At a glance
| Detail | What to do | Why or limit | Basis |
|---|---|---|---|
| Glass in or beside a door, or below 800 mm | Doors and side panels up to 1500 mm (300 mm zone); other glass below 800 mm | Critical location: glass breaks safely, is robust, is a small pane or is protected | Approved Document K 5.1, 5.2, Diagram 5.1 (read) |
| Safe breakage | Class 3 of BS EN 12600 (or Class C of BS 6206) | Class 2 or B only for door panes wider than 900 mm | Approved Document K 5.4 (read) |
| Small annealed panes | 250 mm maximum, 0.5 m² maximum, 6 mm minimum thickness | Leaded and copper lights may be 4 mm where fire resistance is not important | Approved Document K 5.6, 5.7 (read) |
| Large clear glass, non-domestic | Manifestation at 850 to 1000 mm and 1400 to 1600 mm | Logo at least 150 mm high or bands at least 50 mm high | Approved Document K 7.4 (read) |
| Glass in a metal frame | Glass never touches metal: gasket or tape, setting block, edge clearance | Sizes from the glazier and the system | Glazing guidance; no standard read |
| Mirror | Off the wall with air behind, sealed edges, neutral-cure products, grommets on screws | No contact between silvering and bare metal or strong solvents | One maker's guide citing BS EN 1036 |
First four rows: Approved Document K (2013, England), read 7 Nov 2026. The rest is qualitative guidance.
How glass sits in a metal frame
The rule is simple: glass must never touch metal. In a glazed frame the glass is held by soft parts: gaskets or tape on each face, a setting block under the lower edge and a clear gap around every edge. Those soft parts carry the load, take up movement and keep the glass edge, its weakest part, away from a hard surface. Glass makers’ guidance repeats the same points: edge clearance cushions the glass and isolates it from the frame, and missing or poor setting blocks and gaskets let stress build up until the glass breaks.
Why edges matter. A chip or nick on a glass edge is where a break starts. Glass makers report that the edge strength is reduced by damage, and that thermal breaks begin at the edge, usually at right angles to it. Shading at the edge makes the temperature difference across a pane worse, and a dark frame heats the edges hidden inside the rebate. That is why the same glass can be safe in one frame and break in another: the frame colour, the shade and the edge finish are part of the specification. A glazier or the glass supplier checks thermal stress for the actual glass, size and exposure.
Metals move more than glass. Most metals used in interiors expand more per degree than glass does, so a long metal frame grows relative to the pane it holds. The gap and the soft parts take up the difference, and a tight fit does not. For the metals see thermal expansion of metals; the glass coefficient was not read for this sheet and is listed for checking.
The bead goes on the secure side. The part that holds the glass in place, a bead or a clip, belongs on the side that cannot be reached by someone outside, so the glass cannot be removed from the wrong side. A removable bead also lets the glazier replace a cracked pane without cutting the frame.
Fire-rated glazing is a different system. A glass and frame combination that has a fire rating is only that rating as tested, with its own glass, beads, seals and fixings. Do not change any part. See steel and glass screens and doors for how the evidence is read.
Safety glazing and manifestation
Safety glazing applies to every glazed frame in a building. Approved Document K (2013 edition, England) section 5 sets the rule, and the diagram below follows it. Diagram 5.1 marks the critical locations: glazing in a door, glazing in a side panel next to a door, and low glazing in windows and walls. Paragraph 5.2 then gives three routes: glass that breaks safely, glass that is robust or in small panes, or glass that is permanently protected.
Breaking safely. Paragraph 5.3 defines safe breakage by BS EN 12600 section 4 and BS 6206 clause 5.3: a small clear opening only, disintegration into small pieces, or separate pieces that are not sharp or pointed. Paragraph 5.4 sets the class: Class 3 of BS EN 12600 or Class C of BS 6206, and Class 2 or B for door panes wider than 900 mm. A class belongs to a tested product of a given type, thickness and size, so the glazier should state it and the glass should be permanently marked.
