The Sweat Equity column in the San Francisco Chronicle today steered readers to a UC PowerPoint presentation on the Web that provides details on fire-resistance ratings for roofs.

Produced by UC Cooperative Extension wood durability advisor Steven Quarles, the 18-slide presentation gives viewers an understanding of how roof coverings get their fire rating.

The information in Sweat Equity, a regular home improvement column written by brothers Bill and Kevin Burnett, was prompted by a reader question about whether an existing shake roof can be treated for improved fire resistance. In short, the answer is no.

Roofing systems are fire-rated either A, B, C or not ratable, with A the highest rating, or most fire resistant, according to the article.

If homeowners wish to preserve the woodsy and casual appearance of a wood shake roof, the Burnetts said the existing shakes should be replaced with new fire-retardant pressure-treated wood shakes. Alone, the pressure-treated shakes result in a class-B roof covering; in conjunction with an underlying fire barrier material, such as this gypsum or fiberglass, the roof is class A.