Damage caused to timber by the deathwatch beetle is thought to be the cause of the collapse of a window at a historic building.
The hall and inner court yard at Oxburgh Hall has been closed while experts investigate what caused a dormer window to collapse on August 18 at around 8pm.
The roof window fell into the courtyard below, but no-one was hurt.
Caroline Pons, assistant director of operations at the National Trust, said the hall had been closed for safety reasons.
“We need to understand the cause, survey the wider area for damage and make the building safe and water-tight once more. We hope to re-open as soon as we can.”
Members of the public can still visit the grounds and tea room, which remain open.
Two fire crews were called to the hall at Oxborough after the damage triggered the fire alarm.
Staff and members of the donor family who live on site were evacuated.
The spokesman said early investigations have revealed damage caused by deathwatch beetle to be the likely cause.
Full condition surveys of trust buildings are carried out by heritage experts every five years, the spokesman added.