All operational grease extract ductwork systems will require cleaning annually as a minimum unless a fire risk assessment recommends otherwise.


The following factors have an impact on the frequency of cleaning:

  • Type of usage – The type of cooking and volume of catering clearly impact on the level of grease accumulations within an extract ductwork system.
  • Volume of catering – The number of covers catered for within the size of the catering facility impacts on the volume of grease particulates.
  • Type of cooking – The levels of grease that are produced will depend on the type of cooking carried out, for example kitchens that produce high levels of fried or chargrilled food will produce much higher grease levels that those using less intensive cooking methods such as baking and boiling.

Pre-cleaning micron readings are taken to enable cleaning frequency to be confirmed as suitable or adjusted accordingly. To verify frequency of cleaning based on micron readings it is recommended that inspections be carried out at regular intervals. The table below sets out guideline maximum levels of grease deposit and actions recommended.


Surface Grease Deposit Limits


Surface Grease Deposit Limits
WFTT Measurement Recommended Action
200µm as a mean across the system Complete Cleaning Required
Any single measurement about 500µm Urgent Localised Cleaning Required
Typical cleaning intervals as Stipulated by TR/19 (Second Edition, 2013)
Cleaning Intervals (months) Daily Usage
Perceived level of grease production Typical Example Daily Usage: Up to 6 hours Daily Usage: 6-12 hours Daily Usage: 12-16 hours Daily Usage: 16+ hours
Low No significant production of grease laden aerosols during normal daily food production operations 12 12 6 6
Medium Moderate production of grease laden aerosols during normal daily food production operations 12 6 4 3
High Heavy, significant or continual production of grease laden aerosols during normal daily food production operations 6 3 3 2

Insurers may not always pay a claim if the duct system was found to be the cause of the fire spreading, especially if it could be demonstrated that the system was not properly cleaned in accordance with their policy conditions or warranties.