Historic Brick Repair and Pointing, in a nutshell
BY JOSHUA DUNN
Historic Brick Masonry: Threats and Solutions
Historic brick buildings are a common sight in the Kansas City area. These durable buildings have a long service life and with periodic maintenance, many have years of effective use ahead. The primary threat to masonry buildings is water. A seemingly natural reaction to this would be is to paint or apply a sealer to the building to prevent water penetration.
Unfortunately the solution is not as simple as it would seem. Standard paints do not allow the breathability that historic masonry buildings require. Even if walls were sealed from the direct weather, most stone basements in historic buildings soak in water from surrounding soil and through capillary action, that water rises into the walls of the building. Under normal circumstances this water would evaporate out of the building and would not cause concern. However, when this building gets painted with a standard paint, it traps water and vapor inside the walls of the building creating a permanant damp condition. Over time, this could destroy the building and cause secondary problems such as mold and pest concerns. Another cause of water in the building envelope is due to condensation within walls. This is also a concern when considering sealing the building.
The alternative to sealing the building with paint would be to re-point and repair problem spots in the exteror masonry walls. Re-pointing is the process of removing damaged mortar from the face of the walls and replacing with a historically suitable mortar (matched for appropriate color, hardness, and permeability). Done correctly, it will match and blend with the existing building. This approach will allow the masonry to continue to breath and shed rainwater sufficiently while avoiding the problems of a water barrier. This is the way these durable buildings were designed to function. Wall caps and skyward facing masonry must be addressed in connection to the roofing and flashing system. This should be assessed on a case-by-case basis.
If design is a factor in the considerations for using paint on the exterior facade of historic brick buildings, and it is appropriate to the historic design considerations, there are paints and limewashes that are specifically designed to be breathable. It is highly recommended that if you are planning on painting your brick building that you first work with a historic masonry consultant or reputable masonry restoration contractor to ensure that soft mortar will not cause an expensive failure to your new finish, and that the walls are dry enough to paint.
Historic Masonry Re-pointing
1. The Mix
Historic mortar was a lime-based mix rather than a portland cement-based mix as modern mortars are. There are several reasons to re-point with the same type or compatible mix as what was used in the historic masonry. Using harder, less permeable modern day mortars can lead to early wall failure.
as a general guidline, structures that were built prior to 1872 should be repaired with a "straight lime" mortar. Straight lime mortar is generally mixed at a 1 to 3, lime to sand ratio.
Buildings constructed before 1932 generally will need to be repointed with a lime based mortar to match the appearance and strength of the structure.
Most buildings constructed after 1932 generally can be repointed with a modern portland cement based mortar formulated to an suitable strength for the materials that surround the mortar.
The amount of water used in the mix should be minimal to reduce shrinkage but still provide a stiff workable consistancy.
2. The Sand
By volume, sand is the primary component in mortar mixes. Naturally, finding a sand that is similar to the color and size used in the original mortar is important. When exposed to weathering, the thin coat of cement washes off of the surface of the mortar, which leaves the sand as a primary visual surface of the mortar.
Only use pigments that were intended as mortar and concrete color. These should be permanent pigments consisting of iron oxide.
Pointing Execution Guidelines
Old mortar should be cleaned out of mortar joints to a depth of twice the width of the joint or where structurally sound mortar is found. This should generally never be less than a half inch.
Mortar should be firmly pressed into joints with pointing tool to prevent air gaps.
Mortar joints should be finished to match existing joints as closely as possible. Mortar joints should be finished when the mortar is still soft enough to tool but not so wet that it smears the masonry units around it.
Final cleaning can be done with a gentle masonry cleaner suitable for historic applications and the type of masonry used. Waiting until mortar has cured before final wash will provide better final mortar strength.
Masonry Repair, Rebuilding, and Unit Replacement
When considering a masonry restoration or repair, as with any other restoration or rehabilitation project, we must determine the condition of the unit or section to be restored.
An important question is: can it be repaired or does it need to be rebuilt (preserved or restored)? Much of the time the answer is: it can be repaired.
Rebuilding the damaged section may produce a better result if it is extensively damaged however it is always a much more costly and usually more risky option.
A historic masonry consultant or masonry restoration company should usually be sought out to analyze repairs and provide if necessary, a maintenance plan. If you are in the Kansas City metro area, Alliance Restoration would be happy to provide a free initial consultation of your building.