How do I work out which shower door seal I need and replace it?

The reason people cannot find a shower seal is that they go looking for their enclosure's brand, which for a twenty-year-old enclosure no longer exists or never published parts. Seals are a commodity extrusion. What matters is the shape of the cross-section and the thickness of the glass it grips, and both of those are on the seal in your hand. The drawing shows the five profiles you will actually encounter.

What you'll need

• A steel rule or a vernier, to measure the glass
• The old seal, or a photograph of its end-on cross-section
• A sharp trimming knife and a small square, for cutting to length
• Washing-up liquid in water as a lubricant

Step by step

1. Take the old seal off first.Most slide off the end of the glass or peel off along their length. Some sit under an end cap or the door handle — take that off rather than tearing the seal. Keep the old one intact; it is your template and your length measurement.
2. Measure the glass edge.With the seal off, measure the thickness of the glass itself. It will be one of 4, 5, 6 or 8mm. This is the single most important number and it is what the seal's channel is made for. A 6mm seal on 5mm glass falls off; a 5mm seal will not go onto 6mm glass.
3. Identify the profile by looking at the end.Look at the cut end of the old seal against the drawing. A hollow strip with a magnet in it is a magnetic seal — always used in pairs on the closing edges of a hinged door. A channel with a round bulb below it is a bulb seal, used at the bottom. A channel with one or two thin blades is a wiper or fin seal, used on a sliding door's vertical edge. A channel with a wide angled fin projecting inwards is a drip strip, whose job is to throw water back into the tray rather than to seal. Fins on both sides is an H-seal, for where two panels overlap.
4. Measure the length, and note whether it was cut to an angle.Measure the old seal end to end. Look at whether either end was cut at an angle to meet a frame or a curved track — quadrant enclosures often need one end cut to match the curve.
5. Buy generously long.Seals are sold in standard lengths, commonly around two metres, and are cut to fit. Buy the next length up rather than exactly what you measured; the trimming is the easy part and being 10mm short is not recoverable.
6. Fit it wet.Wipe the glass edge clean and dry, then wet the seal's channel with soapy water and slide it on from one end in one movement. Dry fitting a tight seal onto glass takes real effort and tears the lip.
7. Trim in place, square, with a sharp blade.Fit the seal full length first, then mark and cut. A square cut against a small square looks right; a freehand cut looks like a repair. Cut it a millimetre or two long so it compresses slightly into the frame rather than leaving a gap.
8. Check the pair on a magnetic seal.Magnetic seals only work as a matched pair with the magnets facing each other. Replacing one of a pair with a different profile means the door will not hold closed. If one is worn, replace both.
9. Test with the shower running before you call it done.Run the shower for a couple of minutes with the door closed and nobody in it, and look along the outside of the joint. A seal that looks right and still passes water usually means the door is out of adjustment rather than the seal being wrong.

Tips

• Photograph the cut end of the old seal on a sheet of paper with a rule beside it. A supplier can identify a profile from that in seconds and cannot identify anything from a brand name.
• If the seal has a fin that trails on the tray, it goes on with the fin trailing in the direction the door closes, not against it.
• Wash seals as part of cleaning the shower and dry them. Most seals fail from soap and scale hardening the lip rather than from wear.

If it doesn't work