THE CHALLENGE
CBI provided architectural and engineering design services for slate roof replacement, valley replacement, associated copper gutter work and painting at Boyden Hall. This historic structure built-in 1924, contained a predominantly sloped roof covered in green slate with red copper gutters and valleys. Green slate, popular at the time of installation, contains many impurities including iron and gypsum which create weak points because they expand after absorbing moisture. Over many years, water absorption into the slate causes the iron to rust and when iron rusts, it expands. This expansion causes delamination of the slate on the micro level. The same mechanism occurs with the gypsum. Each of these materials, when expanding, create pressure on the surrounding material causing delamination and eventual failure.
CBI first performed test cuts removing existing slate to view the existing conditions of the roof. We observed that the existing slate was installed over one layer of original rosin paper over an existing wood deck. While the existing wood decking appeared to be in satisfactory condition, it exhibited water stains indicating that water infiltration had occurred over the years, most likely due to ice damming. The slate was not in good condition. In order to determine the condition of the slate we tapped the slate, with our knuckles, in order to create a sound. If the sound is that of fine china, the slate is in good condition and of good quality. If the sound is a hollow sound, such as that of a “wet book” then the slate is of poor quality and/or in bad condition. The slate of Boyden Hall fell into the latter category and was in poor condition. We observed many slates that were missing, cracked, and delaminated. We noted dark stains indicating high concentrations of iron and white stains indicating high concentrations of gypsum. This all indicated that this slate was at the end of its useful lifespan and needed to be replaced
CBI’S SOLUTION
CBI recommended a complete slate roof replacement as none of the existing slate could be salvaged. However, due to the University’s budget, we designed a partial roof replacement to address the rear of the building first because it was in the poorest condition and most accessible from the main parking lot. We are now completing the roof replacement on the front of the building which requires careful planning and specific staging and contractor access specifications.
CBI recommended using an un-fading green slate to provide a higher quality slate that matches the original color, edge finish, and size of the existing slate. As part of a complete roof replacement, we also recommended that the existing patina red copper flashings be replaced with new zinc-coated copper flashing, the existing rosin paper underlayment be replaced with new 30# building felt, and that ice & water shield membranes be installed at all roof edges and penetrations. Zinc-coated copper, which has a silver metal finish, was installed in lieu of red copper because it has a longer life span than uncoated red copper and is equally appropriate for the historic building. The zinc coating is essentially a “sacrificial,” layer that will wear away over the course of twenty to thirty years due to acid rain and other atmospheric reactions. The copper beneath the zinc layer has a life span of about fifty years uncoated, so the flashings can last about 80 years. Copper ring nails were used for installing the slate, and sat within the nail hole so the slate was slightly loose and the head of the nail did not protrude above the face of the slate, or hold it down too tightly. Slate must not be fastened to tightly to the deck or against other shingles because it must be allowed ample space to expand and contract and air temperature and humidity fluctuates.
Existing patina red copper gutters were somewhat deteriorated, but were not leaking or structurally unsound. Replacing the existing gutters and gutter dogs with new copper as part of the metal work would have been very expensive because of their size and intricate connection with existing wood gutter trim and cornice. Instead, the gutters and dogs were coated with a liquid-applied waterproofing membrane that consisted of a base coat and gunmetal gray finish coat. We expect that this waterproofing membrane will extend the useful life of the existing gutter system by as much as thirty years.
