I have a 31 year old house that leaks around the Bilco door and from the top of some of the walls when it rains hard. I know that I have to keep the gutters clear to prevent this but it doesn't always help. Around 20 years ago we had a bad problem and had additional waterproofing added on the outside walls. I am going to call in some experts but don't want to do more than necessary as I have my house for sale. What would you recommend?