U.S. General Services Administration
Technical Procedures Disclaimer
Prior to inclusion in GSA’s library of procedures, documents are reviewed by one or more qualified preservation specialists for general consistency with the Secretary of Interior Standards for rehabilitating historic buildings as understood at the time the procedure is added to the library. All specifications require project-specific editing and professional judgement regarding the applicability of a procedure to a particular building, project or location. References to products and suppliers are to serve as a general guideline and do not constitute a federal endorsement or determination that a product or method is the best or most current alternative, remains available, or is compliant with current environmental regulations and safety standards. The library of procedures is intended to serve as a resource, not a substitute, for specification development by a qualified preservation professional.
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We’ve reviewed these procedures for general consistency with federal standards for rehabilitating historic buildings and provide them only as a reference. Specifications should only be applied under the guidance of a qualified preservation professional who can assess the applicability of a procedure to a particular building, project or location. References to products and suppliers serve as general guidelines and do not constitute a federal endorsement nor a determination that a product or method is the best alternative or compliant with current environmental regulations and safety standards.
NOTE: The use of caulks and sealants in the masonry joints of historic structures, is only an appropriate maintenance treatment when it is intended to replace caulking or sealants used as part of the original design.
This standard includes guidance on selecting joint sealants for masonry, based on composition, elasticity and durability. For guidance on installing joint sealants, see “Sealing Masonry Joints to Make Them Airtight and Watertight”.
- Sealants may be used for expansion joints, at intersections of differing materials and to infill gaps where differential settlement/movement is still active.
- Sealants should NOT be used as a substitute for proper masonry repointing (i.e. facade surfaces, etc at vertical and horizontal joints).
The performance of sealants is affected by several factors:
- Physical properties such as porosity, texture and elasticity.
- Chemical properties such as composition and salts.
- Environmental factors affecting durability.
- Joint design affecting performance (i.e. the width to depth is directly related to sealant performance).
Sealants in satisfactory condition should:
- Be pliable enough to conform to masonry joints, cracks, etc.
- Be able to withstand the anticipated joint movement.
- Be insensitive to moisture.
- Be resistant to alkalinity which is often present in stone.
- Be NOT stain or mar the appearance of the masonry.
- Masonry joints to receive sealants should be clean and free of dirt, dust, grease, or oil, salts, chalk or lime and other debris.
In their 1987 report, “The Use of Sealants in Masonry Joints”, The National Institute of Standards and Technology (NIST) - formerly known as The National Bureau of Standards (NBS) - listed the following factors and properties of stone which can affect sealant performance:
- Porosity of masonry and stone
- Permeability of masonry and stone
- Moisture content of masonry and stone in presence of freezing and thawing conditions
- Salt crystallization within masonry and stone pores
- Thermal and moisture expansion of joints
- Surface texture of masonry and stone
Sealants may be classified by composition, elasticity and durability.
- Chemical or composition types of sealants in 1 and 2-part formulations include:
- Oil based
- Acrylic
- Butyl
- Polysulfide
- Urethane
- Silicone
- Polymercaptan
- Classification according to elasticity or joint movement includes three levels as follows:
- Low Movement - Up to 5% movement; this group is generally composed of oil and resin based sealants.
- Medium Movement - From 5 to 12.5% movement; this group is typically comprised of polymer based acrylic types and some urethanes. (Acrylics are not generally recommended for outdoor use.)
- High Movement - Up to 25% movement; this class generally includes the urethanes, silicones and polysulfides.
Classification according to durability is as follows, according to the (NIST) study of sealants:
- Five-Year Durability
- Oil based
- Polybutanes
- Ten-Year Durability
- Buty
- Acrylics
- Hypalon
- NuPrene
- Fifteen-to-Twenty-Year Durability
- Polysulphides
- Silicones
- Polyurethanes
- Mercaptors