Mortar

Mortar is the glue that holds masonry units together, and although all mortars (with the exception of some thinset tile mortars) are composed of the same ingredients -- portland cement, hydrated lime, sand, and water -- not all mortars are the same. They vary in the proportion of ingredients and, therefore, what kind of material they are used with and how you use them.

Mortars are similar to concrete but have more compressive strength and are formulated to bond masonry units, seal them, and account for small differences in their sizes. Lime is added to increase the water-retention properties of the mix and to make it more workable.

Types of mortar

Mortar is classified in the following types:

Type M: This mortar has the highest compressive strength. It is the best choice for masonry that's below grade, subject to frost heave, or in contact with the earth, such as retaining walls and walks.

Type N: This medium-strength mortar is the type most commonly used. It is suitable for general use in above-grade exterior masonry, such as freestanding walls and nonloadbearing structures such as planters.

Type S: This medium-strength mortar is used where lateral strength (the ability to resist bending) is more important than compressive strength, as in structures subject to high winds or other side loads.

Type O: This low-strength mortar is suitable for interior use only.

Thinset Mortars: These mortars are used for tile installation. Some are cement-based, others are epoxies or organic mastics. Use latex-modified thinsets and grouts when tiling outdoors.

Making your own mortar

You can mix your own mortar, using the proportions shown in the table at the bottom of the page, but bagged factory mixes, though more expensive, are better in two respects: Their proportions are more accurate than on-site mixes and their ingredients are more thoroughly distributed throughout the mix. For most do-it-yourself projects, prebagged mortar proves more cost-effective. If you do mix your own, clean sand and clean water are essential.

Pro Tip: Selecting the right mortar

No matter what kind of masonry material you're working with, make sure you have the right mortar and that you mix it properly. For example, the lime in most mortars stains stone so their mortars are lime-free but rich in cement. Mix the mortar for a stone wall a bit on the dry side so it's able to support the weight of the stones. Mix brick mortar a little wetter than block mortar. For tuck-pointing, start by mixing type N mortar with half the recommended amount of water. Let it stand for an hour, then mix in the remaining water. Always ask your retailer for the mortar that meets the requirements of your project.

Estimating mortar

Use these general guidelines to estimate how much mortar you'll need to set brick and concrete block for structures. Because different mortars have different weights per cubic foot (for example, 70, 75, and 80 pounds for Types N, S, and M, respectively), check the specifications on the bag before you complete your order.

Here's what you'll need for a wall with 3/8-inch joints:

  • Concrete block: 6 cubic feet of mortar for 100 square feet of 8x16-inch block.
  • Brick: 4.2 cubic feet of mortar for 100 square feet of 4x8-inch brick.

If you're making 1/2-inch joints, you'll need just under one-third more mortar.

Most brick walls are built with two faces (wythes), each one brick thick. So figure out how much mortar you'll need for one wythe, then double it.

Mortar proportions

Type M
Cement - 1
Lime - 1/4
Sand - 3-3/4

Type S
Cement - 1
Lime - 1/2
Sand - 4-1/2

Type N
Cement - 1
Lime - 1
Sand - 6

Type 0
Cement - 1
Lime - 2
Sand - 9

Mortar is caustic

Wear work gloves, long sleeves, a respirator, and safety glasses when mixing or spreading mortar.

 

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