Where I live the builder had a defect where he installed ZC heating boilers in the living space and left a 2x4 too close to the flue exit of the boiler. Even though it was under drywall, after 30 years the houses started burning down, one by one.
After several fires, the fire marshall determined the cause. What was happening over that 30 year period was that piece of wood was very gradually drying out at the point of contact and becoming more and more scorched to the point that eventually it caught on fire silently inside the walls of the house, all the houses starting from the exact same location. Although my house never caught fire, when we tore out the heater to relocate it, you could see the scorch marks beginning on the 2x4.
Of course with an exposed mantel that's not hidden inside a wall and you'd be more likely to see it happen and take action before it caught on fire. But maybe not.
After several fires, the fire marshall determined the cause. What was happening over that 30 year period was that piece of wood was very gradually drying out at the point of contact and becoming more and more scorched to the point that eventually it caught on fire silently inside the walls of the house, all the houses starting from the exact same location. Although my house never caught fire, when we tore out the heater to relocate it, you could see the scorch marks beginning on the 2x4.
Of course with an exposed mantel that's not hidden inside a wall and you'd be more likely to see it happen and take action before it caught on fire. But maybe not.
