Care and Preservation of Slate Roofs
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I S S U E 4 / 2 0 0 8 V O L U M E 2 5 N U M B E R 4
Journal of architectural
technology published by
Hoffmann Architects, Inc.,
specialists in the rehabilitation
of building exteriors.
Care and Preservation of Slate Roofs
Deborah J. Costantini, AIA and Benjamin J. Robinson, Assoc. AIA
A ccustomed as we are to the
complicated roof systems of today’s
the country, from the humble country
cottage to the imposing cathedral,
construction, with waterproof mem- slate roofs have become an endan-
branes, insulation board, ballast, and gered species, not because they have
adhesives, it can be hard to imagine reached the end of their serviceable
that simple stone slabs nailed to a life, but rather due to a lack of infor-
plain wood roof deck could protect mation about their repair and upkeep.
against the elements for upwards of Because so many roofing professionals,
a century. But that is exactly what a while knowledgeable about contem-
slate roof does, provided it has been porary roofing materials, do not have
properly designed and maintained. Al- much experience with natural stone,
though they cover buildings through- slate roof owners are plagued with
out New England and dotted around a barrage of misinformation—and
incorrectly performed repairs. Walk-
ing on the roof during installation or
selecting the wrong type of fasten-
ers for replacement slates could lead
to cracked stone pieces tumbling off
the roof: dangerous, unsightly, and a
source of leaks. Even worse is the
slate roof owner who dutifully invests
in a full roof replacement, only to learn
that the original roof still had fifty-plus
years of useful life, and that the new
roof, incorrectly installed, is beginning
to crumble after just a few short years.
So when is it necessary to replace a slate
roof? And what maintenance is needed
to keep the roof in good condition? A
variety of new products are on the mar-
ket, from synthetic slate to photovoltaic
shingles. When would these be appropri-
With regular maintenance and appropriate, periodic repairs, a well-designed slate roof can last
ate technologies to consider, and how do
upwards of a century.
they integrate with traditional roofing?
As a Project Architect with Hoffmann Architects, Inc., Deborah J. Costantini, AIA has specialized expertise in the
design and maintenance of slate roofs. Project Coordinator Benjamin J. Robinson, Assoc. AIA develops design
solutions for slate rehabilitation that apply new technologies to the historic roof restoration process.J O U R N A L
Broken slates Damaged flashing Loose gutters
While slate rehabilitation can raise of problem areas, is essential to a slate fractures may not create observable
many questions, caring for a historic roof ’s longevity. problems until months or even years
roof need not be a painful process. down the road, when the slate begins
When should major investigations occur?
After all, the material is durable, and to break apart, permits water entry,
Every 5 to 7 years, a more thorough
the construction elegantly simple. The and eventually slides off the roof.
inspection by a qualified roofing
solution lies in knowing what to look Who should perform the survey?
professional should be conducted to
for, and how to approach any prob- Twice-yearly seasonal inspections for
identify deteriorated conditions. Wait-
lems that do arise. general conditions can be performed
ing until problems are so obvious and
pervasive that they can no longer be by building maintenance personnel,
How and When to Inspect a Slate ignored only leads to more costly re- but more comprehensive roof system
Roof habilitation, with the added likelihood investigations should be conducted by
Why inspect the roof? Adequate of interior and structural damage. A an architect or roofing design profes-
drainage is a critical component of a detailed, routine investigation can help sional experienced with slate rehabili-
properly functioning slate roof system. to identify minor deterioration and tation.
