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Historic Preservation - Technical Procedures

Spectitle:

Guidelines For Rehabilitating Historic Buildings: Metals

Procedure code:

0109108S

Source:

National Park Service, Preservation Assistance Division

Division:

General Requirements

Section:

Reference Standards

Last Modified:

02/24/2012

Details:

Guidelines For Rehabilitating Historic Buildings: Metals



GUIDELINES FOR REHABILITATING HISTORIC BUILDINGS:  ARCHITECTURAL
METALS


U.S. Department of the Interior
National Park Service
Preservation Assistance Division
Washington, D.C.


An illustrated booklet addressing the Secretary's Standards and the
guidelines is available from the U.S. Government Printing Office.
The title is "The Secretary of the Interior's Standards for
Rehabilitation & Illustrated Guidelines for Rehabilitating Historic
Buildings", ISBN 0-16-035979-1.

Each of the guidelines included in the booklet mentioned above have
been separated into individual entries for specific use in HBPP.
This entry represents one of many guidelines included in the
booklet and describes RECOMMENDED and NOT RECOMMENDED applications
of the Secretary of the Interior's Standards as they relate to
Architectural Metals.  For a list of the Secretary of the
Interior's Standards for Rehabilitation, see 01091-04-S; For
general information relating to the purpose, organization and
content of the individual guidelines, see 01091-05-S.  Both of
these entries should be referenced along with the information
contained in this document.


BUILDING EXTERIOR

ARCHITECTURAL METALS:  Cast iron, steel, pressed tin, copper,
aluminum, and zinc

Architectural metal features -- such as cast-iron facades, porches,
and steps; sheet metal cornices, roofs, roof cresting and
storefronts; and cast or rolled metal doors, window sash,
entablatures, and hardware -- are often highly decorative and may
be important in defining the overall historic character of the
building.  Their retention, protection, and repair should be a
prime consideration in rehabilitation projects.


IDENTIFYING, RETAINING AND PRESERVING

1.   Recommended:

    -    Identifying, retaining, and preserving architectural
         metal features such as columns, capitals, window hoods,
         or stairways that are important in defining the overall
         historic character of the building; and their finishes
         and colors.

    Not Recommended:

    -    Removing or radically changing architectural metal
         features which are important in defining the overall
         historic character of the building so that, as a result,
         the character is diminished.

    -    Removing a major portion of the historic architectural
         metal from a facade instead of repairing or replacing
         only the deteriorated metal, then reconstructing the
         facade with new material in order to create a uniform, or
         "improved" appearance.

    -    Radically changing the type of finish or its historical
         color or accent scheme.


PROTECTING AND MAINTAINING

1.   Recommended:

    -    Protecting and maintaining architectural metals from
         corrosion by providing proper drainage so that water does
         not stand on flat, horizontal surfaces or accumulate in
         curved, decorative features.

    Not Recommended:

    -    Failing to identify, evaluate, and treat the causes of
         corrosion, such as moisture from leaking roofs or
         gutters.

    -    Placing incompatible metals together without providing a
         reliable separation material.  Such incompatibility can
         result in galvanic corrosion of the less noble metal,
         e.g., copper will corrode cast iron, steel, tin, and
         aluminum.

2.   Recommended:

    -    Cleaning architectural metals, when necessary, to remove
         corrosion prior to repainting or applying other
         appropriate protective coatings.

    Not Recommended:

    -    Exposing metals which were intended to be protected from
         the environment.

    -    Applying paint or other coatings to metals such as
         copper, bronze, or stainless steel that were meant to be
         exposed.

3.   Recommended:

    -    Identifying the particular type of metal prior to any
         cleaning procedure and then testing to assure that the
         gentlest cleaning method possible is selected or
         determining that cleaning is inappropriate for the
         particular metal.

    Not Recommended:

    -    Using cleaning methods which alter or damage the historic
         color, texture, and finish of the metal; or cleaning when
         it is inappropriate for the metal.

    -    Removing the patina of historic metal.  The patina may be
         a protective coating on some metals, such as bronze or
         copper, as well as a significant historic finish.

