Slate roofing is prized for its durability and classic appearance, but like any natural material, it can attract algae and moss over time. These growths are typically caused by moisture retention and shaded areas where sunlight can’t quickly dry the roof. Algae often appears as dark streaks, while moss is thicker and green, creating a fuzzy layer. Both can trap moisture against the slate, potentially weakening the surface and causing slow water damage if left untreated. To diagnose the issue yourself, do a safe visual check from the ground or a secure spot, looking for discoloration or green patches. Avoid walking directly on loose or fragile slate as it can crack easily. Early detection is key to preventing damage and preserving the life and beauty of your roof.
Simple cleaning is often possible with a gentle rinse or a soft brush paired with a cautiously chosen cleaning product specifically formulated for slate to avoid damage. Avoid harsh chemicals or power washing, which can erode the material or break seals. If growth is persistent, or you notice cracked or loose slates, it’s wise to call a roofer experienced with slate roofing. When speaking with a contractor, inquire about their approach to treating algae and moss on slate and confirm if their method complies with your roofing warranty. Asking, “Do you use slate-safe cleaning treatments?” and “How will you protect the integrity of the slates during cleaning?” can help ensure your roof receives proper care without risking its long-term durability.