WHAT IS REPOINTING?
Whether it is a mass masonry wall, siding, chimney or foundation, the repointing process remains much the same.
It involves removing the deteriorated part of the existing mortar and replacing it with a new mortar which will have the same physical and esthetics characteristics as the original mortar. Repointing can be partial or complete.
In order to help you in the section of the new mortar, it is important to:
- Know the construction date of the project to be restored and;
- Know the cause of the deterioration of the original mortar.
IMPORTANT QUESTIONS TO ASK BEFORE SELECTING THE RIGHT MORTAR:
» Is it a facade or mass masonry?
» Has the building environment changed?
» Are de-icing salts used near the facade or wall?
» Is there are heating source on any side of the facade?
» What time of year is the work scheduled for?
» Is there an available water source nearby?
» Does the team that will carry out the work have the required experience?
THE BEST PRACTICES FOR REPOINTING
1. The old mortar must be removed to an equivalent depth of twice the thickness of the joint or until sound mortar is obtained – at least 20 mm (7/8 inch.).
2. Ensure that the bottom of the cavity joint has a square finish and not a U or V shaped finish.
3. Before repointing, make sure the joint contains no debris or dust.
4. Masonry should be moistened, but no water should remain stagnant at the bottom of the cavity.
5. The new mortar must be installed in successive layers of ¼ inch (6 mm). Mortar is applied fresh on fresh.If the mortar had time to dry in between layers, it must be moistened again before resuming work.
THE SELECTION OF REPOINTING MORTARS
Selecting the right mortar is essential for the success of the project. To do so, you need to make sure that the new mortar is compatible with the original mortar. The following factors should be considered;
1. The new mortar must be equal or weaker in terms of compressive strength than the original mortar;
2. The new mortar should be equal or more permeable to water vapour than the masonry units and the orginal mortar;
3. The new mortar must be durable;
4. The new mortar must be aesthetically similar to the original mortar.
How does one determine the compressive strength of a mortar or its vapour permeability?Basically, two solution sare available:
The first solution is to order a chemical analysis of the physical properties of the original mortar. This analysis will allow you to determine the compressive strength of the mortar, the binder type, the binder/sand proportions, as well as the particle size of the sand used. Unfortunately, these analysis are expensive and few companies have the expertise to do them.
Knowing the construction date of the project will allow you to potentially identify the type of binder that was originally used.
On the other hand, knowing the cause(s) of the deterioration will allow you to validate whether the use of the same type of binder is desirable or if it would be preferable to make modifications to the design of the project before undertaking repointing work.
Remember that it is possible to use more than one type of mortar or binder on the same project.
6. When the mortar is strong enough to withstand thumb pressure but the fingerprint remains present on the surface, you can proceed with tooling the joint.
7. Once the joints are well compacted, it is generally necessary to open the mortar pores with a brush or a tool specially made to this end.
8. If the project requires only partial repointing, be sure to finish the joint similar to the original mortar.
9. Make sure to start the moist cure immediately after finishing the joint.
* In order to avoid damaging the masonry units, it is necessary to supervise the use of mechanical tools for the hollowing out of the joints. The procedure for hollowing out the joints must be fully detailed in the specification documents.It is very important to always erect a mock-up wall of approximately 1 meter by 1 meter before starting the work. This wall, which must be an integral part of the project, will serve as a reference throughout the work.
The second solution, less scientific, but just as much effective is to date the origin of the project. Contrary to European buildings, the history of masonry in Canada is not very old and therefore fewer types of binders were used. We adhere to the philosophy that the new mortar does not have to be identical to the original mortar, but it must be compatible with it. The date allows to identify with enough precision the type of binder that may have been used for the construction of the project. –Refer to document ''Identification guide for different types of binders'' available on the website. Knowing that the proportion of sand is generally three times greater in volume than the amount of binder, the composition of the original mortar is clarified.
The colour of the original mortar also points to the nature of the original binder. Generally, it’s the sand that provides colour to the mortars. Lime mortars tend to be beige shades, while natural cement mortars tend to be more ocre shades. As for the more recent cements, they are generally more grey shades.