Mortar by type, joint and count

Strength and where each is allowed: Type N is 750 psi minimum and Type S is 1,800 psi, and the extra strength comes from more portland cement and less lime. Type N is the default for above-grade work — veneer, chimneys, interior walls — because its higher lime content makes it more workable, more forgiving of movement and better at healing hairline cracks. Type S belongs at or below grade, under lateral load, and beneath stone and pavers, where the joint has to resist being pushed rather than just hold a unit in place.

Yes, and it is the most expensive mistake on this page. Mortar is designed to be the sacrificial part of a wall: when a wall moves, a joint softer than the unit cracks and can be raked out and repointed, while a joint harder than the unit puts the crack through the brick face instead, where nothing can be repaired. That is why repointing a nineteenth-century soft-fired brick with a modern Type S or Type M mix spalls the faces off it within a few freeze cycles, and why Types O and K exist at 350 and 75 psi. Match the mortar to the unit it is bonding, not to the biggest number on the shelf.

About 17 bags of 70 lb preblended mix for 1,000 modular bricks at a 3/8 in joint, once a 25% waste allowance is on it. The geometry underneath that is 8.06 cubic feet of net joint volume, which is 13.5 bags before waste at the 0.6 cubic feet a 70 lb bag sets to. Counter rules that quote 100 or more bricks to a bag are describing a thinner joint, a thin-brick veneer, or an optimism that shows up as a second trip. Change the joint and the figure moves immediately — this is the one input worth being honest about.

Sand is between 2 1/4 and 3 times the combined volume of the cement and the lime, and that one rule generates every recipe in the standard. Take the lime band ASTM C270 sets for each type and multiply the total by three: Type M at 1 : 1/4 gives 3.75 parts sand, Type S at 1 : 1/2 gives 4.5, Type N at 1 : 1 gives 6, Type O at 1 : 2 gives 9. Sand is measured damp and loose, which is how it comes off the pile, and running below the lower bound produces a sticky mix that shrinks rather than a stronger one.

No — the three differ by what aggregate is in them and what job they do. Mortar has fine aggregate only and is deliberately weak and workable, because it bonds units together and must stay softer than they are. Concrete carries coarse aggregate and is a structural material in its own right, so it is placed at a low slump. Grout is the third thing: a fluid mix poured into the cells of hollow block or into the collar joint of a cavity wall at an 8 to 11 inch slump, high enough that it flows around reinforcement without being vibrated. Substituting any one for another gets you a wall that fails a different way each time.

Two and a half hours from the moment water first hits the mix. Inside that window mortar that has stiffened from evaporation can be retempered — water added and remixed on the board — and it loses very little. Past it, the stiffening is hydration rather than evaporation, and adding water to that is adding water to setting cement, which destroys the bond strength while making the mud look fine. Adding cement to a tired batch is worse again. Board life is the reason a crew mixes in half-bag batches on a hot day and full ones in the morning.

Masonry cement is a bagged product; Type N is a specification, and the two get confused because both use the same letters. A bag of Type N masonry cement already contains portland, a plasticizer and air-entraining agents, so it is mixed with sand alone at one bag to three cubic feet and needs no lime. Adding lime to it produces a mortar that meets no specification at all, since the proportions the standard sets assume you started with plain portland. ASTM C91 makes masonry cement in Types N, S and M and stops there, which is why Types O and K have to be batched from cement, lime and sand.