Mortar is the bedding and jointing material that holds masonry together. Getting the mix right matters considerably more than most first-time DIYers expect — a mortar that is too strong can cause the surrounding brickwork to crack and spall; too weak and it will fail to bond and wash out. The correct mix depends on the application and, crucially, the type of brick or block being laid.
This guide covers the standard mixes and methods for UK domestic building work: brickwork, repointing, and blockwork. Specialist applications such as fire-rated mortar for flues, acid-resistant mortar, or lime putty work for pre-1919 buildings require different approaches covered separately.
Understanding the Ingredients
Ordinary Portland Cement (OPC) — the binding agent. It reacts chemically with water and hardens. Use CEM I (formerly BSEN 197) 42.5N or 52.5N grade from any builders’ merchant. A 25 kg bag costs £6–£10 in 2026.
Sand — provides the bulk of the mortar volume, controls workability and strength, and fills the gaps between aggregate. Two types matter:
- Soft (building) sand — fine, rounded grains; used for mortar, pointing, and plastering. Gives a smooth, workable mix.
- Sharp (concreting) sand — coarser, angular grains; used for concrete, screed, and render, sometimes combined with soft sand for mortar.
- For standard brickwork mortar, soft sand or a 50:50 blend of soft and sharp is correct. Pure sharp sand makes a stiff, difficult-to-work mortar. A 25 kg bag of soft sand costs £2–£5; a bulk bag (850 kg) costs £40–£70.
Lime (optional) — hydrated lime (calcium hydroxide) added to a cement mortar improves workability, reduces cracking due to shrinkage, and adds flexibility. Particularly important for soft, handmade bricks. Hydraulic lime (NHL 2, 3.5, or 5) can replace cement entirely for traditional lime-based mortars. A 25 kg bag of hydrated lime costs £8–£14.
Water — must be clean and potable (tap water in the UK is suitable). The amount determines consistency; add gradually.
Plasticiser — a liquid additive (typically air-entraining plasticiser) that improves workability without adding more water. A few millilitres per mix. Never add washing-up liquid — it weakens the final strength despite feeling workable. A 5 L bottle costs £12–£20.
Standard Mortar Mix Ratios
Ratios are expressed as cement : lime : sand (where lime is optional) or cement : sand where no lime is used.
| Application | Mix ratio (cement : lime : sand) | Equivalent (cement : sand only) | Strength class |
|---|---|---|---|
| Engineering brick, hard stock brick | 1 : 0 : 3 | 1:3 | M12 (strong) |
| Standard brickwork, new build | 1 : 0.5 : 4.5 | 1:4 | M6 |
| General brickwork, blockwork | 1 : 1 : 5.5 | 1:5 | M4 |
| Repointing (medium hardness brick) | 1 : 1 : 5.5 | 1:5 | M4 |
| Soft, old, or handmade brick | 1 : 2 : 9 | 1:7 | M2 (weak, flexible) |
| Dense blockwork, concrete blocks | 1 : 0 : 4 | 1:4 | M6 |
| Render (undercoat) | 1 : 0.5 : 4 | — | — |
| Render (finishing coat) | 1 : 1 : 6 | — | — |
Important: never use a mortar stronger than the brick or block being laid. A hard cement mortar (1:3) on a soft handmade brick will cause the faces of the bricks to spall off as the mortar resists thermal and moisture movement more than the brick can. Pre-1919 buildings with original lime mortar should always be repointed with lime mortar, not Portland cement.
Tools You Need
- Cement mixer (for batches over a barrow load) or wheelbarrow and mixing board
- Spot board (a flat board, typically 1.2 × 1.2 m ply) for hand mixing
- Gauging trowel or shovel for measuring and mixing
- Bucket for water
- Bricklaying trowel for application
- Pointing trowel (smaller) for repointing
A concrete mixer hire costs £40–£70/day. For small repointing or DIY brickwork jobs, hand mixing in a wheelbarrow is adequate.
Mixing Mortar by Hand (Step-by-Step)
- Measure by volume — use a bucket, gauging box, or shovel strokes consistently. Consistency of proportion matters more than absolute accuracy.
- Dry mix first — measure out sand and cement (and lime if using). Combine the dry ingredients on the spot board or in the wheelbarrow. Mix thoroughly until the colour is even throughout — no streaks of white cement or grey sand.
- Form a crater — make a well in the centre of the dry mix.
- Add water gradually — pour a small amount into the crater. Work in from the sides, incorporating dry material from the outside in. Do not add all the water at once.
- Check consistency — properly mixed mortar should:
- Hold its shape when balled in the hand
- Slide cleanly off the trowel without slumping
- Leave a slight sheen on the trowel
- Not crack when squeezed (too dry) or slump to a liquid (too wet)
- Adjust if needed — too stiff: add a tiny amount of water. Too wet: carefully incorporate more dry mix from the pile at the edge.
- Add plasticiser — if using, add to the water before pouring, per the manufacturer’s dilution rate (typically 50–75 ml per 50 kg cement).
Mixing Mortar in a Cement Mixer
- Add half the water to the rotating drum.
- Add the sand.
- Add the cement (and lime if using).
- Add remaining water gradually until correct consistency is achieved.
- Allow to mix for 3–4 minutes minimum before testing consistency.
Clean the drum with a mix of aggregate and water immediately after use. Hardened mortar in a drum is very difficult to remove.
Common Mistakes
Adding too much water is the single most common error. Wet mortar is easy to work but weak when cured, prone to shrinkage cracking, and slow to stiffen. The correct consistency feels stiffer than most beginners expect.
Using pure sharp sand produces a mortar that is almost impossibly stiff to work for brickwork. Use soft (building) sand or a blend.
Not pre-blending dry ingredients results in an uneven mix where pockets of unmixed cement create localised hard spots and weak spots.
Using mortar past its working life — once mortar begins to stiffen (typically 1–2 hours depending on temperature), do not add water and retemper it. The cement has begun its initial set. Retempered mortar has significantly reduced strength. Discard and make a fresh batch.
Laying in freezing conditions — mortar must not freeze during its initial cure period (typically 24 hours). If temperatures are below 5°C, either avoid laying or use cold-weather precautions (frost blankets over fresh work, pre-warming the mixing water, using rapid-hardening cement).
How Much Mortar Do You Need?
A rule of thumb for standard brickwork with 10 mm joints:
- 1 m² of brickwork (using standard 215 × 102.5 × 65 mm bricks) requires approximately 0.025 m³ of mortar
- 1 m³ of mortar covers approximately 40 m² of standard brickwork at 10 mm joints
- 1 × 25 kg cement + 4 × 25 kg soft sand makes approximately 0.08 m³ of mixed mortar (sufficient for around 3 m² of brickwork)
For repointing, the volume required is far smaller as you are only filling the joint to a depth of 15–20 mm, not the full brick depth.
Lime Mortar for Historic Buildings
Any building constructed before approximately 1920 was almost certainly built with lime mortar. Repointing with OPC cement mortar creates a far harder joint than the original brickwork can accommodate. The result — over years and decades — is that the bricks themselves crack and spall as the mortar prevents thermal and moisture movement. This destroys the face of period brickwork irreversibly.
For pre-1919 buildings, use a natural hydraulic lime (NHL) mortar or a lime putty mortar, matched in strength and composition to the original. NHL 2 or NHL 3.5 are appropriate for most soft Victorian bricks; NHL 5 for harder engineering brick or exposed, wet locations. Lime mortars take longer to set, require protection from frost for longer, and have different workability characteristics — but they are the correct and conservation-appropriate material.
