How to Make Mortar

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 a general-purpose bagged cement made to BS EN 197-1 (usually labelled CEM I or CEM II) 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 (about 850 kg) costs around £65–£80.

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 £17–£20.¹

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. It is dosed in small measured amounts per bag of cement (see step 7). Never add washing-up liquid: it can entrain too much air and weaken the mortar despite feeling workable. A 5-litre can costs about £5–£9.

Standard Mortar Mix Ratios

Ratios are expressed as cement : lime : sand (where lime is optional) or cement : sand where no lime is used.

Where walls are built to the simple design rules in Approved Document A, the mortar should be at least strength class M4, such as a 1:1:5 to 6 mix of cement, lime and sand by volume.²

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 be repointed with lime mortar, not Portland cement.³

Tools You Need

• Cement mixer (for batches over a barrow load) or wheelbarrow and mixing board
• Mixing board (a sheet of exterior ply) for hand mixing, and a smaller spot board to hold mortar beside the work
• Gauging trowel or shovel for measuring and mixing
• Bucket for water
• Bricklaying trowel for application
• Pointing trowel (smaller) for repointing

Hiring a small cement mixer is cheap: regional hire firms list around £18 a day or £35–£45 a week, excluding VAT. For small repointing or DIY brickwork jobs, hand mixing in a wheelbarrow is adequate.

Mixing Mortar by Hand (Step-by-Step)

1. Measure by volume, use a bucket, gauging box, or shovel strokes consistently. Consistency of proportion matters more than absolute accuracy.
2. 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.
3. Form a crater, make a well in the centre of the dry mix.
4. 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.
5. 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)
6. Adjust if needed, too stiff: add a tiny amount of water. Too wet: carefully incorporate more dry mix from the pile at the edge.
7. Add plasticiser, if using, add to the water before pouring, per the manufacturer’s dilution rate (for a standard liquid plasticiser, typically 125–250 ml per 50 kg of cement).⁴

Mixing Mortar in a Cement Mixer

1. Add half the water to the rotating drum.
2. Add the sand.
3. Add the cement (and lime if using).
4. Add remaining water gradually until correct consistency is achieved.
5. 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 roughly 0.07 m³ of mixed mortar (enough for about 2.5–3 m² of half-brick wall, or roughly 150–180 bricks at about 60 bricks per m²)

For repointing, the volume required is far smaller, as joints are normally raked out to about twice the joint width (around 20 mm for a standard 10 mm joint), not the full brick depth.³

Lime Mortar for Historic Buildings

Most buildings constructed before about 1919 were built with lime mortar.³ Repointing with OPC cement mortar creates a far harder, denser joint than the original brickwork can accommodate. The result, over years and decades, is faster decay of the bricks, because the hard joint leaves the brick as the most permeable part of the wall, so the brick rather than the mortar takes the frost and salt damage.³ 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 generally suit soft Victorian brick; NHL 5 is kept for harder masonry or very exposed, wet locations. Take specialist advice on a listed building. 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.