Why do my greenhouse panes keep slipping out?
Why panes creep down a slope, whether the fault is clips, glazing bars or a frame that is out of square, and what fixes each.
- Difficulty
- intermediate
- Time
- 2 hr
- Read
- 6 min
- Safety
- warning
Short answer
Panes slip because nothing is holding them: missing or tired W-clips, overlap clips that have sprung off, or a frame that has racked so the pane no longer sits square in its rebate. Replace all the clips on the affected slope rather than the one that came off, and if the frame is out of square, fix the base — no amount of clips holds glass in a twisted frame.
In a dry-glazed aluminium greenhouse nothing bonds the glass to the frame. Panes sit in a rebate, overlap each other and are held entirely by small sprung clips. That system works well and lasts decades, provided the clips are all present and the frame is square. Once either is untrue, gravity does the rest slowly, over months, and the first sign is usually a pane sitting a centimetre lower than its neighbours.
Safety
What you'll need
- Greenhouse W-clips and overlap clips
- Cut-resistant gloves and eye protection
- Pliers or a small screwdriver
- Tape measure for the diagonals
- Glazing tape or lap sealing strip (optional)
- Packing or shims for levelling the base (optional)
Step by step
- Secure any pane that has already moved.Before diagnosing anything, get a hand or a temporary clip onto any pane sitting proud or low. A slipped pane is one gust away from falling.
- Count the clips on a sound area and on the failing one.Look at a run of panes that has never moved and note how many W-clips per side and how many overlap clips per lap. Then look at the failing area. Missing clips is the answer far more often than anything else.
- Check the clips you do have.Old clips lose their spring, corrode and open up. A clip that can be pulled off with two fingers is no longer doing its job. Replace the whole slope's worth rather than the obvious failures — they are the same age.
- Check the glazing bar rebate.Look along the bar. Corrosion, distortion, or a bar that has been bent by a knock will not hold a pane properly however many clips are fitted. A damaged bar can usually be replaced individually.
- Measure the frame's diagonals.Across each glazed elevation and across the base. If the diagonals differ, the frame is racked, the openings are no longer rectangular and every pane is sitting at a slight angle in its rebate. This is the cause people miss.
- Check whether the base has moved.A racked frame almost always means a settled base. Level round the perimeter. One corner that has dropped 15mm is enough to start panes creeping.
- Correct the base and re-square the frame.Support and pack the low corner, or re-bed the base properly. Then slacken the frame's corner bolts, push the frame back to square with the diagonals equal, and re-tighten. Doing it in that order matters — squaring a frame on a base that is still out simply re-racks it.
- Refit the panes with a full set of clips.Two W-clips per side per pane as a minimum, and an overlap clip at every lap. Fit them at the same positions the frame was designed for — the glazing bar has locations for them.
- Consider lap sealing strip if water ingress is the complaint too.Small self-adhesive strips fitted at the horizontal laps stop water and dirt being drawn between panes and stop the panes chattering. They do not hold the glass — that is still the clips' job.
Tips
- Buy a box of clips rather than a handful. They are inexpensive, they get lost, and having spares means a slipped pane is fixed the same day.
- Go round the whole greenhouse once a year and push gently on every pane. It takes ten minutes and finds the loose ones before a gale does.
- Anchoring the greenhouse to its base is part of this problem. A structure that shifts in the wind racks the frame, and racking is what starts the panes moving.
- If the greenhouse is old and every clip is tired, replacing all of them in one go is a genuinely worthwhile afternoon.
- Take a photograph of a correct clip arrangement before you dismantle anything, so you have a reference for the positions.
Common mistakes
- Replacing only the clip that failed — Every clip on that slope is the same age and has had the same weather. The next one fails within months.
- Using sealant or tape to hold the glass — It is not strong enough to hold a pane, it makes the next pane replacement much harder, and it hides the movement rather than stopping it.
- Squaring the frame without fixing the base — The base is what put the frame out of square. Correct it and the frame stays where you put it; leave it and the racking returns within a season.
- Pushing a slipped pane back up and walking away — Whatever let it slide is still absent. It will move again, and the next time it may go all the way.
If it doesn't work
One pane keeps sliding down after being pushed back
Cause: Its overlap clips are missing or sprung, so nothing resists it sliding on the lap. — Fix: Fit new overlap clips at both sides of the lap. This is the specific clip that stops downward creep on a sloping roof.
Panes have moved across a whole roof slope
Cause: A racked frame, usually from a settled base, or the frame bolts have loosened. — Fix: Check the diagonals and the base level. Correct the base, re-square the frame, then refit the glass. Clips alone will not hold glass in a twisted opening.
A pane rattles constantly in wind
Cause: The pane is undersize, or the clips have lost tension. — Fix: New clips first. If it still rattles, the pane is more than a few millimetres small and is better replaced at the correct size.
Panes slip only on one elevation
Cause: That side takes the prevailing weather, or the base has settled at that side. — Fix: Level check on that elevation, and use a full set of new clips there. Consider toughened glass or polycarbonate on the exposed side.
Clips keep springing off
Cause: Wrong clip type for the glazing bar profile, or a distorted bar. — Fix: Take an old clip to the supplier — several profiles exist and they are not interchangeable. If the bar is damaged, replace the bar.
Questions people ask
What are W-clips and overlap clips?
W-clips are the small springy Z or W shaped clips that push into the glazing bar and press the pane against the rebate, holding it sideways. Overlap clips are small S or Z shaped clips that grip two overlapping panes together at a horizontal lap, stopping the upper pane sliding down. Both are needed and they do different jobs.
Should I use sealant at the laps?
Not to hold the glass. Purpose-made lap sealing strips are available and they keep water and algae out of the joint and stop rattling, which is worthwhile — but the structural job is the clips'.
How do I know if my frame is out of square?
Measure both diagonals of each glazed elevation and of the base. Equal diagonals mean square. A difference of more than a few millimetres over a domestic greenhouse is enough to affect how panes sit.