For this reason, slate roofs typically maintenance issues which can then What needs to be inspected? Assess the
have a high slope, so that water runs be addressed promptly, before they condition and quality of installation for
off as quickly as possible. When leaves become serious problems. each component of the slate roof system:
and other waste are allowed to ac- How should the inspection be conducted? • Slate. Document cracked, broken,
cumulate on the roof or in gutters, Such surveys should be completed misaligned, delaminated, or missing
water becomes trapped, creating a initially from the ground, with the use slates. “Sounding” a slate by tapping
potential source for leaks. To prevent of binoculars, to assess the overall it with knuckles or a slating hammer
this problem, simple gutter cleaning condition of the roof. However, can help determine its integrity; a
and debris removal in the spring and because some defects are only evident
dull thud can indicate poor condi-
fall, along with inspections for signs on closer inspection, use of a ladder
tion, whereas a full, deep sound
of damage to the drainage or snow or lift to access all roof areas is an
generally means the slate is intact.
guard systems, should be performed important part of a thorough inves-
twice a year. Basic roofing steward- tigation. Avoid walking on the roof! • Flashing. A slate roof is only as
ship, including the early identification Slate cracks easily, and some hairline good as the weakest flashing detail.
Deteriorated roof deck Missing fasteners Bent snow guards
2VOLUME 25 NUMBER 4
Slates may last centuries, but flashing
must be maintained and replaced Synthetic Slate
periodically. Look for loose and
missing fasteners, and flashing that Shingle Roofing:
exhibits deformation, wearing, cor-
rosion, or pin holes. Open seams
When Is It a Viable Option?
and missing flashing members are Why synthetic slate?
also common sources of water Because natural slate roofs can last up to 100 years or more, property owners
entry. Areas to investigate include: are often surprised when roof replacement is recommended. Rehabilitation
ridge, hip, valley, roof-to-wall, drip of slate roofs can be both expensive and time intensive, requiring specialized
edge, and step flashing. craftsmen (slaters), tools, and accessories. Natural slates often break during
• Drainage system. Deterioration of shipment, such that additional quantities must be purchased. Plus, material lead
copper gutters and drainage attach- times can drastically stretch out the timeline of the project.
ments manifests similarly to flashing To circumvent these challenges, some slate roof owners have turned to syn-
damage. Check for the signs of thetic slate shingles as an alternative reroofing option. Synthetic slate roofing
distress listed above at gutters, con- is relatively inexpensive and can be installed quickly by a standard roofing crew,
ductor heads, leaders, attachments, using pneumatic nail guns with standard roofing nails. Lead times for materials
and related flashings. are shorter than those for natural slate. And, overall, the weight of the syn-
• Substrate/roof deck. Inspect the thetic material is far less than that of traditional slate, allowing for a slate-like
rafters and sheathing for moisture appearance on a structure that could not bear the load of natural stone.
staining. Test probes, involving re- What is synthetic slate?
moval of selected slates to expose Numerous manufacturers produce synthetic slate shingles. Product composition
the substrate, can be used to deter- varies from rigid ceramic materials to semi-flexible shingles made with post-indus-
mine if wood rot is present when trial waste rubber and plastics. While the real-time life expectancy of synthetic slate
attic space is inaccessible. is not proven, laboratory testing has projected its useful life to be 40 to 60 years.
• Fasteners. Confirm the condition How do synthetics compare with natural stone?
and material of nails and other Just as with natural slate, the lifespan of a synthetic roof is dependent on the
fasteners. Copper or steel nails are quality of the weakest element. Heavy gauge metal flashings, heavy roofing
generally found on most slate roofs, underlayments and bituminous ice and water barriers are key aspects to a
but replacement slates or flash- successful synthetic slate roof. Properly detailed conditions, such as intersec-
ings may have been secured with tions at dormers, chimneys, vent stacks, and skylights, as well as hips, valleys and
inappropriate fasteners, which may ridges, are important design considerations.
deteriorate prematurely.
Colors, textures, shingle sizes, and edge profiles of existing slate can all be
• Accessories. Check the stability, mimicked with synthetic shingles. To determine if the “look” of the synthetic
proper functionality, and correct is acceptable when compared to the natural stone, product samples and test
installation of snow guards, lightning panels should be assessed in position on the roof. Often, synthetics have a
protection systems, and other safety slight sheen and in some cases appear too regular in their coursing, yielding an
devices. unnatural and potentially unappealing appearance.
