Available Maintain Resources
Selecting Mortar for Historic Buildings
Ensuring that a replacement mortar will be sympathetic to historic masonry is key to a successful repointing project. Our goal is to make sure that the new mortar is a sacrificial element, meaning that it, NOT the masonry units, will slowly deteriorate and can be periodically replaced. This deterioration occurs because the mortar rather than the masonry is handling the stress of building movement and moisture. Mortar should be breathable enough to transfer moisture out of the wall system and flexible enough to absorb the pressure of freeze-thaw cycles and building movement. The selected mortar should be more vapor permeable and “softer” than the original mortar and the masonry units.
New construction mason contractors are generally familiar with Type N or S mortars. These are often specified for modern wall systems because they rely on air barriers and expansion joints, neither of which historic masonry buildings feature. Historically, masonry buildings were load bearing and relied on mortar to 1) transfer vapor and moisture out of monolithic wall systems and 2) to accommodate freeze-thaw cycles and building movement. As a result, relatively rigid and impermeable mortar that is normally used successfully in new construction is detrimental to historic masonry buildings.
Many people believe that Type N or Type O are “soft” enough for historic masonry. This misunderstanding derives from the ASTM standards for property blended (as opposed to proportion blended) which set minimum 28-day PSI requirements of 350 for Type O and 750 for Type N. However, these are minimum requirements and test data shows that property blended Type O and Type N can actually readily achieve strengths of 1200 and 1700 PSI respectively. These strengths are much too high for most historic buildings. Proportion blended mortars, which are not tested for performance, can easily achieve even higher strengths. Additionally, Portland cement is less vapor permeable than lime and can inhibit vapor transfer.
Bidding and Planning
We work with a wide range of contractors, many of whom are working on lime mortar projects for the first time and have many questions on how to bid and plan the project.
- Lime mortar is installed much drier than most modern mortars and should be the consistency of brown sugar. You should never use grout bags or pointing guns. You should also plan on installing lime mortar using back fillers.
- Many specifications call for you to install mortar in fine lifts. Unless you’re installing nonhydraulic lime putty (which needs brief exposure to the atmosphere to initiate carbonation) this is unnecessary. Lifts are only necessary when sagging is a concern.
- Lime mortar requires more attention during curing than cement lime mortars, so you will need to plan ahead in order to keep it protected. Preferably the mortar should be covered with burlap and misted for at least 3-5 days, including weekends. This is especially important if the weather is very hot or windy.
- Lime mortar should be installed when it is 40 degrees and rising and protected from frost for at least a week. Never use admixtures.
- You must plan to work neatly because cleaning is not an option after installation.
- We will be happy to provide referrals if you would like a custom training session.
Lime mortar is a specialty product and you should anticipate budgeting at least 2-3 times the cost of conventional Portland cement-lime mortars.
Custom Color & Aggregate Matching
We have matched thousands of mortars and we retain samples from all of our jobs. When we are asked to match a mortar color, we first check our existing formulas for a match. If we have a match that is approved, we do not charge a color matching fee. Color matching and aggregate matching fees are $350/sample for most projects; custom materials or more complicated projects may have a higher fee schedule. You should anticipate a two-week lead time for custom color matching although it may be faster; much of the timing depends on the workload in the laboratory and the difficulty of the match. We do have to allow the samples to cure, so inevitably there is some unavoidable lead time to the process. If we have to clean samples the lead time is extended.
In some situations, you may bid on a project that doesn’t specify a certain type of mortar. Instead, you may be asked to simply “match existing” or you may be required to commission testing to establish a compatible replacement. When the type of mortar is not specified but lime mortar is likely the best option, it is very difficult to get apples to apples bids. Low bidders who budget for conventional mortars have an advantage over those who favor lime mortar because typical cement-lime mortars are commodity materials with simpler, more familiar installation and curing procedures.
When no mortar type is specified, we suggest submitting two allowances: one for conventional mortar and one for lime mortar. This enables you to demonstrate your price competitiveness while also providing a realistic budget number if lime mortar is the more appropriate material. In too many cases such loose specifications disqualify the bidders who price appropriate materials, only to have the low bidder submit a change order after the job is awarded.
Pricing for preblended custom mortars is dependent on the bag quantity, aggregate, and pigment load so cost is highly variable. In situations where lime mortar is likely to be the most compatible material, we are happy to provide budget numbers based on comparable projects.
Anticipating Leadtime
We advise allowing a lead time of 1-2 weeks to produce a custom blended mortar. Lead times can be shorter, but this production time will need to be factored into your work schedule. During the spring/summer peak these estimates may be longer. If we need to do a custom color match plan for an additional 2 weeks when the development of a new color formula is required.
Mockup Panels
We always recommend installing a mockup panel that is at least 3’ x 3’ to demonstrate how the mortar will be removed without damaging the masonry units as well as any cleaning procedures, and to establish the agreed-upon joint profile and color of the cured mortar. This ensures that the standard of workmanship is accepted by all parties before the job starts. You should budget for this work and include it in your bid.
Maintain - Matrching Historic Mortar in Use
Need Assistance?
Contact us at 773-598-4300 for more information and assistance.
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