Why does my consulting kit contain a tyre depth gauge?

Door fitment. If I can get 4.5mm between the base of your door and the floor, there’s enough room to fit and operate an under door tool. I can get my borescope in with around 5mm. The under door tool goes through this gap and allows me to pull on the handle on the other side of the door.

Why do you care? Fire regulations usually mean that certain types of locks on routes of egress need to open with just the handle; the example we’re all familiar with is a hotel room, where the interior handle will always unlock and open the door. It’s the same for compliance in commercial settings.

What this means is, in many cases, a locked door controlling access to a secure area can be opened from the inside just by pulling on the handle. The under door tool allows me to do this from the locked side.

The under door tool is quite long – a few feet. There are models that collapse down and so on, but they’re a bit of a pain to carry on a consulting job. So I carry the tyre depth gauge.

As for solutions for doors vulnerable to the UDT? Depends on the door, but they’re usually very cheap and easily installed by your own estates team.