Restoring a Copper Masterpiece

May 15, 2002

18 • Interface March 2002
In 1904, John Ireland, Archbishop of the Diocese of St. Paul,
Minnesota, began plans to create a great cathedral to serve the
growing community of St. Paul. Ireland secured land on what
was known as St. Anthony Hill and named Emmanuel Louis
Masqueray the architect. Built in the Classical Renaissance style
of architecture, the Cathedral was adapted from the original plan
of St. Peter’s in Rome, but the floor plan was in the shape of a
Greek cross. The cornerstone was laid on June 2, 1907, and
the structure, still incomplete, was dedicated April 11, 1915
(Photo 1).
The crowning glory
of the cathedral was its
stunning, copper-clad
dome accentuated by a
cross visible to most of
the city. Ireland’s successor,
Archbishop Austin
Dowling, would later
invite artisans from the
Vatican Mosaic Society
to decorate the dome’s
interior with mosaics of
the cardinal virtues,
which today are priceless
works of art. The
solid structure,
including the roof,
was built to last.
In 2000, Harry J.
Flynn, then Archbishop
of the Diocese of St.
Paul and Minneapolis,
sanctioned a two-year,
$35-million restoration
project, including reroofing
the great
dome and all the roof
areas. (See Photo 2.)
The design team consisted
of McGough
Construction, general contractor; Miller Dunwiddie Associates,
the preservation architect; and Roof Spec, Inc. Before the
restoration is completed, exterior granite will have been cleaned,
mortar joints will be tuckpointed, dome safety and access will be
enhanced, and nave lighting will be improved. But the single
largest line item on the project is the roof. Roof Spec, Inc. (RSI)
designed and managed that portion of the project, which
involves replacing the entire roof, including the copper dome,
transept, apses, sacristy, and various flat roof sections.
By Terry Thone
Photo 1 (left): The birth of the Saint Paul Cathedral before the original dome was set. Photo courtesy of Archdiocese of Saint
Paul and Minneapolis archives. Photo 2 (right): By 1989, hard Minnesota weather had taken its toll. Water leaks, apparent
from inside the structure, were endangering the now priceless interior mosaics. A patchwork of green-painted EPDM repairs dotted
the once beautiful roof, and the cross atop the cathedral was rusting. Even the exterior granite walls were darkened by pollution.
A patchwork of green-painted EPDM repairs dotted the once
beautiful roof. The cross atop the cathedral was rusting. Even the
exterior granite walls were darkened by pollution. RSI’s initial
investigation revealed that the 16-ounce copper covering the
dome was peppered with splits and tears, allowing moisture to
infiltrate the structure. Surprisingly, when the copper was
removed in some areas, it was discovered that only an organic
felt or a red, rosin-type paper without felt had been used as an
underlayment. The decking consisted of a “book tile” material
placed in a rail system that extended from the top of the dome
to its base. The system was tied into the granite walls of the
cathedral. In some locations, the book tile substrates were
cracked, as were the concrete bases below the lower gable
roof areas.
Preparing for the project offered several interesting challenges
to all parties involved. A project of this type would normally
call for a single tower crane that could reach the top of the
dome and all its surrounding areas. But the Federal Aviation
Administration (FAA) wouldn’t allow it, as the cathedral, sitting
on the highest point in Saint Paul, is too close to the airport and
would have interfered with flight patterns for the anticipated two
years of construction. The alternative was to erect two articulating
tower cranes, one on the north and one on the south side of
the structure.
The original project development timeline of the various
details and construction processes was initially scheduled to be
approximately twelve months. However, this was subsequently
condensed, allowing only four months from the start to completion
of the details in December of 2000, to allow initiation of
restoration in 2001.
Technical roofing
The roofing contractors, consisting John A. Dalsin & Son,
Inc., of Minneapolis, Minnesota, and Dalco Roofing and Sheet
Metal of Plymouth, Minnesota, removed the architectural copper
sections for restoration and repairs. Many of the architectural
Photo 3: Original, unsupported or hollow architectural copper. Photo 4: Wood framing was constructed in the same configuration as the hollow
copper to act as support. It should be noted that this framing was also
insulated to prevent any potential of condensation.
