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The "Stronger is Always Better" Conundrum
In any modern industry or trade, new ways of doing things evolve over time to improve productivity, efficency, quality and longetivety of a product. The same can be said for the Masonry trade and the mortars we use. Manufacturers, salemen, mason's and engineers continue to improve and modify practices in the industry. One topic that continues to be debated is the use of harder mortars versus softer ones and when they should be used.
Harder Vs. Softer Mortar
A harder mortar is one that is mixed in a ratio that has a greater amount of portland cement than it does lime. Portland cement in the hardened state provides, durability, compressive strength and bond strength. Mortar's classified as stronger, include Type "M" and Type "S" mortars.
A softer mortar is a mixture that contains more lime in relation to portland cement. Lime in the hardened state provides elasticy, autogenous healing, and some compressive and bonding strength. Typically, the softer the mortar, the more lime it has in relation to cement and aggregate. Softer mortars include Type "N", Type "O", and Type "K."
Mortar Common Applications
Mortar Type | Application |
M | Interlock, Foundations, Below grade |
S | Below Grade, Blockwork |
N | Exterior Brick, Non-loadbearing Walls |
O | In Some Historic Resoration |
K | Historic Restoration |
When Things Start to Go Wrong
There is no doubt that anything below grade or loadbearing MUST use a harder mortar like type "S". Its high compressive strength and bond strength allow the masonry systems to handle heavy loads and resist hydrostatic pressures below grade much better.
However, there are many strong-headed masons out there that will add more cement to there mortar mix for exterior brickwork veneer or chimney with the belief that the brickwork will last longer because the mortar is so strong. Rather than use a Type "N" mortar, wich is the correct mortar for this application, they create a Type "S" or Type "M".
Why Harder is Not Always Better
The best rule of thumb to have is to always use the prescribed mortar outlined by the engineer, designer or manufacture. When that is not the case, the next best rule is to never use a mortar that is harder than your masonry unit being laid.
When you install a brick or stone with a mortar that is harder than the brick/stone itself, you are setting yourself up for having damaged units in the future. This is because you want your mortar to be sacrificial. You want it to be softer than the masonry unit so that it will crack and weather before your brick does. It is much easir to repoint a wall than it is to cut out and replace individual bricks.
The Water Problem
Masonry is constantly exposed to harsh elements, especially here in Ottawa. Moisture from rain and snow will penetrate brick or stone. In this event, when the sun comes back out, water will always take to easiest way out which, is through your mortar joints. Thats why repointing is such a common masonry service in Ottawa, because mortar joints erode sacrificially to save the bricks or stones. It is a simple repair to perform and helps bring integrity back to the structure.
However, if a harder mortar was used in this scenario, the water has nowher to go and will sit in the brick/stone and through freeze thaw cycles cause the masonry unit to spall. That is when a brick repair is needed which is a far more intrusive process than a basic repoint because the whole unit will need to be cut out and replaced.
This is a common problem we see on chimneys all around Ottawa. Some chimney repairs and rebuilds we've fixed were after a previous masonry contractor had built it with strong of a mortar, The chimney wasn't even 8 years old yet. The bricks were completely destroyed on the chimney and yet the mortar seemed to be in perfect condition--a tell tale sign the mortar used was way too hard.
Concluding Thoughts
At Kavanagh Masonry, we always use the mortar the manufacturer or designer prescribes knowing that often softer is in fact better. For majority of the brick chimney rebuilds and brick repairs in Ottawa we use a Type "N" mortar which is the best mortar for the longevity of your bricks.
For more information on our Chimney repairs or brick replacement services, please visit
our Services Page. If you would like to get a free estimate on masonry repair services for your Ottawa home, please reach out on our Request a Quote Page.
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