Resources for Property Owners

Brick is the most prevalent building material found in Denver. The earliest buildings in Denver were constructed of wood, but following devastating fires and floods, Denver passed a building ordinance in the early 1860s requiring that all new buildings be constructed of masonry. Fortunately, there were rich clay deposits and several brick manufacturers in the area, leading to a variety of brick colors and patterns.

Brick is a highly durable building material. When fired at high temperatures, the clay mixture forms a protective fireskin on the surface of the brick. This fireskin has a lower porosity than the interior of the brick, reducing water absorption.  When the fireskin is removed through aggressive chemical cleaning, sandblasting, or sanding, the brick becomes much more susceptible to deterioration.

It is important to use the gentlest means necessary to clean brick and remove paint. Use a cleaner or paint remover that is pH neutral (pH of 6-7) or slightly alkaline (slightly more than pH 7) and a natural bristle brush or plastic spatula.  Never use a wire or metal brush, metal spatula, or a sander.  Rinse thoroughly with a garden nozzle or a very low-pressure water wash, no greater than 150 psi. Never let a cleaning product dry on the brick surface.

Tip: Put your hand in front of the water spray. If the water pressure hurts your hand, it’ll hurt the masonry. 

Colorado has an abundance of durable building stone. These include various granites, Lyons sandstone, rhyolite from Castle Rock, and Yule marble. Some buildings were constructed entirely out of stone, and most brick buildings have some sort of stone trim.

It is important to use the gentlest means necessary to clean stone and remove paint. Use a pH neutral (pH of 6-7) or a slightly acidic cleaner (slightly less than pH 7) for granite, sandstone, and rhyolite.  Use a pH neutral (pH of 6-7) or a slightly alkaline cleaning (slightly more than pH 7) for limestone and marble.

Always use a natural bristle brush or plastic spatula when cleaning; never use a wire or metal brush, metal spatula, or a sander.  Rinse thoroughly with a garden nozzle or a very low-pressure water wash, no greater than 150 psi. Never let a cleaning product dry on the stone surface.

Tip: Put your hand in front of the water spray. If the water pressure hurts your hand, it’ll hurt the masonry. 

There are two different types of terra cotta used for architectural purposes: slip-glazed and vitreous glazed.  Slip glazed terra cotta can typically be found on buildings constructed in Denver prior to around 1900, while vitreous glazed terra cotta was used in Denver after about 1900.  Slip glazed terra cotta has a matte finish and usually red.  Vitreous glazed terra cotta has a somewhat shiny sheen and can be a wide variety of colors and textures.

It is important to use the gentlest means necessary to clean terra cotta and remove paint. Slip and vitreous glazes can be easily damaged if a cleaning product is too acidic or alkaline. It is best to use a pH neutral (pH of 6-7) cleaner or paint remover on terra cotta. 

Always use a natural bristle brush or plastic spatula when cleaning; never use a wire or metal brush, metal spatula, or a sander.  Rinse thoroughly with a garden nozzle or a very low-pressure water wash, no greater than 150 psi. Never let a cleaning product dry on the surface of the terra cotta.

Tip: Put your hand in front of the water spray. If the water pressure hurts your hand, it’ll hurt the masonry. 

Mortar is a sacrificial material that helps moisture escape walls and allows for some flexible movement of the masonry structure.  Using a mortar that is too strong will cause the masonry to crack during movement rather than the mortar.  It will also cause moisture to escape through the masonry, which can cause it to spall.

Use a Type O mortar on historic masonry for repointing or repairs. Type O mortar contains 2 parts hydrated lime, 1 part Portland cement, and 8-9 parts sand (measurements by volume). Never use any mortar stronger than a Type N mortar on historic masonry.

Tip: You can add a small amount of pigment or use different colors of sand to match the historic mortar color.

Tip: The contractor must remove the mortar residue from the face of the masonry before the mortar residue dries. Dried mortar smear is nearly impossible to remove from the face of historic masonry.  

Historic wood is much more durable than modern farmed lumber and trim. Historic wood has closer growth rings, giving it a denser structure and making it less susceptible to decay.  The best way to keep historic wood protected is by painting or staining it on a regular basis. Scrape any loose paint or stain from the surface of the wood, rinse the wood with a low-pressure water rinse, and then allow it to fully dry before priming and painting. You can use wood consolidants for any areas of rot prior to painting. 

Metal was used sparingly in Denver on historic buildings. A few of Denver’s earliest neighborhoods had standing-seam metal roofing, though it was an uncommon practice. A limited amount of metal trim was sometimes for ornamentation, and for unique light fixtures.  Cast iron was often used for storefront piers and lintels. Painted, iron-based metals were more commonly used in Denver than copper-based metals, though both can be found on historic buildings in the city.

Architectural metals containing iron will corrode when exposed to oxygen, so it is important to keep these metals painted. Copper-based metals will form a green patina upon exposure to oxygen. This green patina protects the metal from further deterioration and does not need painting. Similarly, aluminum forms a clear patina upon exposure to oxygen, so it does not need to be painted, though it often is painted for aesthetic purposes.  

Clay and concrete roofing tiles became popular in Denver in the early twentieth century.  These durable roofing products typically last 100 years or more if properly maintained.  While very few companies manufacture new roofing tiles, several local roofing contractors maintain salvage yards so customers can repair roofs without requiring full replacement. Careful removal, repair and reinstallation of roofing tiles by a skilled contractor can substantially lengthen the life of a tile roof.

• National Park Service’s Illustrated Guidelines on Sustainability for Rehabilitating Historic Properties (PDF)
• National Park Service’s Secretary of the Interior’s Standards for Rehabilitation
• National Park Service’s Preservation Briefs addressing common rehabilitation concerns
• National Trust for Historic Preservation window weatherization guide