What is Mortar?
Mortar is a workable paste made from cement, sand, and water that binds masonry units (bricks, blocks, and stones) together and fills the gaps between them. Unlike concrete which is a structural material on its own, mortar is primarily a bonding agent designed to hold masonry units together while accommodating slight movements and providing a weather-tight seal.
The composition of mortar determines its strength, flexibility, and durability. Proper mortar selection and calculation are critical for structural integrity and long-term performance of masonry walls, chimneys, and other structures.
Mortar Types
| Type | Compressive Strength (psi) | Common Uses |
|---|---|---|
| Type M | 2,500 | Below-grade, foundations, retaining walls |
| Type S | 1,800 | Structural walls, exterior at/below grade |
| Type N | 750 | General-purpose, above-grade exterior |
| Type O | 350 | Interior, non-load-bearing walls |
| Type K | 75 | Historic restoration |
How to Calculate Mortar
A simplified rule of thumb: approximately 7 bags (80 lb) of mortar per 1,000 standard bricks with 3/8 inch joints.
Coverage Rates
| Unit Type | Units per sq ft | 80 lb Bags per 100 sq ft |
|---|---|---|
| Standard Brick | 6.75 | 5–7 |
| Modular Brick | 6.86 | 5–7 |
| 8" CMU Block | 1.125 | 3–4 |
| 12" CMU Block | 1.125 | 4–5 |
Mixing Ratios
- Type M: 1 part Portland cement : 0.25 parts lime : 3.5 parts sand
- Type S: 1 part Portland cement : 0.5 parts lime : 4.5 parts sand
- Type N: 1 part Portland cement : 1 part lime : 6 parts sand
- Type O: 1 part Portland cement : 2 parts lime : 9 parts sand
Worked Example
A wall 20 ft long × 8 ft high using standard bricks with 3/8" joints:
Bricks = 160 × 6.75 = 1,080 bricks
Mortar = (160 / 100) × 6 bags = 9.6 bags
With 10% waste: ~11 bags (80 lb)
Frequently Asked Questions
How much mortar do I need per brick?
On average, you need approximately 0.006 to 0.01 cubic feet of mortar per standard brick, depending on joint thickness. One 80 lb bag of pre-mixed mortar typically lays about 35 to 40 standard bricks.
What is the best mortar type for a garden wall?
Type N mortar is the most common choice for above-grade exterior garden walls. It offers good bonding strength and flexibility to accommodate minor settling and thermal movement. For walls below grade or retaining walls, use Type S or Type M.
How long does mortar take to cure?
Mortar reaches initial set in 24 to 48 hours and achieves most of its strength within 7 days. Full cure (28 days) is needed for maximum strength. Keep mortar moist during the first 72 hours to ensure proper hydration and curing.
Can I mix mortar ahead of time?
Mixed mortar should be used within 1.5 to 2 hours. After this time it begins to set and loses workability. Do not add more water to re-temper mortar that has begun to harden, as this weakens the bond strength significantly.