Roof and structure are the building components a landlord often stays responsible for under NN and some NNN leases.
Roof and structure are among the most expensive parts of a building, so who is responsible for them has a big effect on the landlord's risk. In an absolute NNN lease the tenant pays for them. In some NNN and NN leases the landlord retains these items, sometimes with limits or tenant reimbursement of costs over time. Buyers should review the building's age and condition, ask for roof warranties, and consider setting aside a reserve when the landlord has this responsibility.