[MassHistPres] water and slate roofs

[Ward Hamilton at Olde Mohawk Historic Preservation]

One of the benefits of a traditional roofing system, like slate, is the passive ventilation the building enjoys. How the wood roof material, beneath, impacted this is apparently minimal if the slate is in good serviceable condition and the attic is dry.  

What are the renovations planned that are going to make the building "very air tight"? How will these changes impact the systems of the envelope and how your building breaths? Does this involve insulation? 

The slate is your roof covering, not the membrane your local builder is suggesting. Removing a roof covering to install a "water barrier" that will then be perforated by thousands of nails when the slate is reinstalled is as bad an idea as it sounds and an enormous waste of money.

Not every advance in building science is good for every building, and some are downright bad. If you make the roof air tight by spraying foam insulation between the rafters then condensation will occur regularly below the slate when the roof stops breathing. The moisture will rot the wood substrate, the fasteners and the slate. 

If you remove all the slate and apply a top of the line, W.R.Grace ice and water membrane to the roof deck then reinstall the slate the wood will be protected.  But the condensation issue will not be resolved and the additional moisture, no longer absorbed by the wood, will accelerate the deterioration of the slate and fasteners and destroy the roof system.

Please contact me directly if I can assist the Town in any way.


Ward Hamilton

Olde Mohawk Historic Preservation
877.622.8973      OldeMohawk.com
Slate Roofing   |   Masonry   |  Restoration

-------- Original message --------
From: Judy Markland <jmarkland at lmstrategies.com> 
Date:01/16/2014  3:24 PM  (GMT-05:00) 
To: MHC listserve <masshistpres at cs.umb.edu> 
Subject: [MassHistPres] water and slate roofs 

Greetings:
Whately has an 1841 Greek Revival town hall that it is hoping to 
rehabilitate with both town and CPA funds.  The roof on the two story 
building is slate and generally in good repair.  Although it has not had a 
complete inspection, the architects estimate that less than 15% needs 
repair.  There are no obvious signs of flaking or delamination.  The slate 
was installed directly over an earlier wood shingle roof, probably in 1871 
when the existing 1-story building was lifted to become the second story.

The consensus is that the roof should be retained and repaired.  However, 
the renovation will make the building very air tight.  A local builder has 
expressed concern that there is no water barrier below the slate, and that 
any water infiltrating will not be able to evaporate as it most likely does 
now, causing timbers and supports to rot.  He suggests removing the slate, 
adding a water barrier, and replacing the slates in good condition.

This will obviously be expensive.  Has anyone had experience with slate 
roofs in tight, energy efficient buildings and, if so, can you recommend a 
course of action?

Judy Markland
Whately Historical Commission


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