Masonry mortar types

What combination of letters indicates mortar types?

Wild guess: M S N W R

Nosmk - Astm C270 :wink:

O N S M K - Astm C 270;)


Some of you are on the right track.

See how we all learn?
This BB is worth millions. ha. ha.


I will give my version of the correct answer this afternoon.

Marcel :slight_smile: :smiley:

What type does a “MASON WORK” with to get the stongest mix?

Thanks Barry, and would you know which one to use on glass blocks and or a chimney?

Marcel :slight_smile:

M for chimneys

Glass block has a different one, 1/1/4 I believe.

Then there is straight lime mortar, which some call L

Well, since Barry gave out the answer, I will try to summarize.

M A S O **N ** W O R K

The five typical mortar mixes designated types M,S,N,O and K are labeled so because each is an alternate letter in the term MASON WORK in descending psi strength. These designations were assigned in 1954 and replaced the mortar designations A-1, A-2, B and C.

M 2,500 psi


S 1,800 psi


N 750 psi


O 350 psi


K 75 psi

Know that a weaker psi mortar is not a “bad” or inferior mortar to one with a higher psi. A lower psi mortar has much better adhesive and sealing powers than a higher one. Mortars are selected on the balance between these attributes as to what is required for the building situation on a particular spot in the job. A type M mortar with its high strength yet poor adhesion and sealing can be a bad choice for one area of the job and just what is needed in another.

Type M mortar

This uses a 3 / 1 / 12 mix and results in a mortar with a 2,500 psi compressive strength. Type M is used for below grade load-bearing masonry work and for chimneys and brick manholes.

**Type S mortar

**This uses a 2 / 1 / 9 mix and results in a mortar with a 1,800 psi compressive strength. Type S is used for below grade work and in such areas as masonry foundation walls, brick manholes, retaining walls, sewers, brick walkways, brick pavement and brick patios.

Type N mortar

This uses a 1 / 1 / 6 mix and results in a mortar with a 750 psi compressive strength. Type N is the normal, general purpose mortar mix and can be used in above grade work in both exterior and interior load-bearing installations.

**Type O mortar

**This uses a 1 / 2 / 9 mix and results in a mortar with a 350 psi compressive strength. Type O is a lime rich mortar and is also referred to as “pointing” mortar. It is used in above grade, non-load bearing situations in both interior and exterior environments.

**Type K mortar

**This uses a 1 / 3 / 10 mix and results in a mortar with but a 75 psi compressive strength. Type K is useful only in historic preservation situations where load bearing strength is not of importance and the porous qualities of this mortar allows very little movement due to temperature and moisture fluctuations. This aids in prolonging the integrity of the old or even ancient bricks in historic structures.

**Straight lime mortar

**This uses a 0 / 1 / 3 mix and is used now only to recreate the construction and review the methods of times past or maybe for purely visual purposes. This mortar was made before Portland cement was available in many areas and so this is what was used. Sometimes you’ll see straight lime mortar called “L” mortar (for lime) but this is not designating it as “type L” mortar as in the MSNOK types. There is no “type L” mortar.

Marcel :slight_smile: :slight_smile:

very nice info Marcel! thanks

Has a permit been pulled for the work in question? :wink:

Thanks Marcel
that’s some great info.