Roof Sheathing Investigation

Making the final decisions on roofing specifications and product details can sometimes be difficult– what was done 30 years ago in good faith may be considered detrimental to the building today.  The decisions we make today will undoubtedly have an impact on future generations of work–whether it will be positive or negative is not always black and white and may not be apparent for years.  Therefore making informed decisions through research and education and maintaining a log of rationale is important when doing any building conservation project.  In making the final decisions on  products and implementations we need to not only follow our own preservation philosophy but we must also consider products that aid in the reduction of continual maintenance, be considered green or environmentally friendly, and not cause harm to the historic fabric of the building (interior and exterior) or the landscape.

For the re-roofing project at Beauport, the team has been discussing wood shingle underlayments– in particular, tar paper, felt, ventilation layers, and waterproof membranes.  The combinations can be endless, but not all products are  appropriate for museums.  Wood roofs are meant to breathe and are generally laid over a substrate that allows air to circulate behind them: skip sheathing — wood strips or battens nailed directly to the roof rafters.  How Beauport’s original roof was laid is a mystery as the entire roof was replaced in the 1940s with red asphalt shingles, but it does not appear that skip sheathing was installed.  Therefore, this application was not considered.       

The specifications for the roof replacement in the 1980s state that the wood shingles were to be placed directly over wood sheathing.  In general, the current roof is in fair to good condition–most leaks can be traced back to flashing details and chimneys.  It was decided to review the current sheathing conditions to determine which underlayments (if any) are required for the roof.   The team decided to investigate roofs that were built at different time periods—1908, 1912, 1917, and 1925.

Wood shingles were removed at the four investigation areas (as shown in the photo above).  The team was interested in learning the condition of the sheathing as well as if any underlayments were installed.  At all four sites, the sheathing was in great condition–three out of four did not include any underlayments at all with wood shingles directly laid onto the wood sheathing.  One investigation site did include felt paper along the eave, which may have been placed there by a worker that did not realize it was not a part of the project.  The wood sheathing was generally 1 x 6 boards laid horizontally and butted up against one another.  The later roof sheathing–1925– was tongue and groove boards. 




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