Proper Mixture of Mortar for Rock Walls

Common Mortar Mixes

• There are six types of mortar for use on rock walls, of which three are the most commonly used. Type M, the strongest, can hold up to 2,500 pounds per square inch of pressure. In general, you should use it for foundation walls and any load-bearing below-ground wall. Type S mortar is a strong mortar that can withstand up to 1,800 pounds per square inch. It is used for retaining walls and also used for some foundation walls. Type N mortar, the weakest common mortar, is also the general-purpose type of mortar. It can hold up to 750 pounds per square inch. It is used for most common above-ground walls.

Uncommon Mortar Mixes

• Type O mortar cannot handle more than 350 pounds per square inch. It is used only for above-ground non-load-bearing walls such as decorative garden walls. Weaker mortars such as type O cannot be used for underground work because they are too weak to withstand the lateral pressure of the soil. Type K mortar can only handle 75 pounds per square inch of pressure. It is primarily used for the preservation of non-load-bearing historic walls where it is more important to match the look of historic mortar than to support weight. Straight lime mortar, which uses no Portland cement, is also very weak and used only in historic preservation.

Weight and Mortar

• To more specifically calculate the precise mixture of mortar that you need, you need to determine the maximum pressure that will be exerted on the mortar. Multiply the height of the wall in feet by 124.8, divide it by 12, and divide it by 12 again. This will determine the pounds per square inch put on the mortar by the wall itself. Add to this number any additional pressures placed on it by other loads the wall supports.

Warning

• Do not use a stronger mortar than you need to, when building a rock wall. The stronger a mortar is, the less flexible it is. This can cause cracking. Stronger mortars are also less permeable to water, which can lead to water accumulation behind the wall with resultant damage to foundations.