screw at an angle or at the wrong height so it passes beside the steel rather than through it. The window holds until the first storm, and then the frame pulls away from the wall.

The fix is a fixing detail, not a stronger screw. A jig that positions the screw at the centre of the reinforcement chamber, a screw length specified from the profile drawing, and a mark on the profile or a groove in the chamber that the installer can find by feel. Some systems use a screw that is designed to self-drill into the steel, which at least gives a audible and tactile confirmation that it engaged.

Checking that it is right

Reinforcement is easy to verify before the window is assembled and almost impossible afterwards, which is why the checks belong in the fabrication process rather than in final inspection.

1. Check the reinforcement section against the drawing: thickness, material and coating, on every delivery.
2. Check the cut length, because a section that is 20 mm short leaves an unreinforced zone at the corner.
3. Check that the section slides into the chamber without forcing, and that it does not rattle. A loose fit means the load is not transferred.
4. Fix the section with screws at the specified spacing so it cannot move during welding or transport.
5. After welding, confirm the corner is reinforced, either by the section continuing through or by a cleat.
6. On site, use the fixing jig and check the first window of each elevation by removing a screw and looking.

The last item is the only one that catches the site error, and it takes a minute. Removing one screw per elevation and confirming it passed through steel is a cheap check that catches the most expensive failure in the product.

Writing the specification

A reinforcement specification that can be checked has four elements: the section (material, thickness, coating), the chamber it goes in, the length and continuity requirement, and the fixing detail. Most specifications have the first and none of the others.

The wind load the calculation is based on should be stated too, because it is what makes the section size defensible. A supplier who is asked for a 2.0 mm steel section without a wind load will either over-specify and charge for it, or under-specify and hope. Stating the design pressure, the span and the serviceability limit turns the conversation into a calculation that both sides can check.