Six fire engines and around 40 firefighters were called to a fire at a hospital building on Brockley Hill in Stanmore.
Half of the roof of a mixed-use building was damaged by fire. There were no reports of any injuries.
Station Commander Chris Haggar said: “Crews worked hard in arduous conditions to bring the fire under control.
“Crews remained on scene throughout the afternoon damping down and assisting our Fire Investigation team.
“One of the Brigade’s 32-metre turntable ladders was used at the scene as a water tower to tackle flames from height.”
The Brigade’s Control Officers took the first call to the fire at 1536 and sent crews from Stanmore, Harrow, Hendon, Northolt and surrounding fire stations to the scene. The fire was under control by 1715.
The fire is believed to have been accidental and caused by hot works being carried out on the roof.