Is it safe to retread old tyres?

Hello Paul, I have observed some people who cut treads with knives onto completely worn out tyres while they are on trucks. Can such tyres work safely for these cargo trucks? Birungi

Hello Birungi, retreading worn out or bald tyres by cutting shallow treads into them with a knife is wrong and endangers the lives of people travelling in such vehicles and other motorists. One must appreciate the role of tyre treads to understand the gravity of this vice. 
Tyre treads are an important part of the outer vehicle tyre circumference designed in patterns according to the purpose of the tyre. Treads give the tyres better control on and off road, especially when it is wet or slippery and when you need to maneuver or stop the vehicle. 
Good tyre treads require depth in order to provide grip and control on slippery terrain or evacuate water to prevent aquaplaning (skidding on water patches) when driving over wet road sections. Deep tyre treads will allow cooling of the tyres to avoid overheating and damage of the inflated inner tyre sections as you drive along. 
Tyres have different layers of rubber (plies) which protect them as well as improve their performance. The outer most layer is the one with the treads. Companies that correctly retread used tyres clean them up and replace the worn out outer layer with a new layer comprising effective and deep treads. 
The process uses machines to bake or vulcanise the tyres and make sure the retreaded outer layer will not fail or peel off when driving.