How should information from the Where might synthetic slate be suitable?
inspection be collected and used? A design professional should be able to offer guidance in choosing an appropri-
To create a record of the survey, ate synthetic or natural slate for a specific building. Is the structure to receive the
both for immediate repairs and for new roof in a historic district or considered to be a landmark building? Can the
long-range maintenance planning, roof structure bear the load of natural slate? What is the budget for the project?
document deterioration conditions There are many questions that an architect will answer prior to recommending a
and sites of previous repair work in specific roofing product, whether synthetic or natural slate.
a written report, supplemented with
Regardless of the material selected, proper detailing of the roof using quality
photographs and drawings where
products, thorough design documents, and certified installers is what yields a
possible. Prioritizing repairs and
(continued on page 4)
successful outcome.
3J O U R N A L
(continued from page 3)
budgeting for larger-scale remediation Install membrane flashings. Replace- areas, slate should be removed in a py-
projects can be simplified with clear ment of metal flashings or gutters ramidal shape. A diminishing number
records of ongoing and emerging provides an opportunity to add or of slates at higher courses allows for
problems. Results of maintenance replace ice and water barriers along proper nail positioning at installation
efforts can also be tracked to deter- eaves or at valleys. Although many and minimizes the use of bibs.
mine the solution’s effectiveness. older slate roofs were constructed To avoid a patchwork of obvious
without membrane flashings and replacement slates, matching by
Maintenance and Repairs remained leak-proof for decades, new color, size, and texture is an essen-
developments in underlayment ma- tial part of any slate replacement
Documenting signs of deterioration
terials provide added security should effort. When available, attic stock
is an important first step in caring
the slate or flashing become dam- can be used, or extra materials can
for a slate roof, but it is insufficient
aged by weather events, fallen trees, be ordered during a replacement
to record problems yet wait until
or other unexpected circumstances. project to maintain stock for future
they require major rehabilitation to
add them to the facility maintenance Secure loose and misaligned slates. repairs. A knowledgeable roofing
schedule. In conjunction with bian- Position the slate and secure it with design professional should be able to
nual inspection of the roof assem- appropriate roofing nails in the over- identify the slate type on your roof
bly, building personnel should also lying slots, covered with a bib flashing. and its origin, and should be able to
perform routine cleaning and repairs, locate matching slate, either salvaged
“A slate roof
guided by a maintenance agenda such or newly quarried.
as the following, but tailored to the Replace damaged safety accessories.
needs of the specific structure type, Snow guards that are bent or broken
occupancy demands, and exposure: is only as good as should be replaced promptly, as large
Clear debris. Remove leaves, branches, drifts of snow are potentially danger-
dirt, and other waste from gutters, the weakest ous, both in themselves and in their
flashing detail.”
valleys, and penetrations. Clear any ob- tendency to loosen and dislodge slates,
structions in gutters and downspouts, which can then fall from the roof.
both seasonally and after major storms. For any repair or maintenance work,
Repair/replace flashings. Open new materials and installation details
seams and missing or inappropriate Alternatively, use a slate hook, which should produce a roof area with a
securement can be repaired, while is a curved, copper or stainless steel serviceable life comparable to that of
deteriorated flashings will need to be metal device which hooks under the existing sections, when possible. Ap-
replaced. Copper flashings can be slate at the exposed edge. Strap hang- propriate and well-performed repair
replaced in kind, with a minimum of ers might seem to function similarly to measures may aid in remedying im-
16-ounce copper, although a heavier, slate hooks, but these are not only large mediate failures, in turn delaying—or
24-ounce weight is preferred for and unsightly, they also tend to deflect, preventing—more extensive repairs or
erosion-prone areas like valleys and allowing the slate to slip out. replacement.
gutters. Lead-coated copper is also Replace deteriorated or missing slates.
