Before revision: 250px|Firewall residential construction, separating the building into two separate residential units, and fire areas Firewall, as seen on the inside of a newly constructed townhouse unit, showing how the firewall extends above the roofline Example of a firewall used to inhibit the spread of a fire at an electrical substation A firewall is a fire-resistant barrier used to prevent the spread of fire.
After revision: 250px|Firewall residential construction, separating the building into two separate residential units, and fire areas Example of a firewall used to inhibit the spread of a fire at an electrical substation A firewall is a fire-resistant barrier used to prevent the spread of fire.