What cutter is used for fitting intumescent strip?

Firstly, what is an intumescent strip?

Intumescent strip is a length of plastic extrusion that contains a core of intumescent material. It’s designed to be fitted around a doorframe or to the top and sides of the door itself. In the event of a fire or extreme heat, the intumescent strip expands to close any gaps around the door. This helps prevent the spread of flames from one room to another. Additional features along the length of strip, such as ‘brush’ or ‘fins’, also help prevent the spread of smoke even before any flame is present.

Preventing the spread of flames and smoke in a building fire, helps to save lives. It is estimated that there are approximately 37,000 house fires each year in the UK. The majority of fire-related deaths are caused by the inhalation of smoke and toxic gases produced by the fire, rather than the fire itself. Improvements in regulation standards over the years have helped slow down the spread of flames and smoke during a fire, and significantly reduce the amount of fatalities each year in the UK.

When installing brand new doors in a refurbishment property or new build, it is not uncommon for ‘full doorsets’ to turn up on site. These complete sets feature a door mounted in the frame with hinges and intumescent strips already installed. However, there are many situations where a door has to be trimmed to fit into an existing frame, so the intumescent strip needs to be refitted on-site. Alternatively, existing doors often need to be retro-fitted with fire and smoke seals to conform to the latest regulation standards. It’s these situations that you will need to reach for your router or trimmer and cut the correct width of slot, so the intumescent strip can be fitted.

Intumescent strip is manufactured in various widths and usually comes with a self-adhesive backing, which readily attaches to common frame and door materials. Typically, you would use 10mm or 15mm wide strips for a 30 minute fire door and 20mm wide strip for a 60 minute fire door. However, it is highly recommended that you check the door manufacturers specifications, as the correct width of strip used is a critical factor in whether the door is compliant to the appropriate building regulation standards or not.

So; what is the best and easiest method of cutting slots for intumescent strip? There are a two popular ways, both requiring a router or trimmer along with the correct type of cutter.

The traditional method was to select a straight plunge-cut router cutter with a width to match the strip. The next stage of the process was to adjust the depth setting on the router to plunge in just deep enough to accommodate the seal. This method is certainly effective but can be ‘fiddly’ to set up and often difficult to operate accurately.

The alternative and more effective method is to select a grooving cutter, designed specifically for routing a slot or channel – perfect for intumescent strip. This type of cutter requires the body of the router or trimmer to be held at 90 degrees to the frame or door. Available in popular widths to suit corresponding widths of intumescent strip, each cutter is manufactured to be dimensionally accurate for the strip to fit perfectly. The bearing guide on the cutter makes operation effortless and it is automatically set to a 4mm depth of cut; which suits most brands of standard intumescent strips on the market today. The Maxitek intumescent strip router cutters are available in widths of 10mm and 15mm. If a 20mm groove is required, simply make a cut using the 15mm cutter and then set the depth 5mm lower to complete the 20mm width of cut.

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