Important notice for new rumoured burglar trend: burning burglar bars

This article is a holiday update for all those with InvisiBars™ & other polycarbonate burglar bars, and will be a good resource for those with aluminium or steel burglar bars too.

As happens each year, new trends emerge to defeat security systems put in place. From crow-bar gangs bypassing alarm systems to wily amateur burglars sending small children in to retrieve household items & keys. View our crazy list of burglary stories here!

This year’s trend has, not surprisingly, finally hit polycarbonate burglar bars. Rumours are spreading about a burglar using a blowtorch or a butane lighter to weaken the burglar bars, and we’d like to outline the effects below to equip you with all the knowledge you require to keep your home safe & sound.

This post will also highlight the weak & strong points of various burglar bar types to help you better understand how to use your burglar bars to protect your home.

Note: Please always ensure that you are aware that burglar bars are your 3rd point of resistance against intrusion. Your first is your perimeter security (electrified fencing, perimeter wall, wall spikes, etc.). Second is your alarm system, and third is your window protection (i.e. burglar bars).

The easiest method of gaining entry to a property through any burglar bars is to cut an opening in them. Please view the outline below & assess the pros & cons of your own burglar bars.

The most common type of burglar bar. Standard indoor burglar bars are usually between 9 & 12mm thick. Outdoor steel bars (spanish bars) are often between 12 & 14mm thick. Most bar types are hollow tubes, though more expensive varieties can be purchased (and are often done so by factories, large industrial buildings, etc.) that are solid.

Pros: Cutting with a hacksaw can be a noisy method of obtaining entry. Steel is a highly visible deterrent. It is not susceptable to certain types of burning.

Cons: Cutting may be done at a single point, after which the bar can be permanently bent out of place to allow a wide opening. Thinner bars are easily bent apart by hand & remain in that position, allowing a wide opening. Time taken, depending on the strength of the intruder can take a few minutes or longer. Steel bars are often spaced over the 120mm safety standard, thus allowing wider gaps for entry. Outdoor installations are highly susceptable to tampering.

Polycarbonate bars come at thicknesses of 5 – 6mm and are 30mm wide. They are spaced for safety at 120mm apart, or narrower.

Pros: Apart from aesthetics, polycarbonate bars are highly resistant to high levels of impact. Higher flexibility allows for greater impact resistance. Bars cannot be permanently bent out of shape, and cutting into them at a single point will cause bars to constantly flap back into place (sawing a large piece out of the bar will prevent this). Bars are installed on inside, limiting opportunities for tampering. Bars are installed individually, forcing intruders to remove them one at a time.

Cons: Sawing into material is slightly quieter than with other bars. Butane lighters can be used to weaken bars at a single point. Less of a visual deterrent.

Aluminium tubing can be installed similarly to polycarbonate bars (riveted/screwed individually) or part of a larger frame with fewer installation points. Tubes are hollow & come in thicknessess of 9-12mm squared.

Pros: Slightly more noisy to cut with a hacksaw. Installed on inner side of window, allowing less opportunity for tampering. Visible deterrent. Not easily burned with a standard butane lighter.

Cons: Faster to cut through than polycarbonate or steel. Bars are set at wider distances apart than polycarbonate, allowing for entry without bending or cutting. Once cut, can be permanently bent to allow a wide entry space. Can be bent out of place by hand. Easier to kick/beat out of place than other burglar bars.


All burglar bars can be burned with a simple blowtorch, but polycarbonate is more susceptible to burning with butane lighters. While this may seem alarming because they are easy to come by, we will list the benefits & hindrances to using this procedure to gain entry to a property.

Please note that, as with every type of burglar bar used, a whole & comprehensive system of burglar preventives is required. Alarms, perimeter security & personal vigilence is required. Any type of burglar bar can be penetrated, given enough time & freedom. Burglar bars are always your last barrier to entry & your personal security is reliant on a network of security measures working together to protect your home & family.

If your burglar bars are installed on the inside of your windows, your windows are kept closed while you are out of the house (and not able to hear or smell attempts to break through them), then your burglar bars are safer from attempts to tamper with them.

Now, let’s address the concerns about access using burning as a method of getting through polycarbonate burglar bars.

The procedure for & result of burning polycarbonate burglar bars with a standard butane lighter is as follows:

Intruder holds lighter flame at its hottest point directly onto single bar for some time to stimulate melting point. Polycarbonate begins to burn & melt but immediately cools & hardens, turning black & hard – thus a hole cannot be made in the bar, but the bar will be rendered more fragile than usual at the place it was burned.

The intruder will need to continually hold the flame on the individual bar for many minutes in order for a sufficient weakening to occur. If an uninstalled bar is then severely twisted, it will break at this weakened point. An installed bar, however, will be required to be beaten or kicked at this burned point, a noisy procedure requiring a great deal of effort.

Fire duration
Small amounts of burning plastic will come away from the bar during the melting process, although the flame is extinguished immediately after separation, as polycarbonate cannot support a burning flame. Black smoke & a strong odour are emitted during this time.

Access to bars
Bars installed at 120mm apart allow for little space in which an intruder can safely apply a flame to a bar without holding an exposed hand very close to & directly beneath the burning material.

Once the weakened bar is broken with force, the opening will allow an entry space of 270mm, and bars will constantly flap back into their original position owing to their flexibility.

All-in-all, intruders will be able to cause a small amount of damage to the bars with a flame, but will additionally require the standard use of force & sufficient time & privacy to do so. A more efficient method of burning through the bars would be to use a blowtorch – a procedure which can be used on any type of burglar bar.

Any Questions?

At Thorne – Trusted Barrier Systems, our commitment to security is not lipservice. We are dedicated to creating safer, more comfortable living environments that are sustainable, affordable & easy-to-maintain. If you have any questions about burglar bars or other security systems, visit our blog or Facebook page. Or drop us a line on


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