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Mortar, Thinset, Grout and Concrete Are Not Interchangeable

Four cement-based products, four completely different jobs. Substituting one for another is the most common and most expensive mistake in masonry and tile work.

Two distinctions that catch people out:

• "Mortar" means two different things in tile and in masonry. Tile thin-set mortar is an adhesive. Masonry mortar is a bedding and jointing material. They are not related products and cannot be swapped.
• "Grout" also means two different things. Tile grout fills joints between tiles. Masonry grout is nearly liquid concrete poured into block cells. Buying the wrong one is a wasted trip and a wasted day.

Also watch the bag labelling. Mortar mix and mason mix already contain sand. Masonry cement and Portland cement lime do not - you add the sand. Mixing a bag of masonry cement neat gives you an unworkable, over-strong paste.

Mortar Types M, S, N, O and K

The types are named from every second letter of the phrase MASON WORK, which is the only reason the sequence looks random. They differ in compressive strength, and - more importantly - in how much movement they tolerate.

The rule that governs all of it: mortar must be weaker than the unit it holds. Mortar is the sacrificial element. When a wall moves - and every wall moves - you want the joint to crack, because a joint can be raked out and repointed. If the mortar is stronger than the brick, the stress goes into the unit instead and the faces spall off. That damage is permanent.

This is why Type S or M on soft reclaimed or historic brick is a genuine act of destruction, and why Type N is the correct default for ordinary above-grade residential brickwork despite being the weakest of the three common types. Reach for Type S when the wall is at or below grade, when it is structural block, or when wind and seismic loads demand the extra flexural bond strength.

Sand proportion is not a free variable either. In every mix above, sand runs 2-1/4 to 3 times the combined volume of cement and lime. Over-sanded mortar is weak and crumbly; under-sanded mortar shrinks and cracks.

Masonry Mortar Coverage: Bags per Brick, Block and Stone

Coverage figures published by manufacturers vary widely, because joint width, unit absorption, bedding method and how much mortar hits the ground all move the number. These are planning figures - check the bag before you order.

Two shortcuts worth memorising: about 3 bags of 80 lb mortar per 100 brick, and about 9 bags per 100 block. Over a thousand brick, budget roughly 8 bags of masonry cement plus a ton of masonry sand.

Past about 300 block or 1,000 brick, stop buying pre-mixed. Masonry cement plus bulk sand is typically half the cost, and a mixer beats opening thirty bags by hand. Below that, pre-mixed saves you a sand delivery, a tarp and a pile in the driveway.

One 80 lb bag of pre-mixed mortar yields roughly 0.6 to 0.7 cubic feet of mortar and takes about 5 quarts of water. One 70 lb bag of masonry cement mixed with about 3 cubic feet of damp sand yields roughly 3 cubic feet.

Thin-Set Coverage by Trowel Notch

Thin-set coverage is set by the trowel, not by the tile. A bigger notch leaves a thicker bed, which is why a 50 lb bag can cover 95 square feet or 25 depending on which trowel is in your hand.

Coverage requirements are a code issue, not a preference. Industry standards call for at least 80% mortar contact in dry interior areas and 95% in wet areas and all exteriors, with full support at tile corners and edges. Pull a set tile up early in the job and look at the back - if the ridges have not collapsed into a continuous bed, go up a notch size or start back-buttering.

Two more rules that decide whether the floor lasts:

• Comb in one direction, never in swirls. Straight ridges collapse and push air out. Circular trowel patterns trap air pockets under the tile, which is exactly where a large tile cracks.
• Respect the open time. If a skin has formed on the ridges and it does not transfer to your finger, the bond is gone. Scrape it off and re-comb. Hot, dry or windy conditions cut open time to a few minutes.

Use polymer-modified thin-set (ANSI A118.4 or A118.15) for almost everything - porcelain, large format, over membranes, and in wet areas. Unmodified A118.1 is only correct in specific systems where the membrane manufacturer requires it. If you are working out tile quantities too, our tile calculator handles the layout and waste side.

How to Estimate Tile Grout

Grout volume depends on four things: tile length, tile width, tile thickness and joint width. Bigger tiles have less joint per square foot; thicker tiles and wider joints have more.

