Four fire engines and around 25 firefighters were called to a flat fire on Westfield Gardens, Romford.
The roof and rear decking of a terraced house were completely destroyed by the blaze. Part of the ground floor and half of the first floor were also damaged. Firefighters used a 32m turntable ladder as an observation platform and used a thermal imaging camera to check for fire spread.
Four adults and one child left the building before the Brigade arrived. There are no reports of injuries.
Control Officers took the first of six calls at 1405 and mobilised crews from Ilford, Dagenham, Hainault and surrounding fire stations to the scene. The fire was extinguished by 1631.
The fire is believed to have been accidental and caused by refracted sunlight.
A London Fire Brigade spokesperson said: “Fires caused by refracted and reflected sunlight can happen all year round and are actually quite common.
"Our advice is to make sure that you keep mirrors, crystals, glass ornaments and other reflective items out of direct sunlight at all times.”