The short answer
Glazing bars leak because the seals that hold the glass in place wear out. The cap on top of each bar, the rubber gaskets either side and the ends where bars meet the ridge and eaves all age in sun and frost. Repair works when the bars are sound and only the seals have failed. Replacement is the answer when the metal has corroded or the frame has moved.
How a bar keeps the rain out
A glazing bar is more than a support. It is a small drainage system. The panes of glass or polycarbonate rest on rubber gaskets along each bar, and a cap is screwed down on top to clamp them and cover the joint. Any water that gets past the cap is meant to run down a channel inside the bar and out at the eaves.
So there are three layers of defence: the gasket under the glass, the cap on top and the internal channel. When one fails, the next copes for a while. When two fail, water appears inside the room, usually as a drip or a damp line along the bar.
Because the bar is also long and runs the whole length of the roof, a small failure at the top can show as a drip at the bottom. The place where water appears is often not the place where it gets in, and tracing it takes patience. Wetting the roof from the ridge down, in sections, while someone watches inside is one way to find it, and it is how we narrow it down on a survey.
A simple home test is to look for tide marks on the frame or the sill, which show where water has been running, and to note whether the drip stops soon after the rain does or carries on for hours. A long drip suggests water trapped in a channel.
Caps, gaskets and sealant
Rubber gaskets harden with age, shrink away from the glass and crack, particularly at corners. Caps can lift where the screws have loosened, or the sealant under them can fail. UV light and temperature swings do this slowly, and old roofs fail in places that have taken the sun for years.
A dab of silicone along the bar looks like a quick answer, but it often traps water instead of stopping it. Sealant over a blocked drainage channel pushes the leak somewhere else. The channel needs to run clear, and the gasket needs to seat properly, before any sealant is used.
Cracked or slipped glass units are another cause. A unit that has moved leaves a gap at the gasket, and the movement can be a sign of a problem with the bar, not the unit. We check whether the unit sits true and whether the bar behind it is straight.
Repairing seals is practical when the bars are sound. It is poor value on a roof where every gasket has failed, because the same fault returns elsewhere within a short time.
Ends, ridge and eaves
The ends of the bars are where leaks tend to start. At the ridge the bars meet a ridge piece, at the wall they meet a flashing (the strip of metal that bridges the gap between roof and wall), and at the eaves they meet a cill or gutter. Each junction has end caps and seals, and each can fail.
Water also tracks back along the underside of a bar and drips at the wall. A damp mark running down the brickwork above the glazing points to the wall flashing, not the bars, and our guide to conservatory wall flashing covers that junction.
Frame movement makes it worse. If the bars have bowed, the frame is out of line, or the glass has shifted, the gaskets cannot seal evenly. A bar that sags in the middle collects water instead of shedding it, and the joint at the ridge opens as it flexes.
We photograph each end of each bar that we inspect, so you can see the gaps and cracks yourself and compare them with what needs doing. A flashing that has lifted from the wall needs re-dressing and sealing at the joint, and that is a separate job from the bar itself.
Repair or replace
A repair makes sense when the bars are straight, the metal is sound and the failure is limited to caps, gaskets or end seals. Refitting gaskets, replacing caps, clearing the drainage channels and renewing the end details can bring a roof back to dry, and the bill is a fraction of a new roof.
Replacement makes sense when the bars have corroded, the frame has moved or the glazing is single-glazed and misted. HomeBuilding puts a polycarbonate roof at about 10 years and The Eco Experts at 10 to 15, so a roof near that age with several failed seals is nearing the end. Glass roofs last longer, but the seals around them do not.
We tell you which of the two you are looking at, with photographs of the bars, and the written total covers whichever you pick. For the bigger picture, see conservatory roof repair and the conservatory roofs hub.
If you cannot tell whether repair or replacement fits, send photos of the bar ends and the caps, and a note of when the leak appears.