Multiply that volume by roughly 0.10 to get pounds of dry grout per square foot, which builds in mixing losses and partly filled tile backs. The calculator above does it for you.

Sanded or unsanded is decided by joint width, not by preference. Use unsanded grout below 1/8 in - sand will not fit and will not pack. Use sanded at 1/8 in and above, because unsanded grout shrinks and cracks in a wide joint. At exactly 1/8 in either works, and most installers reach for sanded.

Sanded grout can scratch polished marble and some glass tile. Where that is a risk, use a fine-sanded or a high-performance grout rated for the material rather than defaulting to unsanded in a wide joint.

Epoxy grout costs three to five times more, covers less per unit, and is worth it in a commercial kitchen, a shower floor or anywhere staining is unacceptable. It also has a punishing working time. If you have never grouted before, do not learn on epoxy.

Mixing, Working Time and When to Get Help

1. Add the powder to the water, not the other way round. Roughly three quarters of the water first, then the powder, then trim the consistency with the rest.
2. Mix at a slow speed. A 1/2 in drill at 600 rpm or less. High-speed mixing whips air into the mix, which weakens it and makes thin-set slump off the notch.
3. Slake it, then remix. Let thin-set and grout stand 5 to 10 minutes so the polymers hydrate, then remix briefly without adding water. Skipping the slake is why a mix that seemed fine turns crumbly halfway through the bucket.
4. Mix only what you can use. Masonry mortar has a 90 minute working life - about 2-1/2 hours in cool weather. Thin-set is typically 2 to 4 hours in the bucket and far less on the floor. Grout is often 30 to 60 minutes.
5. Retemper masonry mortar with water only, and only once. Never add more cement, and never retemper thin-set or grout - once they start to set, adding water destroys the bond permanently.
6. Watch the weather. Below 40F masonry work needs heat and protection; above 90F pre-wet absorbent brick and work in the shade. Both extremes wreck the bond in the same way, by starving or flooding the cement of water at the wrong moment.
7. Tool masonry joints at thumbprint hardness. Too early and you smear the joint, too late and you burn it. A concave joint sheds water best.
8. Grout the tile after the thin-set has cured - normally 24 to 48 hours. Grouting over uncured thin-set traps moisture and shows up as blotchy, uneven grout colour that no amount of cleaning fixes.

When the job needs an engineer and a permit

Bag counts are the easy part of masonry. The structure is not:

• Any load-bearing masonry - a wall carrying floor, roof or beam loads - needs a structural design, and mortar type is specified by that design, not chosen at the store.
• Any wall retaining soil needs engineering above your local threshold, commonly around 4 ft measured from the bottom of the footing, though some jurisdictions set it at 3 ft or 30 in and any load above the wall usually forces engineering at any height. Verify the trigger with your building department. See our retaining wall calculator.
• Foundation and basement walls have prescribed reinforcement and grouting based on wall height, backfill and soil class. Our concrete block calculator covers the quantities; the design comes from an engineer or a code table.
• Chimneys, fireplaces and freestanding walls are permitted work in most jurisdictions, and repointing historic masonry may need approval in a conservation district.

This page gives material quantities only. It is not a structural design and does not replace an engineer or a permit.

Related calculators: brick, concrete block, retaining wall, tile, stucco, concrete bags and concrete volume.

Frequently Asked Questions

How many bags of mortar do I need per 100 bricks?

About 3 bags of 80 lb pre-mixed mortar, or 4 bags of 60 lb, at a standard 3/8 in joint. Using masonry cement instead, one 70 lb bag plus about 200 lb of sand lays roughly 125 brick. Widening the joint to 1/2 in pushes it to about 4-1/2 bags per 100 brick.

How many bags of mortar do I need per 100 blocks?

About 9 bags of 80 lb pre-mixed mortar per 100 standard blocks with face-shell bedding, or 12 bags of 60 lb. Fully bedded block takes closer to 13 bags of 80 lb. With masonry cement, budget 3-1/2 bags plus roughly 1,000 lb of sand per 100 block.

What is the difference between mortar and thinset?

They share a name and nothing else. Masonry mortar is a bedding and jointing material made of cement, lime and sand, mixed stiff enough to stand on a trowel. Thin-set is a tile adhesive made with fine sand and polymers, mixed to hold the ridges of a notched trowel. You cannot set tile in masonry mortar or lay brick in thin-set.

