Flat rooflight security and laminated glass

What getting in through a rooflight actually involves

Worth being concrete, because the mental picture people have is usually wrong.

Somebody has to get onto a flat roof, which on a rear extension means over a fence and up a water butt, a bin store or an adjacent lean-to. They then have to work on an exposed horizontal surface, standing over the opening they are attacking, in full view of every upstairs window that overlooks the roof, making noise, with no quick way back down.

Then they have to defeat the glass. Toughened glass breaks readily and disintegrates, which sounds bad until you remember the laminated pane below it is still there and still bonded together. Getting through that takes sustained, loud, two-handed effort with a heavy tool. It is the slowest and most visible option available, which is exactly why it is rarely chosen.

Toughened and laminated do different jobs

These two words get used as though they are interchangeable safety features. They are not.

Toughened glass is heat-treated so the surface is in compression. It is around four to five times stronger than ordinary annealed glass of the same thickness, and when it finally does break it shatters completely into small blunt granules. That makes it safe to walk into and useless as a barrier, because a broken toughened pane leaves an empty frame.

Laminated glass is not stronger. Two panes bonded by a polyvinyl butyral interlayer break at much the same load as one thicker pane would. The difference is what happens next: the fragments stay stuck to the interlayer, so the sheet cracks but stays in the opening, still spanning, still a barrier. It has to be cut or torn out piece by piece.

Security comes from the laminate. Safety underfoot comes from the toughening. A good rooflight has both.

Reading a glass specification: what 6.8 and 11.5 mean

Laminated glass is written as a decimal, and the decimal is the interlayer.

So a specification reading “6mm toughened outer, 16mm argon cavity, 6.8mm laminated inner” tells you the inner pane is two 3mm sheets with a 0.76mm interlayer between them. That is the ordinary build-up on a domestic flush rooflight and it is a sound one.

If a quotation says only “laminated” with no decimal, ask for the figure. The difference between 6.4 and 11.5 in resistance to sustained attack is very large, and both are legitimately described as laminated.

Interlayer thickness, and the number worth asking for

Interlayer thickness is where the security actually lives, and it is the cheapest upgrade on the whole specification.

A single 0.38mm ply holds the fragments together and satisfies the safety requirement. It does not resist much determined attack. Two plies, at 0.76mm, roughly doubles the energy needed to tear through and is the point at which the glass starts being a genuine deterrent. Four plies, at 1.52mm, is what security-rated glazing uses, and it takes a long time and a great deal of noise to defeat.

Stiffer interlayers exist too. Ionoplast interlayers, sold under trade names, are considerably more rigid than standard butyral and hold a broken pane more firmly, which is why they turn up in walk-on and balustrade glazing.

On a roof reachable from a garden, asking for a 1.52mm interlayer in the inner pane is a sensible and inexpensive request.

EN 356 and what the P ratings test

If you want a rating rather than a description, EN 356 is the standard that gives you one.

The P1A to P5A classes are drop tests. A 4.11 kilogram steel ball is dropped onto the glass from increasing heights: 1.5 metres for P1A, 3 metres for P2A, 6 metres for P3A, 9 metres for P4A, and for P5A three separate strikes from 9 metres. The glass passes if the ball does not go through.

The P6B to P8B classes are attack tests, where a machine swings an axe at the pane and the count of blows to make an opening decides the class. Those are commercial and institutional ratings and they are rarely necessary on a house.

For domestic work, P2A or P4A on the inner pane is a strong and achievable specification. A standard 6.8mm laminate is generally around P2A. Getting to P4A means a thicker laminate and it is worth asking the price of.

Overhead glazing has to be laminated anyway

This is the point that reframes the whole subject, and it is why we push back gently when people ask about bolt-on security for a rooflight.

Glass over your head that breaks must not fall into the room. That is why the inner pane of any properly specified overhead unit is laminated: if it fractures for any reason, thermal stress, impact, a fixing error, it stays in the frame. The safety requirement and the security requirement happen to want the same thing.

So a correctly built flush rooflight is already a laminated barrier before anyone thinks about security. The decision left to you is how thick the interlayer is, not whether there is one.

Older domed units are the exception. A single skin acrylic or polycarbonate dome is not a security barrier in any meaningful sense, and it can be cut through quietly. That is the real security argument for replacing an old dome with glass.

The fixed unit against the opening unit

A fixed rooflight has no moving parts and no lock to defeat. The glass is bonded and mechanically retained in a frame that is screwed down through its flange into the kerb timber, and there is nothing to lever.

An opening unit introduces a hinge, a seal line, a drive and a closed position that has to be held. That is not a reason to avoid opening units, but it is the part of the assembly worth asking about.

Electrically operated units are better than manual ones here. A chain or spindle actuator holds the sash closed through a geared drive that cannot be back-driven by pushing on the glass, so there is no handle to force and no latch to slip. Manual pole-operated units use a locking catch, and the quality of that catch varies considerably between manufacturers.

Locks, restrictors and the hinge side

Where a unit does open manually, three details are worth specifying.

Multipoint locking rather than a single central catch, so force applied at one corner does not lever the whole sash. Concealed or security hinges, so the hinge pins are not accessible from outside when the unit is shut. And a restrictor that limits the opening to a ventilation position, which is useful both for security when the house is occupied and for anyone worried about a child in the room below.

