Mortar is the glue that holds masonry together, and picking the wrong type or mixing it badly will undermine even the best brickwork. The bag at the store does not tell you much beyond the type letter and the yield. This guide fills in the gaps so you know what to buy, how to mix it, how much you actually need, and how to handle the curing window without ruining your joints.
The single-letter designations can be confusing. They come from the word MASON, taking every other letter: M, A, S, O, N. The letters used for mortar types are M, S, N, and O, in decreasing order of compressive strength.
Type N is the general-purpose pick for above-grade exterior work. It has moderate compressive strength (750 psi) and good flexibility, which means it can handle minor movement from temperature swings and settling without cracking the bricks themselves. Use it for garden walls, planters, chimneys above the roofline, and non-load-bearing partition walls. Most residential brick projects that are not in contact with soil call for Type N.
Type S is stronger (1800 psi) and more resistant to water penetration. It is the go-to choice for below-grade walls, outdoor steps, patios laid in mortar, and anything in contact with the ground or subject to lateral pressure from soil. If your project involves a retaining wall under four feet or a brick walkway, Type S is the right mortar.
Type M hits 2500 psi and is reserved for heavy-duty structural applications: foundations, load-bearing walls below grade, and retaining walls under significant soil pressure. Its high strength comes with low flexibility. In residential work, you rarely need Type M unless you are building a foundation or a structural wall that carries roof load.
Type O is the weakest (350 psi) and softest mortar. It is used almost exclusively for interior, non-load-bearing walls and for repointing historic masonry where matching the original soft mortar is important. Using a harder mortar to repoint old soft brick can cause the bricks to crack because the mortar is stronger than the unit it surrounds.
Pre-mixed bags (like Quikrete or Sakrete mortar mix) already contain the right ratio of Portland cement, hydrated lime, and sand. Just add water. For site-mixed mortar, the traditional ratios by volume are:
Type N: 1 part Portland cement, 1 part hydrated lime, 6 parts sand. Type S: 1 part Portland cement, 0.5 parts hydrated lime, 4.5 parts sand. Type M: 1 part Portland cement, 0.25 parts hydrated lime, 3.375 parts sand. Sand should be clean masonry sand, not play sand or all-purpose fill sand. The grain shape and size of masonry sand gives mortar its workability and bond strength.
The most common mixing mistake is adding too much water. Mortar should hold its shape on the trowel and not slump off the end. If it slides, it is too wet. If it crumbles and will not stick to the trowel, it is too dry. Add water in small amounts and mix thoroughly between additions. The ideal consistency is often compared to peanut butter: stiff enough to hold a ridge when you drag a trowel through it, but plastic enough to spread smoothly. Mix in a wheelbarrow or mortar tub with a hoe for small batches, or rent a paddle mixer for larger projects.
Coverage depends on the masonry unit, joint width, and joint depth. For standard modular brick (8 x 2.25 inch face) with 3/8-inch mortar joints, plan on approximately 7 bags of 80 lb mortar mix per 100 square feet of wall area. For concrete block (16 x 8 inch face) with 3/8-inch joints, the rate drops to roughly 3.5 bags per 100 square feet because the units are larger and there are fewer joints per square foot. Wider joints increase consumption proportionally. A half-inch joint on brick uses about a third more mortar than a 3/8-inch joint on the same wall. Always round up when ordering. Running out mid-course is far worse than having a partial bag left over.
Mixed mortar stays workable for about 90 minutes under normal conditions (60 to 80 degrees Fahrenheit, moderate humidity). High heat and low humidity shorten that window to as little as 45 minutes. Cold weather extends it but introduces a different problem: if mortar freezes before it cures, the bond fails completely. Do not lay mortar when air temperatures are below 40 degrees or expected to drop below 40 within 24 hours of placement. In hot weather, dampen your bricks or blocks before laying them. Dry masonry units suck moisture out of the mortar too fast, weakening the bond and causing the joint to crack as it shrinks. A quick dunk in a bucket of water for bricks, or a light misting from a hose for blocks, makes a noticeable difference in joint quality.
Mortar does not dry. Like concrete, it cures through hydration. The chemical reaction needs moisture to proceed correctly. If joints dry out too fast in sun and wind, the mortar will be weak and chalky. Keep fresh mortar joints out of direct sun for the first 24 to 48 hours if possible. Mist the wall lightly with a hose two or three times a day for the first three days in warm weather. Covering the wall with damp burlap or plastic sheeting also works. Full cure takes about 28 days, though the mortar reaches working strength within a few days. Strike (tool) your joints when the mortar is thumbprint firm. Press your thumb into the joint surface. If it leaves a clean impression without mortar sticking to your thumb, the timing is right. Tooling too early smears the joint. Tooling too late cracks it. A concave joint profile (made with a rounded jointing tool) sheds water best and is the standard choice for exterior walls.
Inconsistent joint thickness is the most visible sign of sloppy brickwork and it happens when the mason does not use a gauge rod or string line. A gauge rod is a straight piece of lumber marked at each course height (brick height plus joint). Hold it against the wall every few courses to check that your joints have not crept thicker or thinner. Another frequent issue is mixing too large a batch. If you cannot use a batch within 90 minutes, it starts to stiffen and the temptation is to add water and remix. This process, called retempering, weakens the mortar. One retemper is generally acceptable; a second one drops the strength enough to matter. The safer approach is to mix smaller batches and keep a consistent pace. Finally, skipping the buttering step on the brick ends leads to hollow head joints that let water into the wall. Every brick should have mortar applied to the end before it gets pressed into place.
