How to replace a pane of greenhouse glass

Inspired by an Alitex greenhouse maintenance guide. This walk-through draws on the Alitex video "How to change a piece of 4mm toughened glass in a traditional Alitex greenhouse". Alitex are a Hampshire-based UK greenhouse manufacturer with decades of experience and their video shows the clip system clearly. The basic technique applies to most traditional aluminium-framed greenhouses.

1. Put on leather gloves and safety glasses before you start

Broken greenhouse glass is thin - usually 3mm or 4mm horticultural glass - and it can shatter into sharp fragments without much warning, especially if there are cracks running through it. Leather gloves will protect your hands far better than ordinary work gloves, and safety glasses are sensible whenever you are handling glass above head height.

Do not rush this step. A cut from glass in an awkward position is a straightforward DIY job turned into a trip to A&E.

2. Release the glazing clips

Most aluminium greenhouses use small metal or plastic W-shaped glazing clips that grip the bottom edge of each pane and secure it to the frame or to the pane below. You will usually find one or two at the base of the broken pane.

Use a flat-headed screwdriver or a small pry tool to ease the clips off. They can be reused if they are in good condition. If any are corroded or bent, replace them - a new pack of glazing clips costs almost nothing and worn clips are how panes fall out in the first place.

3. Remove the broken glass carefully

Once the clips are off, work the broken pane loose from the frame. If the glass is cracked but still largely intact, it will often come out as a single piece. If it has shattered, pick out the larger fragments by hand (with gloves) and collect them in a sturdy cardboard box or bucket - never a bin bag, which gives no protection against puncture.

Small fragments in the channel at the base of the frame can be swept out with a stiff brush. Clean the frame rails while you have the chance: any grit or debris left in there will scratch the new glass.

4. Measure the opening accurately

Measure the width and height of the opening at two or three points - frames can be slightly out of square, particularly on older greenhouses. Take the smallest dimension in each direction and subtract 5mm to give the glass a few millimetres of clearance on each side.

Note the thickness of the existing glass too - usually 3mm horticultural glass or 4mm toughened glass on better-quality structures. If you are replacing into an Alitex or similar traditional greenhouse, the manufacturer may be able to supply the exact size; otherwise, a local glass merchant will cut to order within a day or two.

5. Slide the new pane into position

Greenhouse panes typically overlap slightly - each pane laps about 10mm over the one below. Start from the bottom and work up if you are replacing panes in the same section, so each new pane can overlap the one beneath.

Slide the new pane into the top channel of the frame first, then lower it into the bottom channel. It should sit flush with the surrounding panes. If it sits proud or recessed, check the measurement - the glass may be slightly oversize.

6. Fit the glazing clips and check for movement

Refit the glazing clips at the base of the new pane, pressing them firmly until they snap into position. Give the pane a gentle push at each corner to confirm it is secure. There should be no rattle and no movement in any direction.

On roof panels in particular, make sure every clip is properly engaged. A pane that is not fully secured can slide out in strong wind, which is a hazard rather than just an inconvenience.

When to call a handyman

Call Richard if the pane is in the roof and working at height is involved, if the frame rail is damaged and needs attention before the glass can be refitted, or if you are dealing with a large run of broken panes rather than just one or two. Greenhouse glass on a pitched roof is straightforward enough on the ground; less so on a ladder with glass over your head.