Constructed in 1936, Hansee Hall is the longest continuously active residential building, also one of the largest residential structures (111,364 square feet) on the University of Washington Seattle campus. Designed in the collegiate gothic style by John Graham and David Myers, with supervising architects of Bebb & Gould, it is a cast-in-place concrete structure with a multi-toned brick veneer, multi-light steel windows, and Vermont slate roof.
Overall appearance is a Commercial Standard slate roof approximately 3/16” thick laid in a Dutch Lap pattern. Hip slates are smaller slates set in a Comb Ridge method laid with butt joints and roof dormers with fantailed hip detail. Open valleys are copper. Copper slating nails in two sizes, 1 1/2” at the field and 3 1/4” at the ridge were used. Slate courses consist of various sizes ranging in widths from 9 inches to 20 inches and consistent lengths of 20 inches. Tile colors range from grey to green yielding upon weathering to a red tone, as well as some purple accent slate. The dormer walls and roof are clad in slate. All copper sheets used for flashing, window surrounds, profiles, gutter-linings, etc., were crimped by passing through heavy rolls to form 3/16-inch, V-shaped corrugations or ridges in the sheets in the direction of the short dimension following standard specifications of the period.
Scope
In 2023, interior plaster walls and plaster ceilings were exhibiting signs of water intrusion and the University of Washington was concerned the water migration was due to the 86-year old slate roof. Peter Meijer Architect, PC (PMA) was retained review previous building envelope reports, to evaluate the existing conditions of the roof, recommend a path to mitigate the water intrusion, and to provide a professional opinion on the “remaining life expectancy” of the existing slate roof system.
PMA approached the process from both a research perspective and an as-built investigation perspective. Our first step was to commission a laser scan point cloud and photogrammetry record of the entire roof design, surfaces, and exposed conditions. Combing the laser scan and photogrammetry precision eliminated the need to access the entire slate surface, as well as, provided documentation of all visible deficiencies with a accuracy of 1/8”. These tools allow for more methodical review of the roof surface within an office environment and provide a higher degree of accuracy for capturing deficiencies in difficult access areas.
After the digital captures, PMA guided the selective removal, with the assistance of a premier slate roof contractor, of slate units in specific locations corresponding with different environmental exposures, different locations within the field, along the ridge, and adjacent to open valleys for further assessment of the installation, under layment conditions, and laboratory testing. The National Slate Association, Technical Bulletin #7, Slate Roof Condition Assessment Surveys was referenced against PMA experience and standard slate roof assessment methodology.
Initial field observations indicated that the comb ridge slates were orignally installed incorrectly without an overlap, without elastic sealant, and installed with a ½” wide gap the length of the ridge exposing the wood structural deck. From the ground, the gable ends of the roof appeared to have decayed material extruding from beneath the slate tile field. Upon closer observation, it was noted that a ¾” thick slate damp proof course is set in mortar on top of the masonry veneer walls. There is no gable end edge flashing thus exposing the bedding layers of the damp proof course to weather and causing the cleavage planes to expand, shear, and fail. Visually, the green slate that has changed tone to red appear to be more weathered than the other slate colors. In readily accessible roof areas, the slate roof field had numerous incorrect repairs including face nailing, the use of galvanized nails, and broken underlying slate courses, evidence of traffic on the slate without adequate protection. However, the only deficiency aligning with interior water intrusion were the open ridges at cross gables.
PMA sent slate samples to several labs to conduct standard ASTM C 406 Standard Specification for Roofing Slate grading including C 120 Test Methods of Flexure Testing of Slate, C 121 Test Method for Water Absorption, and C217 Test Method for Weather Resistance of Slate. Petrographic examination was also conducted specifically looking for Calcite content and the presence of Carbonaceous ribbons.
Evaluation Process
As roof system observations and assessments were performed, and a review of the photogrammetry and laser scan were completed, it became apparent that the source of interior water was not resulting from an aged slate roof. Except for very specific locations, the slate roof was in good condition. Field tile failures were less than 4%. (Slate roofing industry standards recommend replacement when failure rates are approximately 20%) Original copper flashing systems were sound with no pin holes or galvanic corrosion noted. Felt underlayment was in place, continuous, not broken, but very brittle due to age. (PMA did not test the underlayment for asbestos content (a common component of early under layments) as hazardous material content was not a contributing factor of water infiltration. The construction means and methods conformed closely to the original design documents allowing PMA to evaluate original design details for water tightness.
As a result of the field assessment preliminary findings, UW raised the question of ‘what is the remaining service life of the existing slate roof system.’ Would the system last another 20 – 25 years (the equivalent time of a typical new (non-slate) roof system warranty). To better evaluate future life expectancy, PMA augmented the on-site assessment with laboratory testing. ASTM C406 establishes the “grade” of new roofing slate resulting from ASTM C120 & ASTM C121 testing methods. The grade ranges are: Grade 1 = 75yr +; Grade 2 = 40 – 75 yrs; Grade 3 = 20 – 40 yrs. Color fastness grades are categorized into: Unfading / semi-weathering / weathering. Color fastness does not correlate to longevity of the slate unit. The United States slate industry has no empirical measurements for aged slate roof service life evaluation.
Testing
Due to a lack of knowledge of slate materials, lack of familiarity with slate construction techniques, and inadequate assessment approach, field observations of archaic / historic slate are often misinterpreted as deficiencies. PMA demonstrated through research and technical analysis that the existing slate roof had substantial life and only targeted repairs were needed. This information saved millions of dollars and allowed limited resources to be placed elsewhere on the UW campus.
Slate roofs have been documented to last for 1,000 years. The oldest extant slate roof is the Saxony chapel in Bradford-on-Avon. Slate roofs originating from the same quarry as UW Hansee Hall are documented to be over 200 years old. Therefore, in theory, Hansee Hall slate has a potential life expectancy of another 100 years. A critical aspect of determining the life expectancy of archaic/older material, is understanding of the limitations associated with modern ASTM test methods. ASTM standards are not always accurate prediction of slate quality. The ASTM tests are designed for new, recently quarried slate. PMA used our experience and broadened our research into longevity by including testing means and methods from Europe and other countries outside the United States with longer years of slate testing. The standards in use on the market to certify slate are the European EN 12326 (parts 1 and 2), and the American ASTM C406, C120, C121 and C217. Further compounding the varying results from each of the testing standards is that some studies have shown there is a low correlation between this expected service life and the laboratory test results.
Water Absorption testing is the only common test approach to all the test standards. As slate ages, the interstitial water within the stone layers evaporates increasing the ability of the slate to absorb more water without damage. Water Absorption tests remain one of the essential tests for determining longevity in the field. Heavy reliance on Water Absorption tests results from the widely accepted fact that Freeze /Thaw test results have little to no impact on slate roof lifespan.
The diverse experience of PMA staff, including conservators, architects, preservation planners, and designers, coupled with our 20 year expertise of state and local building codes, historic construction means and methods, and familiarity with architectural styles and eras, gave the University of Washington a building envelope firm able to translate the value of the existing historic resource into economic and longevity terms.
Blog post written by PMA Founder and principal, Peter Meijer.