CLAY TILE AGING
Clay tile aging rates are related to their manufacturing quality. Clay tile firing results in a process called vitrification. Vitrification causes clay to change at the molecular level into a glass-like material.
Highly-vitrified clay is less absorbent than poorly vitrified clay. Because absorbent tile holds more water, and holds it longer, these elevated moisture levels encourage various types of biological growth, including mold fungi, moss, lichen, and algae.
Old, poorly-vitrified clay tile will become brittle and fragile sooner than high-quality tiles and should not be walked (look for biological growth and broken tiles).
Many inspectors would assume that the darkened areas of the tiles in the photo below are mold, but in fact they are a glass applied during firing. This is a poorly-vitrified clay tile.
This is a lower quality tile in Colorado. Flakes have detached in a process called “spalling”.
The photo below shows spalling on a home in the high desert of California.
Spalling is caused by absorbed moisture freezing. The expansion of water to ice causes flakes to pop loose.
Old clay tile
Clay expands and contracts at a rate different from mortar. Eventually this will cause the bond between clay and mortar to fail and tiles will become displaced… another sign of old tile. In this photo not only are tiles cracked, broken and displaced, but mortar has crumbled and collected in the pan tiles that form water channels. This type of tile is called “pan and cover”.
WALKING CLAY TILE
Clay tile in decent condition can usually be walked. With high-profile tiles, again, step on the headlap. Flat interlocking clay tiles may be difficult to distinguish from flat interlocking concrete tiles and again, inspectors should step on headlaps. Flat, butting clay tiles are often lower quality and inspectors should only walk on these with extreme care or not at all, especially if they are old.
Headwall flashing should be installed at both the underlayment level and at the level of the tile surface (tile “face). The same is true for sidewall and penetration flashing. On this roof, no flashing is installed at the surface of the tile.
The plumbing stack vent is not completely covered by the lead.
OVERHANG at ROOF
Tile should overhang the roof edge enough to divert water off the roof.
Rakes should have rake tile
Rake tile is fastened with drywall screws. Corrosion–resistant fasteners should be used. No other fasteners should be visible.