It began just before 14:00 GMT at the property in Grove Lane on the outskirts of Elmswell, near Bury St Edmunds.
Progress at the scene has been "slow due to the weather conditions", Suffolk Fire and Rescue Service said.
Nine crews remained at the property into the evening, removing the thatch and damping down hot spots. At its peak 16 crews attended the blaze.
The cause of the fire is not yet known.
A fire has ripped through the first floor of a thatched house in a Suffolk village.