| Glass | How it breaks | In a metal frame | Check before you specify |
|---|---|---|---|
| Annealed float | Large sharp shards | Cut edges are the weak point; fine only where the small-pane or robustness rules apply | Thickness and area limits (Approved Document K 5.5 to 5.7) |
| Toughened (BS EN 12150-1) | Small blunt pieces | Edge work and holes must be done before toughening; it cannot be cut afterwards | Whether heat-soaked glass is wanted; marking |
| Heat-soaked toughened (BS EN 14179-1) | As toughened | Used where spontaneous breakage would be a hazard; it lowers, and does not remove, the chance | Who tests, and what the report says |
| Laminated (BS EN ISO 12543) | Pieces stay on the interlayer | Edges must be protected from the cleaning and sealant chemistry the interlayer cannot take | Class and thickness from the glazier |
| Fire-rated glazing | As tested | Only in the tested frame and beads | Classification report and field of application |
The table describes general behaviour from glass-maker guidance. It is not a class table: a specific product reaches a class only on test evidence. Our glass types sheet and glass shelves and table tops explain the types in more detail.
Manifestation. A large clear screen or door in a shop, office or other non-domestic building must be seen. Section 7.4 asks for manifestation at two levels, 850 to 1000 mm and 1400 to 1600 mm above the floor, contrasting with the background from both sides, as a logo at least 150 mm high or bands at least 50 mm high. Frames, mullions, transoms and large handles can do the job (7.3).
Where Approved Document K stops. It covers glazing in walls, doors and windows. A free-standing glass shelf or tabletop is furniture. BSI lists BS 6262-6:2005, the glazing code for special applications, as excluding furniture glazing, so shelf glass is specified with the glazier and, where bought to a trade guide, the furniture trade body’s own guidance.
Mirrors, shelves and fittings
Mirrors fail at the back and the edge. A silvered mirror has a thin metal layer and paint on its back, and both can be attacked by damp, by strong solvents and by the wrong adhesive. One maker’s installation guide, which cites BS EN 1036 for full detail, gives the practical rules: set the mirror at least 5 mm off the wall so air can circulate, and never fix it into a recess; seal all processed edges; use only neutral-cure silicones and adhesives that say they are suitable for mirror, never acid-based glues or silicone with an algaecide; use grommets on screws so the glass never touches bare metal; and keep the room well ventilated and free of solvent fumes while fixing. It also says mechanical fixings should be used with the adhesive, because adhesive alone can fail, and that mirrors should not go on new plaster or masonry or a freshly painted wall.
BSI lists BS EN 1036-1:2007 as current and under review: mirrors made from silver-coated float glass for internal use. A mirror used outside, or in a very damp room, needs extra protection for the backing and edges from the maker. A metal frame around a mirror needs the same air gap and a frame that does not trap water against the edge.
Shelves and brackets. A glass shelf in a metal bracket needs a soft liner between the glass and the metal, polished edges, and fixings into the wall or carcass, never through the glass unless the glass has been drilled for it. Toughened glass cannot be cut or drilled after toughening, so every hole, notch and edge is final when it leaves the glass supplier. No hole size, edge distance or load is given here: the numbers vary between suppliers, and none was read in a primary document. The glass shelf sheet lists the questions to ask.
Patch fittings, clamps and point fixings. Where glass is held by a metal fitting, the fitting carries load through soft bushes and liners, never glass to metal. The detail needs an engineer or the fitting supplier when the glass carries load, such as a glass door, a balustrade or a shelf with a heavy load. See balustrade and handrail rules for glass in guarding.
Isolation, movement and who confirms
Isolate dissimilar metals. Glass does not corrode, but the fittings around it do. Where a brass, steel or aluminium part meets another metal near a wet area, fit an insulating washer or gasket so the two metals do not touch; the galvanic corrosion sheet explains which pairs matter.
Choose sealants and gaskets for the metal and the finish. Sealants and glazing gaskets can stain, soften lacquer or attack plated and silvered surfaces. Ask the partner to confirm compatibility with the finish (lacquered brass, plating, anodising) and with the glass edge or mirror backing, and to name the product in the sample approval.
Allow for movement and tolerance. Visible metalwork has size tolerances that add to glass tolerances; see tolerances for visible metalwork. A frame drawn to the exact glass size has no clearance. Draw the opening larger than the glass by the clearance and tolerance the glazier gives.
