Dark Shading On Roofs

Here is a great article written by our friends at Pacific Roof Cleaning

“Stud striping” or “structure striping” is a visible pattern that can be seen on building exteriors, typically on the north side and other shaded areas. This phenomena is a common site on building exteriors, particularly in areas where there is consistent moisture due to humidity, coastal marine layers, low lying valley fog, etc.

The dark shading is due to the proliferation of the cyano bacteria, gloeocapsa magma (GM). The pattern affect is the result of the direct contact transfer of heat from structural members such as studs, plates, headers, etc. The bacteria requires moisture to grow and flourish. The transfer of heat from framing dries the exterior surface quicker than areas where there are open or insulated bays. The result over time is a pattern, clearly showing the location of structural members below the exterior surface.

Structural striping is not limited to walls. It can also be evident on composition roofs, with rafters being the heat transferring members. While GM has infested all roof types, a pattern due to the bacteria can typically be found at unheated areas such as eaves and overhangs. This perimeter roof shading is particularly obvious when the attic space has not been well insulated and allows heat from the home into the attic area.

In general, wood and tile roofs do not show a patterned growth of GM. This is because wood shingle and shake roofs have moderate insulating properties. Properly installed tile roofs create a natural thermal break and generally do not come into direct contact with heat transferring structural members.
Darker colors tend to hide the bacteria while lighter colors make the shading more obvious. As the bacteria develops and darkens with time, structural framing, heated and non-heated areas and even areas with irregular insulation become clearly visible. Solid building materials such as brick and mortar are equally affected by GM but there is no pattern due to internal heat transfer.
Commercial surfaces accentuate structural striping

Smaller residential homes with many windows, doors and comparatively smaller surface areas tend to obscure the pattern effect. On the other hand, commercial buildings with large monotone surfaces accentuate the effects of structural striping. Typically, commercial building walls show the consistent stud pattern that results from both metal and wood studs.

The best way to remove stud striping

The safest and most effective method to remove the bacteria that causes stud striping is using a chemical “soft wash” cleaning. This is true for at least two reasons. The first reason is that soft wash cleaning eradicates the bacteria that is the source of the problem. While using high pressure may remove the visible bacteria from the surface, it does not kill it. The great results from the soft wash method last much longer than cleaning with pressure. Another reason for using the soft wash method is that it causes no damage. Many of the exterior surfaces where stud striping is evident can be damaged from the use of high pressure. Chemical cleaning uses no more pressure than a garden hose; yet the results are fast, thorough and effective.