4.   Recommended:

    -    Cleaning soft metals such as lead, tin, copper,
         terneplate, and zinc with appropriate chemical methods
         because their finishes can be easily abraded by blasting
         methods.

    Not Recommended:

    -    Cleaning soft metals such as lead, tin, copper,
         terneplate, and zinc with grit blasting which will abrade
         the surface of the metal.

5.   Recommended:

    -    Using the gentlest cleaning methods for cast iron,
         wrought iron, and steel -- hard metals -- in order to
         remove paint buildup and corrosion.  If handscraping and
         wire brushing have proven ineffective, low pressure dry
         grit blasting may be used as long as it does not abrade
         or damage the surface.

    Not Recommended:

    -    Failing to employ gentler methods prior to abrasively
         cleaning cast iron, wrought iron or steel or using high
         pressure grit blasting.

6.   Recommended:

    -    Applying appropriate paint or other coating systems after
         cleaning in order to decrease the corrosion rate of
         metals or alloys.

    Not Recommended:

    -    Failing to re-apply protective coating systems to metals
         or alloys that require them after cleaning so that
         accelerated corrosion occurs.

7.   Recommended:

    -    Repainting with colors that are appropriate to the
         historic building or district.

    Not Recommended:

    -    Using new colors that are inappropriate to the historic
         building or district.

8.   Recommended:

    -    Applying an appropriate protective coating such as
         lacquer to an architectural metal feature such as a
         bronze door which is subject to heavy pedestrian use.

    Not Recommended:

    -    Failing to assess pedestrian use or new access patterns
         so that architectural metal features are subject to
         damage by use or inappropriate maintenance such as
         salting adjacent sidewalks.

9.   Recommended:

    -    Evaluating the overall condition of the architectural
         metals to determine whether more than protection and
         maintenance are required, that is, if repairs to features
         will be necessary.

    Not Recommended:

    -    Failing to undertake adequate measures to assure the
         preservation of architectural metal features.


REPAIRING

1.   Recommended:

    -    Repairing architectural metal features by patching,
         splicing, or otherwise reinforcing the metal following
         recognized preservation methods.  Repairs may also
         include the limited replacement in kind -- or with a
         compatible substitute material -- of those extensively
         deteriorated or missing parts of features when there are
         surviving prototypes such as porch balusters, column
         capitals or bases; or porch cresting.

    Not Recommended:

    -    Replacing an entire architectural metal features such as
         a column or a balustrade when repair of the metal and
         limited replacement of deteriorated or missing parts are
         appropriate.

    -    Using a substitute material for the replacement part that
         does not convey the visual appearance of the surviving
         parts of the architectural metal feature or that is
         physically or chemically incompatible.


REPLACING

1.   Recommended:

    -    Replacing in kind an entire architectural metal feature
         that is too deteriorated to repair -- if the overall form
         and detailing are still evident -- using the physical
         evidence to guide the new work.  Examples could include
         cast iron porch steps or steel sash windows.  If using
         the same kind of material is not technically or
         economically feasible, then a compatible substitute
                   material may be considered.

    Not Recommended:

    -    Removing an architectural metal feature that is
         unrepairable and not replacing it; or replacing it with
         a new architectural metal feature that does not convey
         the same visual appearance.


NOTE:  THE FOLLOWING REPRESENTS PARTICULARLY COMPLEX TECHNICAL OR
DESIGN ASPECTS OF REHABILITATION PROJECTS AND SHOULD ONLY BE
CONSIDERED AFTER THE PRESERVATION CONCERNS LISTED ABOVE HAVE BEEN
ADDRESSED.

DESIGN FOR MISSING HISTORIC FEATURES

1.   Recommended:

    -    Designing and installing a new architectural metal
         feature such as a sheet metal cornice or cast iron
         capital when the historic feature is completely missing.
         It may be an accurate restoration using historical,
         pictorial and physical documentation; or be a new design
         that is compatible with the size, scale, material, and
         color of the historic building.

    Not Recommended:

    -    Creating a false historic appearance because the replace
         architectural metal feature is based on insufficient
         historical, pictorial, and physical documentation.

    -    Introducing a new architectural metal feature that is
         incompatible in size, scale, material, and color.

                         END OF SECTION