March 2002 Interface • 19
Photo 5: Rear apse roofs in production. Note that the apse roof in lower left
corner of picture shows the Ice and Water Shield and sections of 30 pound felt
underlayment. In the background, other apse roofs receive new flat seam copper.
Photo 6: Copper installation being completed on apse roof.
copper elements adorning the roof were originally unsupported,
stamped hollow copper (Photo 3). As new and refurbished copper
elements were replaced where possible, they were given a solid
substrate of wood blocking. The previously unsupported decorative
copper, which was also predominantly stamped hollow, was
filled with spray-applied polyurethane foam to provide support
and to minimize condensation.
The new substrate for the dome was constructed as follows:
1. Two layers of 3/4″ marine grade plywood secured to the
steel rail system with self-tapping screws;
2. An initial layer of 40 mil self-adhering membrane (ice and
water shield) applied for temporary protection;
3. An additional layer of ice and water shield and two individual
layers of number 30 organic finish felt (ASTM
D-226 Type II);
4. Prior to the installation of the new copper, a PyroKure®
600 fire-retardant paper installed to act as a slip sheet
(Photos 4, 5, and 6).
The fire-retardant paper was selected in lieu of a typical red
rosin, primarily to reduce the potential of flame risk or damage
when soldering flat seam copper sections.
The dome is basically segmented into various pie-shaped
sections, which were number coded, extending from the top
lantern to the base of the ring. These were separated by architectural
copper pieces that, at the time of original construction,
were formed from lead molds. This resulted in there being literally
hundreds of individual copper sections that had to be
removed and numbered for reinstallation. They were then lowered
to the onsite fabrication shop for repair and restoration
(Photos 7 and 8).
Elsewhere at ground level, originally stamped architectural
elements that could not be reproduced were cleaned with
microblasters, a method similar to sandblasting that uses highpressure
water with tiny glass beads. (Micro-blasting is abrasive
enough to restore the granite surface to its original beauty, but
gentle enough not to damage the copper. (See Photo 9.) Once
cleaned and repaired, the architectural copper pieces were filled
20 • Interface March 2002
Photo 9: Microblaster at ground level cleaning the architectural copper that
will be repaired and refurbished prior to reinstallation.
Photo 8: Architectural dental and copper lions’ feet directly below the lantern
that support the cross.
Photo 7: Segmented architectural copper. Note copper straps that were
installed at a later date due to the original tack solder having failed.
Photo 10: Architectural copper lions’ feet, which had been removed from the top
of the dome, lowered to the ground for cleaning and refurbishing, and filled with
polyurethane foam prior to reinstallation.
with spray-applied polyurethane foam, again to provide rigidity
and reduce the potential for condensation (Photo 10). This
process provided an additional benefit: as the foam expanded, it
showed small, previously unseen splits and fissures that required
additional repairs prior to reinstallation.
Much of the dome’s exterior skin consisted of interlocking
copper panels (see Photo 11). But as the interlocking sections
approached the base of the lantern, the copper changed into a
Bermuda-style panel (see Photo 12). As the skin was replaced,
architectural elements were reinstalled (see Photo 13). The ornamental
work was not only “tack soldered,” but secured with hexhead
stainless steel (#304) screws through copper tabs into the
Photo 11: Segmented flat seam copper having been reinstalled at the base of the dome.
Photo 14: New method of installation of the architectural copper elements. As was
observed in Photograph 7, additional strapping had to be installed due to eventual
failure of the tack solder. In this situation, not only was tack solder employed, but
additional hex-head screws were also used and secured with epoxy glue.
March 2002 Interface • 21
Photo 12: Completed Bermuda-style roofing at the very top of the dome.
Photo 13: Reinstallation of architectural copper after it was cleaned, refurbished
and filled with foam.
Cathedral Restoration Details:
• The original copper was 16 oz. The majority of
the new material is 20 oz.
• Architectural copper and copper columns are filled
with polyurethane foam. Over 28,000 board feet
of foam have already been used.
• To date, the restoration has used over 1,500
pounds of solder.
• The Cathedral Restoration will require:
√ Over 100,000 lbs. of new copper
√ 2,000 rolls of 30-lb. Felt
√ 2,000 rolls of Ice and Water Shield
√ 1,000 rolls of PyroKure 600 red rosin sheet
√ Over 42 miles of new mortar joints and tuckpointing
in the granite blocks
substrate. After initial placement of the screws, they were
backed out so an epoxy adhesive could be injected into the pilot
hole before the screws were re-set to lock them in place (see
Photo 14).