Which mortar type should I use, N or S?

Type N for general above-grade residential brickwork - chimneys, veneer, garden walls. Type S at or below grade, for structural block, foundations, and veneer in high wind or seismic areas. Type S is stronger at 1,800 psi versus 750, but stronger is not automatically better: mortar should always be weaker than the unit it holds so the joint cracks instead of the brick.

How much thinset do I need per square foot?

It depends entirely on the trowel notch. A 50 lb bag covers about 95 sq ft with a 1/4 x 1/4 in notch, 70 sq ft with 1/4 x 3/8, 45 sq ft with 1/2 x 1/2, and 30 sq ft with a 3/4 in U-notch. Back-buttering large format tile to reach 95% coverage cuts that to 20 to 25 sq ft per bag.

How much grout do I need for tile?

Grout volume scales with joint width, tile thickness and how small the tiles are. A 25 lb bag covers roughly 200 to 230 sq ft of 12 x 12 in tile with 1/8 in joints, about 440 sq ft of 24 x 24 in tile, and only 40 to 60 sq ft of 1 in mosaic. Doubling the joint width roughly halves the coverage.

Should I use sanded or unsanded grout?

Joint width decides it. Unsanded below 1/8 in, sanded at 1/8 in and above. Sand will not pack into a narrow joint, and unsanded grout shrinks and cracks in a wide one. At exactly 1/8 in either works. Sanded grout can scratch polished marble and some glass tile, so use a fine-sanded or high-performance grout rated for that material instead of defaulting to unsanded.

How long can I use mortar after mixing it?

Masonry mortar has a working life of about 90 minutes, or 2-1/2 hours in cool weather. Thin-set is usually 2 to 4 hours in the bucket but far less once it is combed onto the floor. Grout is often 30 to 60 minutes. Retemper masonry mortar with a little water once if it stiffens - never retemper thin-set or grout, because adding water after they start to set destroys the bond.

What is the mix ratio for mortar?

By volume, portland cement : lime : sand. Type M is 1 : 1/4 : 3-3/4, Type S is 1 : 1/2 : 4-1/2, Type N is 1 : 1 : 6, and Type O is 1 : 2 : 9. In every case the sand runs 2-1/4 to 3 times the combined volume of cement and lime. Over-sanded mortar is crumbly; under-sanded mortar shrinks and cracks. Pre-blended masonry cement is simply these proportions already made up.

How much does a bag of mortar cover in cubic feet?

An 80 lb bag of pre-mixed mortar yields roughly 0.6 to 0.7 cubic feet and takes about 5 quarts of water. A 60 lb bag yields about 0.45 to 0.5 cubic feet. A 70 lb bag of masonry cement mixed with about 3 cubic feet of damp sand yields roughly 3 cubic feet of mortar - which is why bulk mixing gets cheap fast.

Can I use mortar mix to fill concrete block cores?

No. Core fill has to be ASTM C476 masonry grout - a fluid mix with an 8 to 11 in slump that flows past the block webs and completely surrounds the rebar. Mortar is far too stiff and leaves voids exactly where the steel needs encasement. Ordinary concrete is also wrong for the same reason.

How much mortar coverage do I need under a tile?

Industry standards call for at least 80% mortar contact in dry interior areas and 95% in wet areas and all exteriors, with full support at every corner and edge. Comb in straight lines rather than swirls so the ridges collapse and push air out, and lift a tile early in the job to check the back before you have set fifty of them.

How much mortar do I need for stone veneer?

Budget roughly one 80 lb bag per 15 square feet for manufactured stone veneer, covering the setting bed and the joints. That figure rises sharply with deep raked joints and irregular natural stone, and drops for a dry-stacked look with no visible joints. A separate scratch coat over lath adds about one bag per 25 sq ft.

Why does my grout colour look blotchy?

Usually one of three things: grouting before the thin-set had cured, so trapped moisture drove different drying rates; inconsistent water in the mix between batches; or washing too soon and too wet, which pulls pigment and cement out of the surface. Wait 24 to 48 hours after setting, measure the water for every batch, and wait for the initial set before sponging.