Building components · Openings & Frames
Glazing Pocket
Explains the pocket as a designed and drained space with its own behaviour, rather than as the incidental gap left where glass meets a frame.
Educational reference entry. What this element has to do, and whether any particular arrangement of it is adequate, depends on the whole assembly, the loading, the exposure, local requirements, the manufacturer's instructions and qualified professional design.
Overview
What a pocket is
Every glazed element has a recess along each edge into which the glass or the sealed unit is set: bounded on the outside by the section's own upstand, on the inside by whatever retains the unit, and at the base by the surface the unit's weight is carried on. That recess is the glazing pocket, and it is formed in the frame, the sash, the mullion or the transom rather than added to it.
The reason it is a position worth naming is that it is not a sealed space. Water passing the outer line of a glazing seal arrives in the pocket, and in most modern glazing arrangements it is expected to. What happens next — whether the pocket drains and ventilates outward, or whether the water stands at the edge of the unit — governs how long the unit's own edge seal survives, which is why a pocket is commonly designed as a drained channel rather than as a slot.
Boundaries
How it differs from nearby elements
The distinctions that keep this entry from being a restatement of its neighbours.
- It is not a glazing bead. The bead is a separate fitted length that closes one side of the pocket and can be taken off; the pocket is the recess itself, and it exists in the section whether the retaining piece is a demountable bead, a clipped cap or a structural bond.
- It is not a frame packer. A packer is a discrete piece placed inside the pocket to hold the unit off the section and transfer its weight at defined points; the pocket is the space that both the packer and the unit occupy, and one is an element while the other is a void.
- It is not the perimeter gap around a window frame. That gap sits between the frame and the opening in the wall and is insulated and sealed; the pocket sits within the frame section between it and the glass, and in external work is commonly drained.
Terminology
Common names and aliases
These names describe the same element. The library keeps one entry per component so that a trade term or a regional name never becomes a second, thinner page.
- glazing rebate
- glazing channel
- glazing groove
Dependencies
What it depends on
What has to be right elsewhere for this element to do its job.
- Having a route out for water that reaches it, since a pocket that collects without discharging holds water against the edge of the unit.
- Clearance being left around the unit rather than the glass bearing directly on the section, because glass and the surrounding material move differently.
- Where the unit is seated within it, since the weight of the glazing has to be delivered to points the section can carry it at.
- Whether the arrangement it belongs to is a drained and ventilated one or a fully sealed one, because the two treat the same recess as doing opposite jobs.
Considerations
Topics that genuinely apply here
Subjects worth discussing about this element. These are discussion topics, never measured values, ratings or performance claims.
In a drained glazing arrangement the pocket is part of the water path, not a defect in it, and the outlets that let it discharge are as much a part of the detail as the seals that let water past in the first place.
The edge of a sealed unit is the part of it that fails first, and standing water in the pocket is a recognised route to that happening earlier than it otherwise would.
Glass cannot be gripped. The pocket has to hold the unit while letting it move relative to the section, which is why the unit sits on and against discrete points rather than being bedded solid.
The pocket is where a unit made to one tolerance is received by a section made to another, and it is sized around that difference rather than around the glass alone.
Assemblies
Building systems this element is met in
The role this component fills inside each assembly. A role describes what the element does there — it is not a statement that this component suits that system in any particular building.
Clarifications
Commonly misunderstood
- That the pocket is sealed and water never reaches it. In most contemporary glazing it is drained precisely because water is expected to get past the outer seal, and treating it as a sealed void is how drainage paths get blocked.
- That packing the pocket solid makes a window tighter. Filling it removes the drainage path and restrains the unit, which are the two things the recess exists to avoid.
Conversations
Questions for qualified professionals
Questions worth asking someone with the project in front of them. This page cannot answer them.
- Is this glazing arrangement a drained one, and if so where does the water in the pocket discharge?
- How is the unit seated and supported within the pocket, and at what points is its weight carried?
- What keeps the glass from bearing directly on the section as the two move relative to each other?
- If a unit is replaced, how is the pocket cleared and its drainage confirmed before the new unit goes in?
Go deeper
Related Build Design Hub guides
Planning guidance behind the decisions this element involves.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this element comes up.
What this entry does not do
- This entry explains a position. It states no dimension, edge cover, clearance, drainage size, glass type or performance value, and none can be inferred from it.
- Glazing pockets are designed as part of a specific window or facade system, and that system's requirements govern; nothing here describes any particular arrangement.
Openings & Frame Elements
The frames, heads, jambs, sills, thresholds and closers that form and finish an opening through a wall.
Browse all openings & frames entries →