I've been an active volunteer for a non profit and can say that it simply could not function without some full time staff.  At an absolute minimum you  need someone to answer the phone,  coordinate the volunteers, order supplies and pay the bills.  If you are building houses this is like being a general contractor.  You can see the 990 for this branch of Habitat for Humanity at Charity Navigator. The highest paid employee is listed on these forms. That person make a little over $60,000 a year.