Restrictors have a second function on a rooflight that people forget: they hold the sash against wind. A large sash swung fully open in a gust puts real load into the hinges and the frame, and a restrictor takes that out of the equation.

The frame, the fixings and the kerb

The glass is usually the strongest part of the assembly. What lets a rooflight down is what the glass is held by.

A frame screwed down at long centres into thin deck boarding can be levered off the kerb as a whole unit, which defeats the glass entirely without touching it. The fix is ordinary good practice: a properly built timber kerb off trimmed joists, fixings at close centres through the frame flange into solid timber, and the membrane termination dressed over the flange so the fixings are not exposed to a bar from above.

On replacement work, this is the part we look at hardest. A twenty-year-old kerb that has been standing in water at its base is soft timber, and soft timber holds no screw. That is a structural point and a security point at the same time, and it is covered in more depth on upstands, kerbs and warm roof build-ups.

Non-fragility is a safety class, not a security class

You will see rooflights described as non-fragile, and it is easy to read that as a security claim. It is not.

Non-fragility is tested to ACR[M]001, the industry test for large roofing elements, and it measures whether a person falling onto the assembly from a working height goes through it. The classes run Class A, B and C, with Class B being the common domestic standard. It exists to protect a roofer walking backwards, not to resist a burglar.

The two properties often travel together, because a laminated pane in a strong frame tends to pass both, but a unit can be non-fragile and still poor against sustained deliberate attack, because the tests measure different things. Ask about the glass make-up and the EN 356 class separately.

What a rooflight can take underfoot is a different subject again, dealt with on what is a walk-on rooflight.

Reachability, which decides more than the glass does

Ask one question before any of the rest: how does somebody get onto that roof?

A rooflight in a second floor roof, with no adjoining structure, reachable only by ladder carried across a garden, is not a realistic target and does not need a specification argument. A rooflight in a single storey rear extension with a flat garage roof next to it, a side gate that does not lock and a fence you can stand on, is a different proposition entirely.

Fixing the access is usually cheaper and more effective than upgrading the glass. A side gate with a decent lock, a bin store moved away from the wall, a water butt relocated, thorny planting along the fence line where the climb would start. Those change the calculation for anyone assessing the back of your house, and they cost a fraction of a glass upgrade.

Garage and outbuilding roofs at ground level

Garages, workshops and detached stores hold tools, bikes and equipment, and their roofs are usually the most accessible on the plot.

The units fitted to them are often the weakest too, because the room is unheated and a cheap dome was the obvious choice. A single skin dome over a garage full of tools is the softest point on many properties, and it does not announce itself as one.

Where a garage genuinely holds something worth taking, the sensible specification is a fixed unit with a laminated inner pane, no opening sash, and fixings through the frame into a solid kerb. Daylight without an access route. Where the building is a garden store with a lawnmower in it, a triple skin dome is a proportionate answer, and we will say so rather than sell you glass.

Alarm contacts and glass-break sensors

Three options exist and they suit different situations.

Opening contacts on an opening sash are simple, cheap and reliable, and they should be specified at the time the unit is ordered so the cable route exists. Retrofitting a contact into a finished frame is awkward.

Acoustic glass-break detectors mounted on the ceiling nearby listen for the frequency signature of breaking glass. They work on fixed units where there is no sash to contact, and they are the usual answer for a fixed rooflight.

Alarm loop glass, with a fine conductive wire embedded in the laminate that breaks the circuit when the pane is cut, is the highest-integrity option and has to be ordered as a special. It is rarely proportionate on a house, but it exists and it is worth knowing about for a home office or a studio.

What an insurer usually asks for

Policies vary and we are not going to pretend to speak for any insurer, but there are patterns worth checking before you order.

Most domestic policies concern themselves with final exit doors and accessible ground floor windows, and treat rooflights as ordinary building fabric. Where a policy does name rooflights, it is generally in the context of an accessible flat roof, and what it asks for is laminated glazing, key-operated locking on any opening light, or both.

If your policy has a specific wording, read it before ordering rather than after, because the difference is usually one line on a specification and no difference at all to the installation. Adding a thicker interlayer at order stage is straightforward. Changing the glass in a fitted unit means a new unit.

Not advertising what is under the glass

A clear rooflight over a home office at night is a lit display of the room and its contents, visible from every window that overlooks the roof and from any elevated position nearby.

That is not an argument against clear glass. It is an argument for thinking about which rooms get it. A kitchen, a hallway or a living room over-lit at night is unremarkable. A studio with equipment in it, or a garage workshop with tools laid out on a bench, is worth obscuring, and an acid-etched inner pane does that while keeping essentially all the daylight.

The same logic applies to blinds, which are worth having over any rooflight in a room you would not leave the curtains open in.

What we specify as standard, and what we ask

Our default on a flush flat rooflight is a toughened outer pane and a laminated inner pane, on a built and insulated timber kerb, with the frame fixed at close centres into solid timber and the membrane dressed over the flange. That is the specification because it is the correct one for overhead glazing, and it happens to be a good security specification as a consequence.

Where the roof is reachable, we will suggest a thicker interlayer and, if the unit opens, multipoint locking and a restrictor. Where it is not reachable, we will tell you that plainly rather than quoting an upgrade you do not need.

Tell us how the roof is reached, what is in the room below, and whether you want it to open. Call 01865 704245 or email info@heritageskylightsoxford.co.uk and you will get a range by email, then a fixed price after a survey.