Who confirms what. The glazier or glass supplier confirms glass type, thickness, class, edge work and setting. The partner workshop confirms the frame, the finish and the fixings. A structural engineer confirms any glass that carries load. Approve the whole detail on a shop drawing and a mock-up (shop drawing approval); the sample is for checking appearance, clearance and fixings, and does not prove a glass class.
Specification wording
Paste into your schedule or specification and replace anything in [square brackets] with your own figures. Wording is a starting point: your specification governs.
Glass in [frame ref]: [type and thickness confirmed by the glazier for the rebate, size and exposure], bedded on [glazing gasket or tape] on both faces and set on [setting blocks of material and number] at [positions]. Edge clearance [ ] mm on every edge; no glass-to-metal contact. Beads on the [secure] side, fixed with [fixings]. Glass edges [polished or arrised]; [thermal stress checked by the glazier for frame colour [ ] and shade]. Glass permanently marked with the maker and the standard class.
Glass at [position refs, Approved Document K Diagram 5.1] to break safely to BS EN 12600 Class [3] [or robust or small-pane under ADK 5.5 to 5.7, agreed in writing]. Large clear areas: manifestation at [850 to 1000] mm and [1400 to 1600] mm above finished floor, [logo at least 150 mm high or bands at least 50 mm high] contrasting from both sides. Evidence of the class and the glass marking before manufacture.
Mirror [ref] to BS EN 1036-1, edges [polished] and sealed with [edge sealer approved by the mirror maker]. Fixed with [neutral-cure adhesive stated suitable for mirror backing] and [mechanical fixing with non-metal grommets], set not less than [5] mm off [painted, fully dry] wall with air circulation behind; not fixed into a recess. No acid-cure or algaecide silicone, no strong solvents. Glass shelves: [soft liner between glass and bracket; fixings into the wall, not the glass; holes and edges formed before toughening].
Terms used on this page
- Setting block
- A small soft block under the lower edge of a glass pane that carries its weight and keeps the edge off the frame.
- Critical location
- A glazed position where people are likely to be hit by or fall into the glass, mapped by heights in Approved Document K Diagram 5.1.
Standards and sources
- Approved Document K (2013 edition, England): Sections 5 and 7 and Diagram 5.1 read 7 Nov 2026.
- BS EN 12600:2002, BS 6206:1981, BS EN 12150-1, BS EN 14179-1, BS EN ISO 12543-2, BS 6262-6:2005, BS EN 1036-1:2007: BSI listings: all current (some under review); BS 6206 largely replaced per secondary sources. None read.
Questions we are asked
Can glass touch a metal frame?
No. Glass makers' guidance is that edges should never touch the frame or metal parts such as screws. Gaskets or tape, setting blocks and clearance keep them apart, and the glazier sets the sizes for the glass and the frame.
Does a glass shelf need safety glass?
Approved Document K covers glazing in walls, doors and windows, not free-standing furniture, and BSI lists BS 6262-6 as excluding furniture glazing. Shelves are normally toughened or laminated, but the glazier sets the type and thickness for the span and load, and the furniture trade body publishes a best practice guide for glass in furniture.
Why does glass crack near a metal frame?
Usually the edge. Chips weaken it, a dark frame heats the hidden edge, shade makes the temperature difference across the pane worse, and a tight fit or missing setting block puts stress on it. Toughened glass tolerates more but is not immune.
Can I put a mirror in a metal frame in a bathroom?
Only if the maker agrees. Silvering is attacked by damp, solvents and the wrong adhesive, so ask the mirror maker for a product and edge sealer suited to damp rooms, keep an air gap behind, and use neutral-cure products and grommets on screws.
What do I need to ask a partner for glass in a metal frame?
The glass type, thickness and class with permanent marking, the gasket and block materials, the clearances, the sealant and its compatibility with the finish, and a shop drawing and sample. For fire-rated glass, the classification report and field of application too.
Download the factsheet
A two-page A4 PDF with the glass-in-frame section, the Approved Document K routes, the detail rules for mirrors and shelves and paste-in wording.