Flat rooflight security and laminated glass

How laminated glass resists an attack

Laminated glass is two or more sheets bonded either side of a plastic interlayer, almost always polyvinyl butyral, under heat and pressure in an autoclave. The interlayer is what does the work.

Hit a toughened pane and it dices into thousands of blunt fragments and leaves the frame in one go, taking about a second. Hit a laminated pane and the glass cracks, but the fragments stay bonded to the interlayer. The pane is now opaque, damaged and still in the opening. Getting through means cutting the interlayer, blow by blow, along the whole perimeter of a hole large enough to climb through. That takes minutes, considerable effort and a lot of noise, on a roof, in the open. The delay is the security, not the glass.

Reading the EN 356 classifications

Security glass is graded to BS EN 356 and the grade tells you what the glass was tested against.

P1A to P5A are the drop ball classes. A 4.11kg steel ball is dropped from increasing heights onto the pane, and the class records how many strikes it resisted. P1A is a single drop from 1.5 metres, P5A is nine strikes from 3.9 metres. P6B to P8B are the axe classes, tested with a mechanical striker, and those are commercial specifications rather than domestic ones.

For a reachable domestic rooflight, P2A or P4A on the outer pane is a sensible bracket. P2A corresponds roughly to a 6.8mm laminate, P4A to an 8.8mm one. Insurers who ask for anything at all usually ask for P1A as a minimum on accessible glazing.

Where the real weak point sits

It is not the glass, on any decent unit. It is the junction between the frame and what it is fixed to.

A flat rooflight is mechanically fixed down to a kerb, and the security of that fixing depends on the fixing pattern, whether the screws are concealed, and whether the kerb itself is sound timber rather than something that has gone soft at the corners. On a unit fitted twenty years ago onto a kerb that has been under a felt covering ever since, the timber is the variable. A survey looks at the kerb before it looks at the glass, which is the correct order.

Fixed units against opening ones

A fixed rooflight has no mechanism, so there is nothing to lever and nothing to defeat. It is inherently the more secure of the two.

An opening unit brings a hinge line and a locking point into the design. Modern flat rooflight actuators are self-locking chain or spindle drives, which cannot be pushed open from outside because the drive train resists back-driving. Manual openers usually run a multi-point espagnolette into keeps in the frame. Both are perfectly adequate on an accessible roof provided the specification says so. What is worth avoiding on a reachable roof is a simple friction stay with a single central catch.

Frame fixings and how they are concealed

On a well built installation the fixings are not visible from outside, and that is not only a matter of looks.

The unit is set onto the kerb, fixed through the frame into the timber at the manufacturer’s stated centres, and the covering plus the external trim then sits over the fixing line. A screw head visible from the roof surface is a screw head somebody can put a driver on. Stainless fixings are used because a corroded screw in a wet kerb junction loses its head long before it loses its shank.

Laminate as an obstacle rather than a barrier

It is worth being precise about what laminated glass promises, because the marketing occasionally overpromises.

No domestic glazing is impenetrable. What laminate does is convert a one second event into a several minute one, and burglary is overwhelmingly a crime of speed and opportunity. Someone who has to stand on a roof making noise for four minutes is someone who leaves. That is the whole mechanism. Anybody selling glass as unbreakable is selling something else.

The acoustic and solar benefits that come with it

Specifying a laminated outer pane pays twice, which softens the cost.

The interlayer damps vibration, so rain noise on the glass drops noticeably against a monolithic outer pane. And the PVB interlayer blocks close to all ultraviolet, which slows the fading of flooring, worktops and furniture under the rooflight. On a south facing flat roof that second benefit is not trivial. Solar control coatings can be applied to a laminated pane without difficulty, so nothing is given up thermally. The wider glazing options are covered under energy efficient rooflights.

Weight, and what it does to the installation

Laminated glass is heavier than monolithic glass of the same nominal thickness, because it is two sheets plus an interlayer.

A double glazed unit with a 6mm toughened outer and 6.8mm laminated inner runs around 25kg per square metre once the frame is included. Upgrade the outer to 8.8mm laminate and the same unit is closer to 35kg per square metre. On a 1000 x 1500 rooflight that is a jump from roughly 40kg to 55kg, which changes how many people are needed and whether the unit goes up a ladder or up a scaffold. It also feeds into the kerb and the deck design on a marginal structure.

Alarm contacts and glass break detection

Where a house has an alarm, an accessible rooflight is worth putting on it, and there are two ways to do it.

A magnetic contact on an opening unit reports the sash leaving its frame and is straightforward, though the cable needs to be run before the reveal is boarded. An acoustic glass break detector listens for the specific frequency signature of breaking glass and covers a fixed unit as well as an opening one. On laminated glass a break detector is arguably more useful than on toughened, because a laminated pane can be cracked repeatedly without ever leaving the frame, so the alarm is what tells you it is happening.

Domes and polycarbonate on an accessible roof

Plastic rooflights behave differently under attack and the difference runs both ways.

Polycarbonate is extraordinarily tough and shrugs off impact that would crack glass, which is why it is used for glazing that expects to be hit. What it resists poorly is a blade. A polycarbonate dome can be cut open with a sharp knife in a way no laminated glass unit can, quietly, without the noise that makes a roof a bad place to work. Acrylic is the opposite: more brittle under impact, and it can be broken out of a clipped kerb more readily than most people assume. On a garage roof reachable from a boundary, an old single skin dome is the least secure thing on the building, and swapping it for a glazed unit improves the security, the thermal performance and the light in one visit.

Internal grilles, and when they make sense

Rarely, in a house. They are an industrial answer, they read as one, and they undo the point of putting glass in a roof.

Where they do appear is on a rooflight over a garage used to store equipment, or over a workshop or an outbuilding where the daylight matters and the view does not. There, a galvanised mesh or bar grille fixed to the inside face of the kerb, below the unit, is honest and effective. It is fixed to the structural kerb rather than to the rooflight frame, so the unit can still be lifted out without dismantling the grille.

Insurance requirements and what a policy usually asks

Standard household policies almost never specify anything about rooflights, and that is worth knowing before paying for a specification nobody asked for.

Where a policy does carry a glazing condition, it typically applies to accessible glazing at ground floor level and asks for laminated glass to a stated class, most often P1A. High value contents policies can go further and ask for P2A or a specific locking standard on opening lights. If security glazing is being specified because an insurer requires it, the certificate from the glass supplier stating the EN 356 class is the document that satisfies them, and it wants keeping with the completion paperwork.

What a specification line should actually say

A quote that says “toughened and laminated glass” tells you almost nothing, because that describes every domestic rooflight sold.

A specification worth comparing names the outer pane thickness and treatment, the cavity and its fill, the inner pane thickness and treatment, and the security class if one is being claimed. Something in the form of “8.8mm toughened laminate outer, EN 356 P4A, 16mm argon filled cavity with warm edge spacer, 6.8mm laminated inner with soft coat low-E” is a real specification. It can be checked against the glass certificate on delivery, which a phrase cannot.

Upgrading an existing rooflight for security

Where a rooflight is already in place and the concern is new, perhaps a neighbouring extension has made a roof reachable, there are three routes and they differ a lot in cost.

Applied security film on the inner face gives some fragment retention and is the cheapest option, though it does not approach the performance of a genuine laminate and it discolours over time. Improving the locking on an opening unit, or fitting an alarm contact, addresses the mechanism rather than the glass and is usually money well spent. Or the sealed unit is changed for one with a laminated outer pane, which on many systems can be done without changing the frame at all, since the unit is glazed into the sash rather than bonded to it. A replacement unit into a sound frame and kerb is often a single visit.