commonly used for historic restora- To remove a broken or damaged slate, When to Replace a Slate Roof
tion, because of its durability in repair a slate ripper can be used to extract A properly installed and well main-
applications. the existing nails. The slate then slides tained slate roof has a life expectancy
To extend the life of existing flash- out, and a new one can be inserted of 60-125 years or more, depend-
ings, by sometimes as much as ten to using either the nail-and-bib or slate ing upon the building location, slate
twenty years, appropriate coatings hook techniques described above. type, and roof pitch. However, poorly
can be applied, such as an elasto- When multiple slates have deterio- performed repair work or neglected
meric roofing membrane (e.g. GAF rated in a given area, substrate issues flashings and other details can signifi-
Topcoat). Check first to be sure the may be the source of the problem cantly shorten a slate roof ’s lifespan.
coating is compatible with the flash- and should be addressed before To determine the appropriateness and
ing material and with the slate roof replacing the overlying slates, lest the cost-effectiveness of repairs as com-
system. problem recur. For larger replacement pared with replacement, a roof design
4VOLUME 25 NUMBER 4
professional will conduct a thorough
evaluation of the roof assembly. Integrating Solar Power with
Repair is usually indicated when:
• Less than 20% of slates on a given
Traditional Stone
roof or roof slope are damaged; Is it possible to combine alternative
• The roof is not complex, with mini- energy roofing technologies with
mal flashing details; traditional slate roofing systems?
• Overall, the slate is in good condi- Surprisingly, the answer is yes. Photovoltaics
COURTESY ATLANTIS ENERGY SYSTEMS, INC.
tion, and flashings are at least in fair (PV) in roofing have become increasingly
shape; mainstream; from integrated solar modules
• The roof still has a significant bal- in EPDM membrane to rack mounted rigid
ance of its expected serviceable life rooftop solar panels, Building Integrated
remaining; and/or Photovoltaic (BIPV) systems harness the
power of the sun and create clean and free
• Historic significance makes preserva- electricity for a building owner.
tion a priority.
One way to integrate a PV system with a slate roof is with a product called
Replacement should be considered “Sunslates.” Sunslates are dimensionally the same as a slate shingle and are
when: installed the exact same way by a roofer. They clip together in series to cre-
• 20% or more of slates on a given ate a low voltage electrical circuit, and an electrician connects the system to
roof or roof slope are damaged; the building’s electrical panel.
• Slate deterioration areas are perva- A variety of local and federal rebates and incentives are available to offset
sive and throughout multiple slopes; the cost of a BIPV system, generating return on the investment in a short
period of time. Your roofing design professional can guide you through the
• Flashings in need of replacement
process.
are complex, requiring excessive
manipulation and maintenance of the If a slate roof rehabilitation is imminent, and alternative energy methods
slates; desired, BIPV roofing may be the answer. Consider integrating Sunslates or
another solar roofing technology with your traditional slate roof. Then, sit
• Multiple areas or challenging loca-
back, relax, and watch you electric meter spin backwards!
tions of damaged slates necessitate
extensive removal, risking breakage
of additional slates; and/or
poor attic ventilation, or other sources copper, selected for all flashings, gutters,
• Slate is in fair to poor condition of moisture; isolated replacements — for downspouts, and metal roofs, to avoid
overall, with little remaining service- example, of a single roof slope — may corrosion by galvanic action between
able life. be sufficient. dissimilar metals.
Because the detailing and craftsman- In the case of older roofs, planning Finally, availability of matching new or
ship of original slate roofs contribute ahead and budgeting for eventual re- salvaged slates should also be consid-
to the character of historic buildings, placement can prevent hasty decisions ered in the repair/replacement decision
they should be repaired rather than and inferior detailing when problems process. Even when replacement is
replaced whenever possible. However, become urgent. An architect with ex- necessary, temporary repairs may have
because the majority of slate roofs perience in slate roof design can specify to suffice to allow time for locating,
were installed around the turn of the a new slate roof which matches, as ordering, and delivering the slates.
