How do I replace a broken pane of greenhouse glass?

An aluminium greenhouse is dry-glazed: the panes are not puttied or bedded, they simply sit in a rebate in the glazing bar and are held by sprung clips, overlapping each other like tiles so water runs down the outside. Once you understand that, replacing a pane is a fifteen-minute job. The two things that go wrong are measuring from the broken pane and forcing a pane that is slightly too big.

Safety

What you'll need

• Replacement pane, cut to size
• Cut-resistant gloves and eye protection
• Greenhouse W-clips and overlap clips
• Pliers or a small screwdriver for the clips
• Tape measure
• Dustpan, brush and a sturdy bag for the fragments
• Glazing tape or a strip of rubber, if the frame uses it (optional)

Step by step

1. Clear the broken glass out completely and safely.Gloves, eye protection, long sleeves. Take the large pieces out first, supporting each one as you free it, then work the small fragments out of the glazing bar rebates with a screwdriver. Bag everything and check the border soil underneath thoroughly.
2. Remove the old clips.W-clips spring into the glazing bar and hold the pane against the rebate; overlap clips hold two panes together at the horizontal lap. Ease them out with pliers and keep them — they are reusable if undamaged, and cheap to replace if not.
3. Measure the opening, not the broken pane.Measure between the inside faces of the glazing bar rebates, at the top, middle and bottom of the opening, and take the smallest figure. Frames are rarely perfectly square, and the broken pane may itself have been the wrong size, which is sometimes why it broke.
4. Order the pane slightly undersize.Around 2 to 3mm smaller than the opening in each direction. Glass needs clearance to be slid in and to accommodate the frame moving. A pane cut to the exact opening will not go in, and a pane forced in is a pane under stress.
5. Get the right glass.Standard horticultural glass is usually 3mm plain float. Toughened glass of the same thickness is a substantial safety improvement, particularly at low level or where children use the garden, and it is worth the modest extra. Polycarbonate sheet is another option and is discussed below.
6. Work out which way the pane goes in.On most aluminium frames a pane slides in from above, behind the pane below it and in front of the pane above, so the laps shed water outwards. Look at an intact area of the same run before fitting — getting the lap the wrong way round makes the roof leak down the inside of the glass.
7. Slide the pane in and settle it.Support it with both hands, slide it into the rebates and let it settle down onto the pane below with the correct overlap. It should sit in place without being held. If it is fighting, take it out and check the size rather than pushing.
8. Fit the clips at the same positions as before.W-clips at the sides to hold the pane against the rebate, overlap clips at each lap. Two per side on a standard pane is normal. Clips that are missing or fitted in the wrong places are the reason panes slip.
9. Check it from inside and out.Look along the run for a consistent overlap and even gaps. Then push gently on the pane — it should not move in its rebate at all.

Tips

• Take one intact pane out and take it to the glass merchant as a pattern if the frame has several sizes and you cannot work out which is which.
• Buy two panes when you buy one. Delivery is the expensive part and you will break another.
• Fit toughened or polycarbonate at low level even if the rest of the greenhouse is float glass. That is where people and children make contact.
• Keep a small stock of W-clips and overlap clips. They spring off into borders and disappear, and a pane held by three clips instead of four is the one that slips.
• Do the job on a still day. A large pane of glass in a breeze is difficult and dangerous to control.

Common mistakes

• Measuring the broken pane — It may have been the wrong size to begin with, and pieces of broken glass do not reassemble to the original dimensions accurately. Measure the opening.
• Forcing a tight pane in — The glass is now loaded by the frame. It will crack at the first frost, the first gale or the first door slam.
• Lapping the pane the wrong way — The whole system depends on each pane overlapping the one below on the outside. Reversed, the lap channels water inside.
• Leaving fragments in the border — Greenhouse glass in soil surfaces for years and cuts hands during planting. Sift the area under the break properly.

If it doesn't work

Alternatives

• Toughened glass: The same thickness and the same fitting method, several times stronger, and it breaks into small blunt granules rather than shards. The obvious choice for low-level panes, for a greenhouse in a family garden, and for anywhere exposed. Costs more and cannot be cut once toughened, so measure carefully.
• Twin-wall polycarbonate: Light, effectively unbreakable in normal use, and it insulates noticeably better than single glass. It transmits slightly less light, it scratches, it discolours over many years, and it flexes in wind so it needs the clips and any specified retaining strips fitted properly. Excellent for exposed sites and for low-level panes.
• Horticultural float glass: The traditional and cheapest option, and what most greenhouses came with. Best light transmission, cheapest per pane, and it breaks dangerously. Fine for high panes where nobody will fall against them.

Questions people ask