Six fire engines and around 40 firefighters were called to a fire at a flat on Bromyard Avenue in Acton.
Two flats, the hallway, balconies and part of the roof of the fifth floor were damaged. One woman left the affected flat and a further 29 people left the building before the Brigade arrived. There were no reports of any injuries.
The Brigade's 999 Control Officers took 28 calls to the fire.
The Brigade was called at 1148 and the fire was under control by 1318. Fire crews from Acton, Hammersmith, Chiswick, Ealing and surrounding fire stations were at the scene.
The Brigade's fire investigators believe the fire was caused by a chemical reaction in a container of roof sealant, which ignited and spread.