The four types and what they are for
Mortar types are named by the alternating letters of the phrase MASON WORK: M, S, N, O, K. Each step down is weaker and more forgiving. The proportions below are the ASTM C270 proportion specification, measured by volume, not weight.
| Type | Portland : lime : sand | Minimum strength | Where it belongs |
|---|---|---|---|
| M | 1 : 1/4 : 3-3/4 | 2,500 psi | Below grade, retaining walls, paving, hard exposure |
| S | 1 : 1/2 : 4-1/2 | 1,800 psi | Foundations, below-grade walls, high wind or seismic |
| N | 1 : 1 : 6 | 750 psi | General above-grade work, most repointing |
| O | 1 : 2 : 9 | 350 psi | Interior, non-load-bearing, historic soft brick |
Those strengths are the minimums for the property specification. A mix made to the proportion spec usually tests well above them. The number that matters for repair work is not the minimum but the comparison: the mortar should be weaker than the units it sits between, so that movement cracks the joint rather than the brick. A joint is cheap to cut out. A spalled brick face is not.
Why sand volume and mortar volume come out nearly equal
The arithmetic that surprises people is that a cubic foot of finished mortar takes about a cubic foot of damp loose sand. Adding cement and lime does not add much volume, because the paste occupies the spaces between the sand grains rather than pushing them apart. So the sand figure is the yield figure, and the portland is a fraction of it: for Type N at 1 : 1 : 6, every cubic foot of mortar wants one sixth of a cubic foot of portland, which is one sixth of a 94 lb bag, and the same again in lime.
Two conventions make the shopping list work. A 94 lb bag of portland cement is one cubic foot. Hydrated lime is measured at 40 lb per cubic foot, so a 50 lb bag is about a cubic foot and a quarter. Both of those are trade conventions rather than laboratory constants, and bulk density does move with how the material has settled in the bag.
Repointing volume, and the depth that actually matters
Repointing only refills the outer part of the joint, so the volume is small. On modular brick with 3/8 joints the joints are about 18 percent of the wall face. Cut back 3/4 of an inch and 100 square feet of wall needs 144 x 0.18 x 0.75 cubic inches per square foot, which works out near 1.14 cubic feet net, or 1.4 with a quarter added for waste, so three bags of 80 lb premix.
The rake depth is not a quantity decision, it is a durability one. Cut back at least twice the joint width, and never less than about 5/8 of an inch, or the new mortar is a skin that will pop off in the first hard freeze. Grinding to depth is what takes the time on this job. A wheel that wanders into the brick arris does damage you cannot repoint away, which is why the slow method with a hand chisel is still standard on anything with value.
Colour matching is the other thing this calculator cannot do for you. Sand is what makes mortar the colour it is, far more than the cement, and a local sand from a different pit will read as a different wall from across the street. If the patch has to disappear, mix a sample, let it cure a week, and hold it against the wall dry before you commit to the batch.
Questions people ask
What is the difference between Type N and Type S mortar?
Type S has more portland and less lime, so it is stronger and stiffer: roughly 1,800 psi against 750. Type S belongs where the wall is loaded or wet, which means foundations, below-grade walls and anything carrying real structural load. Type N is the general above-grade mortar and the usual choice for repointing, because it stays softer than the brick and lets the joint take the movement. Stronger is not better in masonry. It just moves the failure somewhere more expensive.
Can I use Type S to repoint an old house?
Usually you should not. Brick made before roughly the 1930s is often softer than modern Type S mortar, and a hard joint between soft bricks concentrates every bit of thermal and moisture movement onto the brick face, which spalls. The conventional guidance for historic solid masonry is Type N at the hardest, Type O commonly, and sometimes a straight lime mortar with no portland at all. If the building has heritage status, the specification is usually dictated and you should not be guessing at it.
How many bags of mortar do I need per 100 blocks?
Geometrically, about 1.3 cubic feet net for 100 standard 8x8x16 blocks laid on the face shells with 3/8 joints, which is 1.6 cubic feet once you add a quarter for waste, or 3 bags of 80 lb premix. That happens to land on the trade rule of thumb of 3 bags per 100 block, which is reassuring, though the two agree by coincidence rather than by derivation. On brick the same comparison does not agree at all, and the bag figure comes out roughly double the geometry.
Is premixed mortar the same as making it from portland, lime and sand?
Close enough for most jobs, and much easier for small ones. Bagged mortar mix is usually a masonry cement or mortar cement blended with sand, and the bag will state the type it meets. Site mixing wins on cost once the volume gets past roughly 20 cubic feet, and it wins on control if you are matching a colour or deliberately mixing something softer than what is sold in bags. It loses on consistency: batch-to-batch variation in a site mix shows in the finished wall as colour banding.
How long can I keep mortar workable?
Plan on about two and a half hours from the time water hits the mix, less in heat and wind. You can retemper once by adding a small amount of water and remixing, to replace what evaporated, but you cannot retemper mortar that has started to set from hydration, and adding water to it just makes weak mortar that looks fine. Mix batches sized to what you can actually lay, which for most people working alone is a lot less than a full bag at a time.