Freeze-Thaw Damage

Freeze-thaw damage is the gradual breakdown of porous materials when water that has soaked into them freezes, thaws and freezes again. Clay and concrete roof tiles, natural stone and slate, masonry chimneys, mortar and some roof coatings are all vulnerable in climates where temperatures cross the freezing point many times each winter.

Mechanisms

Water expands by about 9 percent when it freezes. If a pore is saturated and freezes from all sides, that expansion creates pressure that can crack the material. Research on frost weathering suggests that a related process called ice segregation is often more important: water is drawn through tiny capillaries toward growing ice lenses, which gradually pry the material apart. Ice growth can also pressurize unfrozen water in pores, a process called hydrofracture.

What it looks like

• Spalling: flakes or layers breaking off the surface of tiles, slate or brick.
• Cracking of tiles and mortar bedding at ridges and hips.
• Delamination of slate along its natural layers.
• Crumbling chimney masonry and mortar joints.
• Cracking of asphalt and pavement-like surfaces where water enters existing cracks.

Material resistance

Water absorption is a key indicator. Clay tiles with low porosity absorb little water; some manufacturers achieve moisture absorption below 2 percent, while concrete tiles average much higher absorption and are commonly resealed every few years. Standards specify freeze-thaw performance for roof tiles: ASTM C1167 in North America defines three grades of clay roof tile with differing resistance to weathering, and other regions have equivalent European and national standards. Choosing the grade suited to the local climate is important where winters are wet and cold.

Contributing conditions

• Prolonged wetness, such as from blocked gutters, shaded north-facing slopes, moss and lichen;
• Ponded water on low-slope roofs, which can freeze and break up the surface;
• Ice dams that keep eaves saturated;
• Poor ventilation beneath tiles and slates.

Prevention

Specifying frost-resistant materials, keeping roofs draining freely, removing moss, maintaining mortar and flashings, and ensuring adequate ventilation under tiles all reduce freeze-thaw damage.