Hi

we have recently been replacing existing fire doors to meet regs. We have been keeping to the 3mm gaps however, the doors are not self closing as a result of the smoke strips.

The strips we use are from Howdens - does anyone else have the same issue?

Perhaps there is another supplier out there that have softer bristles 

We also noticed that a previous contractor had cut the strips in deeper to  overcome this issue but we feel this is not the solution as it was clear the router bit used was not the ones specifically designed for intumescent strips not to mention how far away the intumescent was from the frame.

The face of intumescent strip should be flush with the surface of the door leaf edge or door frame. So making a deeper groove is not an option. 

To reduce friction as the door self-closes, you could use alternative smoke seal types such as those with a blade or you could use perimeter seals or seals fitted to the face of the rebate stop.