Monday, November 3, 2014

When the time is right: Appropriate use of modern materials in a traditional roof system

In my capacity as an historic preservation contractor and consultant, I am often afforded the opportunity to become involved in exciting and challenging projects.  Recently, we were awarded the contract to restore the clay tile roof turrets at the Longy School of Music’s Zabriskie House.  Now a part of Bard College, the Zabriskie House is actually the historic Edwin H. Abbot House with a sympathetically designed addition built in 1969-70.  The deteriorated condition of the turrets, as well as lead-coated copper gutter linings and masonry dormers, had attracted the attention of the Cambridge Historic Commission and a commitment to the proper restoration of these systems was struck between the CHC, building owner and a private donor.

Before I can specify historically appropriate treatments, I need to don my consultant’s cap and dig into the history of a building to best understand its evolution.  Developing the background story will typically answer questions and fill in the blanks when examining traditional building systems.  An 1890 newspaper clipping held by the CHC reports that “[t]he stately home of Mr. Abbot, with its walled-in grounds, on the site of the old Arsenal, promises to be the most costly private dwelling in the city.”  An examination of records held by the Massachusetts Historical Commission and from the Library of Congress’ Historic American Building Survey reveal that the firm of Longfellow, Alden and Harlow designed the Richardsonian Romanesque portion of the building and that Norcross Brothers was the builder of record.

Alexander Wadsworth Longfellow, Jr., was the nephew of the famous poet and an important figure in the architectural history of the United States. After graduating from Harvard in 1876, he studied architecture at the Massachusetts Institute of Technology and the École des Beaux-Arts in Paris, after which he worked as a senior draftsman in Henry Hobson Richardson's office.  After Richardson's death in 1886, Longfellow partnered with Frank Ellis Alden and Alfred Branch Harlow to found the firm of Longfellow, Alden & Harlow.  With offices in Boston and Pittsburgh, the firm designed many important buildings including the Carnegie Library of Pittsburgh and the City Hall in Cambridge, Massachusetts.

Norcross Brothers Contractors and Builders was a prominent nineteenth-century American construction company, especially noted for their work, mostly in stone, for the architectural firms of H.H. Richardson and McKim, Mead & White.  Following the death of Richardson, the brothers became the contractor for many of McKim, Mead & White's projects.  As had been the case with Richardson, much of the value of the Norcross Brothers to architectural firms derived from Orlando Norcross's engineering skill. Though largely self-taught, he had developed the skills needed to solve the vast engineering problems brought to him by his clients. For example, the size of the dome at the Rhode Island Capitol was expanded very late in the design process, perhaps even after construction had begun, so that it would be larger than the one just completed by Cass Gilbert for the Minnesota Capitol.

The Edwin Abbot House is an interesting interpretation of the Richardsonian Romanesque style.  Whereas the great majority of such buildings feature rusticated, pink Milford granite in an ashlar pattern, trimmed with East Longmeadow brownstone, Longfellow created a unique spin for Mr. Abbot.  While trimmed with the brownstone, the field of the walls features coursed Weymouth granite of slightly varying heights.  The motif of orange, brown and golden hues of the stone is continued in the brick wall surrounding the property.  The roof is covered in a flat, square orange-red clay tile.  This is typical of Richardsonian Romanesque buildings which are almost exclusively roofed in clay tile, Monson black slate, Granville, New York, red slate, or some combination thereof.  It should be noted that because their need for stone was outpacing the supply, Norcross Brothers eventually acquired their own quarries in Connecticut, Maine, Massachusetts, New York, and Georgia.

The roof framing system of steel and terra cotta blocks is relatively rare but makes perfect sense when considered in context with what the latest flooring technologies of the era were.  A network of steel beams was bolted together to form the rafters, hips and ridges of the frame. Across each was welded rows of double angle irons (or, inverted T beams.)  Within these channels, in beds of Portland cement, was laid the terra cotta block.  The tile were then fastened directly to the blocks with steel nails.  Because of the ferrous nature of the fasteners, the normal passage of moisture vapor caused the nails to rust and expand slightly, anchoring them securely in place.  Whether this element of the design was intentional or simply fortunate happenstance, the result made for a long-lasting roof.

What doesn’t last forever in traditional roofing systems like slate and clay tile is the sheet metal flashing assemblies.  Over the years there must have been numerous failures which lead to the decision to remove the clay tile from the broad fields of the roof and replace them with red asphalt shingles in the 1980’s.  Confronted with the dilemma of securing the new shingles to the terra cotta substrate, a decision was made to sheath the roof with plywood.  Holes were punched through the blocks and toggles used to fasten it to the roof.  In one area where the asphalt shingles were removed, more than 50% of the plywood exhibited varying degrees of rot due to the normal passage of moisture vapor from the interior spaces.

Fortunately, the turrets had survived the renovations from thirty years before.  A conical turret, in the rear, and an eight-sided hip-roofed turret on the north sided required only repairs which, while extensive, did not require addressing issues with the substrate.  The sixteen-sided turret, on the primary façade of the building, was in poor condition.  Over the years, “repairs” included the use of non-matching tiles, red roofing cement, tar, caulk, and even red slate.  A scaffold was erected to allow safe, unfettered access to the entire turret and the process of removing the tile began.  Care was taken to conserve as many tiles as possible for use in repairing the other two turrets.

The substrate was examined closely and, save for thousands of tiny craters created by the original nails, found to be sound.  A new system for fastening the tiles had to be devised so that they could be attached to the terra cotta blocks. It was critical that the new method allow for the replacement tiles to be securely fastened and resist the damaging forces of escaping moisture vapor.  Cement board, comprised of 90% Portland cement and ground sand, was fastened to the blocks with ceramic-coated masonry screws.  The entire turret was then covered with a self-adhering membrane.  The replacement tiles were carefully matched and sourced from a salvage dealer in Illinois and secured with stainless steel fasteners.  The flat tiles, no longer manufactured new, are referred to as “Cambridge” tiles for their prevalence on the roofs of great homes and institutional buildings in and around Cambridge, Massachusetts.

While I typically advocate for the retainage of all historic fabric when preserving and restoring traditional building systems, there are exceptions.  In the case of the Abbot House roof, we encountered once-modern technologies which pointed us toward contemporary means and methods.  Rusting steel nails in the terra cotta block worked brilliantly for the initial installation but seemed ill-conceived for a second go around.  The use of non-ferrous fasteners and a new substrate that is impervious to moisture infiltration will guarantee the turret’s service life for the next 125 years or more.
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