Four fire engines and around 25 firefighters were called to a fire at a pub under refurbishment in Longwood Gardens, Ilford.
Hainault Watch Manager James Morley, who was at the scene, said: "A member of the public saw flames and smoke and immediately called 999.
"The pub was under refurbishment so thankfully it was empty at the time. We believe that building work earlier in the day had heated the timber frame of a roof extension, which later smouldered and ignited felt."
Around half of a ground floor extension was damaged by fire.
The Brigade was called at 2030 and the fire was under control at 2109. Fire crews from Hainault, Woodford, and Ilford fire stations attended the scene.