Repointing Mortars for Older Masonry Buildings - Site Considerations

This Update presents information critical to the successful application of repointing mortars for the conservation of older masonry buildings .

This Update should be read in conjunction with Update No. 67: Repointing Mortars for Older Masonry Buildings — Design Considerations.

Repointing of older masonry buildings typically involves the use of low-strength mortars and requires more care and attention to detail than modern masonry construction. Many factors come into play both in the selection of suitable mortars and in their application. Design and selection issues were addressed in a previous Update. Here, the focus is on application, including scheduling of the work, qualifications of the workers, preparation of the mortar joint, mixing, application of the mortar, and curing. The specifications for a project should describe in detail what is expected in each of these areas.

Figure 1. Repointing is the process of removing the outer (deteriorated) layer of mortar and replacing it with new mortar.

Pre-construction Considerations

Before construction begins there are several important aspects to consider, to ensure a successful repair.

Selection of qualified contractors. Selection of contractors experienced in the restoration of older masonry is vital. Accreditation of the contractor gives assurance that the workers executing the job have received appropriate training by a recognized organization. In the case of historic structures a minimum number of years experience in conservation work would be a good starting point. For example, PWGSC has suggested that the mason have over five years of experience in a similar type of work following accreditation, or work under the supervision of someone with such experience. Quality of previous jobs could also be used to assess the qualification of the contractor. Some colleges and historic masonry conservation organizations offer training courses on masonry restoration. Mortar manufacturers also offer hands-on training for use of their products.

Seasonal considerations. Time of year and weather can be a constraint on a project. Low-strength mortars are sensitive to extreme climate conditions during application and curing. For example, freezing conditions and high temperatures leading to rapid drying will have a detrimental effect on the durability of the mortar. In northern climates the window of opportunity for repointing can be quite short.

Building occupancy issues. It is important to ensure that repointing work does not cause undue disruption for building occupants or visitors. Scaffolding may need to be in place for extended periods and joint preparation can be very noisy and dusty. These conditions can affect the operation of the building and need to be taken into account in the scheduling of the work.

Test panels. The contractor should prepare representative test sections in an out-of-the-way part of the building. This way the contractor can set the benchmark for an acceptable standard of quality as work proceeds. These test panels can be used to assess the mason's skill and confirm the mortar colour, texture and joint finishes that have been selected for the job.

Joint Preparation

Preparing the joint is the critical first step in the repair work. It must be done carefully so as to enable good compaction of the fresh mortar into the joint and good bonding to both the masonry units and the bedding mortar. Incorrect procedures, improper tools or inadequate skills can result in damage to the edges of masonry units, which not only affects their visual character but can also increase the chance of weather damage.

A clear rectangular space must be raked in the joint for placing the repointing mortar (Figure. 2). In general the depth of the raking should be twice the thickness of the joint. With thin mortar joints, only the deteriorated areas should be raked to avoid damaging the masonry units. Loose material further inside the joint should also be removed. The deeper the joint, the more difficult it is to compact fresh mortar properly into it. With extensive or deep raking of joints the stability and stresses in the wall should be evaluated before deciding on the area of wall to be repointed at any one time.

Figure 2. Correct and incorrect ways of raking the joint in preparation for repointing

Manual tools such as hand chisels or small pneumatic chisels are traditional methods of removing old mortar, as they minimize the risk of damage to the masonry units. Power saws and grinders should not be used on masonry with thin joints, nor on vertical joints, as there is a higher risk of damage to the edges of the masonry units. However, on wide and uniform horizontal joints, careful use of a power grinder or saw in the middle of the joint can help break hard mortar; the raking of the joint is then completed with a hammer and chisel. This may also be necessary with softer stones laid in a harder existing mortar, using the grinder to cut the centre of the joint to facilitate removal of the mortar. Before power tools are used a test should be carried out to confirm the procedure and the operator's experience.

After raking, the joints should be brushed or vacuumed, then washed out with a water jet from the top of the masonry area down, to remove any remaining debris and dust.

Source : http://irc.nrc-cnrc.gc.ca