20th century, many are now reaching closely as possible, the existing roof, in
the end of their serviceable life. Before slate type, coursing style, and detailing,
rushing into a full-scale replacement,
Preservation of Slate Roof Character
even for a complex roof. All materi-
however, consult an experienced als used, from flashings to nails, should Before major repair or replacement
slate roof design professional to rule have a lifespan comparable to that of projects are undertaken, documenta-
out worn flashings, clogged gutters, the slate, with a single metal, such as tion of a slate roof ’s unique detailing
5J O U R N A L
Problem Diagnosis Solution Design
and style should be made, such that Hips and ridges, the upper unions of eaves or valleys with insufficient drain-
new sections can cleave to the original two adjacent roof slopes, could be age, deteriorate more rapidly than do
design as closely as possible. In ad- covered with specially cut slates in a areas that can dry out quickly. Use of
dition to matching the size, shape, variety of arrangements, or might be soft slates also shortens a roof ’s useful
color, and texture of the individual ornamented with flashing formed of lifespan.
slates themselves, which may vary at sheet metal or terra cotta. Valleys, at Installation problems. Improper nailing
different points along the roof slope the internal angle of two roof slopes, of slate, with nails driven in too tightly
or randomly along courses, replicating shed the bulk of water runoff and so or with nail heads left exposed, can
detailing along hips, ridges, and valleys must be particularly well protected. crack the stone or puncture overly-
is also important to preserving the As such, they may employ metal flash- ing slates. Plain steel or galvanized
character of a slate roof. ing, with slates overlapping at the sides nails used instead of solid copper or
(open valley), or they may have slates stainless steel will rust prematurely,
Standard grade roofs, which are the
overlying a metal lining (closed valley). necessitating roof replacement even
most common, use uniform slates of
consistent length and width, with even Construction style may also vary as to while the slate itself is still sound.
cleavage at the edges. Headlaps, the type of sheathing, ranging from wood Design flaws, such as failure to leave
overlap of each slate with the courses boards or battens to concrete and sufficient headlap or sidelaps, permit
above it, and sidelaps, the lateral spac- steel. Even where materials and slate water to pass through joints between
ing of slates in relation to the courses coursing have been duplicated ac- slates, leading to leaks.
above and below, are regular. curately, failure to appropriately specify
Repair problems. Roofing mastic ap-
Random width or textured roofs use and secure the substrate may threaten
plied to damaged slates or flashings
slates of various thicknesses, widths, the longevity of a slate roof installation.
may be a tempting quick fix, but the
and tail lengths to create a less uni- black paste is not only unsightly, it is also
form appearance. Cleavages may be Pitfalls to Avoid prone to crack and to accelerate metal
uneven at the bottom, and slates may Design problems. Insufficient pitch corrosion. Mastic is also difficult to re-
range in color and texture. leads to slower water runoff, which move, making future repairs a challenge.
Graduated roofs employ thicker, larger permits moisture entry into the slate Replacing solid wood sheathing with
slates at the eaves, and gradually dimin- by capillary action. Roof areas that cheaper plywood or pressure-treated
ish in slate size along the slope, with the are permitted to remain in prolonged
smallest, thinnest slates at the ridge. contact with water, such as along (continued on page 8)
6VOLUME 25 NUMBER 4
Slate Roofing
Experience in working with historic
materials, along with technical expertise
in applying new technologies, makes
Hoffmann Architects’ project archi-
tects and engineers uniquely suited to
restoring and replacing slate roofing.
Resolving deterioration at the outsides
of buildings is the firm’s exclusive focus.
As such, our project teams commit to
a detail-oriented approach to roofing
projects, beginning with a thorough
investigation into the slate, flashing, and
underlayment conditions. Compre-
hensive document preparation and
construction administration see that
installation is completed as designed.