By December 1, 2001, the copper dome was 90 percent complete—
well ahead of schedule (Photos 15 and 16). With the dome
nearing completion, the scaffolding was adjusted so work could
focus on the lantern (See Photo 17). Initial repairs to the lantern’s
base had to be reworked after shearing and tearing of the new
copper were discovered as a result of wind loads and the scaffolding’s
extreme weight (Photos 18 and 19). The revised design
incorporated a fabricated copper “skirt” that solved the wind
load problem by allowing movement within the lantern. Because
of the extension of the copper skirt, an umbrella was effectively
fabricated to allow the movement to occur without actually having
copper attached at the base.
At the same time that work was being completed on the
uppermost lantern and cross section, the two lower bell towers
were being repaired and refurbished and the transept gable roof
Photo 15: Overview from the top lantern of the completed architectural copper
sections, Bermuda, flames, and dome.
Photo 16: Additional view of completed copper re-roofing, extending from the
base of the lantern.
Photo 18: Copper joint at base of spire that supports the cross, which had failed
due to the wind loads and possibly the additional load imposed by the scaffolding.
March 2002 Interface • 23
Photo 17: Overview of scaffold surrounding not only the dome but also the
lantern to accommodate the refurbishing and reinstallation of new copper.
Photo 19: Area of concern illustrated in Photograph 18.
areas re-roofed (Photos 20 and 21). The transept roofs received
the same substrate preparation as the dome. However, due to the
massive size of the bell towers and the fact that they were predominantly
ornamental and not necessarily requiring waterproofing,
these were simply repaired in place with new copper being
attached to the existing framework. To provide additional support
and rigidity to the bell towers, however, wood members
were installed from the interior to allow for the attachment of
the new copper pieces as required.
In addition to its use in the roofing project, 20-ounce, leadcoated
copper was also applied to the numerous granite ledges
throughout the building. This not only helped the granite
masonry to blend in with the copper, but also helped minimize
the potential of the masonry being stained by the red copper
and its eventual bright blue-green patina (Photo 22).
Restoration of Archbishop John Ireland’s cathedral is now
more than six months ahead of schedule. But there are still
numerous roof sections needing work. Remaining repairs and
replacement will involve flat seam copper that will incorporate
the aforementioned redundant underlayment system and, at
some locations below the transept or gable roof, a liquid-applied
membrane (Hydrotech 6125) with granite pavers.
When completed, the restoration will provide renewed
appreciation for the vision and efforts of St. Paul’s first Roman
Catholic Archbishop, John Ireland, and for the legacy he left all
who visit St. Paul’s Cathedral in Minnesota’s capital city. ■
For further information about the Cathedral’s restoration, including its copper
work, masonry restoration, and scaffolding, visit the Cathedral link at or This site also offers videos about
many aspects of the project.
• Minnesota Historical Society
• Historical Records of the Archdiocese of Saint Paul
and Minneapolis
• Copper and Common Sense, Revere Copper Products, Inc.
Terry Thone is the CEO and
founder of Roof Spec, Inc., based in
Saint Paul, MN, with offices in
Elmira, NY, and Salt Lake City, UT.
Mr. Thone has more than 30 years
of experience in roof engineering
and consulting. He specializes in the
review and evaluation of existing
roof systems, conducts failure investigations,
and provides technical
reports containing conclusions and
recommendations for
replacement/repair, specification writing, and roof prioritization
reviews. Terry is often called on for expert courtroom testimony
concerning roofing systems. He is a member of RCI
and is a Registered Roof Consultant. Roof Spec is currently
engineering a number of significant Minnesota restoration
projects, including the Saint Paul Cathedral, the Governor’s
Ceremonial Mansion, the James J. Hill House, and the Basilica
of Saint Mary.
Photo 22: Lead-coated flat seam copper being installed at horizontal
ledges of granite masonry.
24 • Interface March 2002
Photo 21: Overview of completed gable roofs for the east transept.
Note the dormers that have been reinstalled and were originally
shown in Photograph 9 being cleaned by microblasters.
Photo 20: Completion of copper refurbishment and installation
above the copper dome and at base of columns that support the
spire for the cross.