Roof washing, handled by roofers first

The dark streaks running down South Mississippi roofs are almost always algae — commonly Gloeocapsa magma — and it thrives in exactly what we have here: heat, humidity, shade, and long summers. It takes hold on the north- and shaded-facing slopes first, which is why one side of a roof can look years older than the other.

It matters beyond appearance. The organism feeds on limestone filler in asphalt shingles, holds moisture against the roof surface, and darkens the roof so it absorbs more heat. Moss and lichen are a further step: moss holds water like a sponge at the shingle edges, and lichen roots into the granule surface, so removing it badly does more damage than leaving it alone.

That's the whole reason we now offer this as a roofing service. A clean roof is a cosmetic result; a roof that's still intact afterward is a roofing outcome, and those two things are not automatic.

Not every home in South Mississippi wears asphalt. Standing seam and exposed-fastener metal, and the tile roofs found on some coastal and Mediterranean-style homes, collect the same staining in the same climate — but none of them react to cleaning the way a shingle roof does. Treating all three the same way is how finishes get ruined and water ends up somewhere it was never meant to go.

On metal, the concern is direction and detail rather than granules. Panels are built to shed water one way, and laps, seams, and fastener penetrations are designed around water running that direction; forcing water uphill across a lap, or straight into an exposed fastener and its washer, works against the way the roof was assembled. The factory finish is its own consideration — it's the layer carrying the corrosion and color performance, and it isn't something to scour.

Tile brings a third set of problems. The tiles themselves are durable, but the underlayment beneath them is what actually keeps water out, and a tile roof walked carelessly cracks pieces that then have to be matched and replaced. Growth in the channels and along the laps is common and it is removable — but the roof has to be accessed and staged by someone who knows what tile will and won't take.

• Metal: water is kept moving with the panel, not driven back against laps, seams, or fasteners.
• Metal: the factory finish is treated as the protective layer it is, not as a surface to scrub down.
• Tile: the working assumption is that the underlayment below — not the tile — is the waterproofing.
• Tile: breakage is a real cost, so access and staging get settled before anyone steps on the roof.
• Any covering: if the evaluation says the roof shouldn't be washed, that's the answer you get.

Regrowth is normal, and how fast it returns depends on the things around your roof more than on the roof itself. Shade is the biggest driver in our climate, followed by anything that keeps the surface damp — overhanging limbs, heavy pine straw, blocked gutters that back water onto the roof edge.

• Trim back limbs that shade or drop debris onto the roof, where it's safe to do so.
• Keep pine straw and leaf litter off the roof surface and out of the valleys.
• Keep gutters flowing so water leaves the roof edge instead of sitting at it.
• Address any attic ventilation problem — a hot, damp roof deck doesn't help.
• Have the roof looked at after major storms, when debris and damage arrive together.