Hoffmann Architects’ specific experi- Wellesley College, Tower Court (pictured), Severence Hall and Clafin Hall in Wellesley, Massa-
ence with slate roofing includes: chusetts. Slate Roof Replacement and Building Envelope Rehabilitation
New York Public Library
Queensborough Community College Smithsonian Institution
St. George Library Center
Oakland Building Arts & Industries Building
Staten Island, New York
New York, New York Washington, District of Columbia
Slate Roof Investigation
Synthetic Slate Shingle Roof Replacement Slate Roof Repairs
Columbia University
Casa Hispanica University of Maryland, Eastern Shore
New York, New York Physical Education and Health Building
Slate Roof Rehabilitation, Part of Princess Anne, Maryland
60-Building Master Plan Slate Roof Investigation
Goodwin Square Hotel Lockwood Mathews Mansion Museum
Hartford, Connecticut Norwalk, Connecticut
Slate Roof and Façade Rehabilitation Slate Roof Rehabilitation
Former Southern New England Scholastic Inc.
New York, New York Fairfield University, McAuliffe Hall (pictured),
Telephone Company
Slate Roof Replacement and Cast-Iron and Bellarmine Hall in Fairfield, Connecticut.
Central Offices
Façade Rehabilitation Slate Roof and Building Envelope Rehabilitation
Various Locations, Connecticut
Slate Roof Replacement and Masonry
Repairs at Five Offices
7J O U R N A L
(continued from page 6)
hundred-year-old
roof is a testament to JOURNAL is a publication of
nature’s endurance Hoffmann Architects, Inc., specialists in
and to the craftsman- the rehabilitation of building exteriors.
The firm’s work includes investigative
ship and skill of its
and rehabilitative architecture and
artisans and caretak- engineering services for the analysis
ers. To preserve such and resolution of problems within
a historic treasure, and roofs, facades, glazing, and structural
to avoid the unneces- systems of existing buildings, plazas,
sary expense of re- terraces and parking garages.
peat repairs, it’s worth
Please send news, technical information,
taking the time to find address changes or requests for free
an experienced slate subscriptions to Editor, Hoffmann
design professional to Architects JOURNAL, 2321 Whitney
Failure to match the color, profile, and texture of existing slates evaluate the roof and Avenue, Hamden, CT 06518.
when selecting replacements can lead to an unattractive patchwork design a maintenance
and rehabilitation For information on the services
appearance.
offered by Hoffmann Architects, Inc.,
strategy. Learning contact our Marketing Coordinator at
wood can lead to shrinking and resultant the fundamentals of slate roof protec- 800-239-6665 or:
displacement of overlying slates, as can tion will enable the slate roof owner
uneven or loosely nailed wood boards. or facility manager to stay abreast 2321 Whitney Avenue
Hamden, CT 06518
Thin, substandard replacement flash- of developing problems, to prevent 203-239-6660
ings can create serious water infiltra- premature deterioration where pos-
sible, and to budget and plan for major 500 Fifth Avenue, Suite 830
tion problems, even if the surrounding New York, NY 10110
slate is in excellent condition. At- rehabilitation. 212-789-9915
tempting to save money by skimping Investigating contemporary innova-
on flashing details is poor economics: 2611 Jefferson Davis Highway, Suite 200
tions in products and techniques may Arlington, VA 22202
the expense of duplicate repairs is be appropriate for your roof—al- 703-253-9800
greater than the cost of doing it right though there is nothing wrong with
the first time. sticking to tried-and-true materials For more information on the topics
discussed in this article, visit us on the
and methods. After all, traditional slate web at www.hoffarch.com or contact
Protecting Your Investment roofs use natural materials to provide us at email@hoffarch.com.
Slate, installed correctly and main- distinctive character and sound wa-
terproofing that lasts for generations. Editor: Alison Hoffmann
tained with due diligence, is one of the
Production: Margaret Bailey
most durable of building materials. A Now that’s sustainable design.
Hoffmann Architects, Inc.
2321 Whitney Avenue
Hamden, CT 06518
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