Question 1

What mortar mix should I use for general bricklaying above DPC?

Accepted Answer

For general bricklaying above DPC, use a 1:1:6 mix (cement:lime:sand) or a 1:5-6 mix with plasticiser (cement:sand + plasticiser). This is BS 5628 Designation (iii) mortar, suitable for most above-ground brickwork including garden walls, house walls, and internal blockwork. Lime or plasticiser makes the mortar more workable and allows slight movement without cracking.

Question 2

What mortar mix should I use below DPC?

Accepted Answer

Below DPC, use a stronger 1:0.5:4-4.5 mix (cement:lime:sand) or 1:3-4 mix with plasticiser. This is BS 5628 Designation (ii) mortar. For very exposed locations or engineering brick work, use a 1:0.25:3 mix or 1:3 straight cement:sand — this is Designation (i), the strongest standard mix. Below DPC mortar must resist ground moisture and be durable against frost.

Question 3

Can I use washing-up liquid instead of plasticiser?

Accepted Answer

This is a common practice but is not recommended. Washing-up liquid creates air bubbles that weaken the mortar and can reduce its strength by up to 50%. Purpose-made plasticisers (such as Feb Mix or Sika Mortar Plasticiser) are designed to entrain a controlled amount of air without significantly reducing strength. The cost of proper plasticiser is minimal compared to the risk of weak mortar.

Question 4

What is the difference between mortar and render mix?

Accepted Answer

Render mixes are similar to mortar but typically richer (more cement) to provide weather resistance and a durable external finish. A standard render scratch coat uses 1:0.5:4-4.5 or 1:4 with plasticiser. The finish coat is slightly weaker (1:1:6 or 1:5-6) so that any cracking occurs in the finish coat rather than the base coat, making it easier to repair. Render should always be weaker than the substrate it is applied to.

Question 5

How much mortar do I need per square metre of brickwork?

Accepted Answer

For half-brick (stretcher bond) walls using standard metric bricks with 10 mm joints, you need approximately 0.02 m³ of mortar per m² of brickwork. For one-brick (English or Flemish bond) walls, approximately 0.04 m³ per m². For blockwork using 440 x 215 mm blocks, approximately 0.01 m³ per m². These figures assume 10 mm joints and include minimal waste.

BS 5628 Designation	Cement:Lime:Sand	Cement:Sand + Plasticiser	Application
(i) — Strongest	1 : 0.25 : 3	1 : 3	Engineering bricks, manholes, retaining walls
(ii)	1 : 0.5 : 4-4.5	1 : 3-4	Below DPC, parapets, freestanding walls
(iii) — General purpose	1 : 1 : 5-6	1 : 5-6	Above DPC brickwork, general blockwork
(iv) — Weakest	1 : 2 : 8-9	1 : 7-8	Internal non-load-bearing walls only

Coat	Cement:Lime:Sand	Thickness (mm)	Notes
Scratch coat (1st)	1 : 0.5 : 4-4.5	8-12	Scratched to form key for next coat
Floating coat (2nd)	1 : 1 : 5-6	6-8	Straightened and ruled
Finish coat (3rd)	1 : 1 : 5-6	3-6	Sponge or wood float finish; heritage three-coat work
Pebbledash base	1 : 0.5 : 4-4.5	12-15	Aggregate thrown onto wet base coat

Location	Mix Ratio	Notes
General repointing	1 : 1 : 5-6	Match existing mortar strength
Exposed/severe locations	1 : 0.5 : 4-4.5	Chimneys, parapets, coastal
Historic/lime buildings	1 : 2.5-3 (lime:sand)	NHL 3.5 or NHL 5 hydraulic lime

Type	Mix Ratio	Thickness (mm)	Application
Bonded screed	1 : 3 (cement:sand)	25-40	Bonded to concrete slab with PVA/SBR
Unbonded screed	1 : 4 (cement:sand)	50-65	Over DPM on concrete slab
Floating screed	1 : 4 (cement:sand)	65-75	Over insulation (underfloor heating)

Mortar Mix Ratios — Bricklaying, Blockwork, Rendering and Pointing

Bricklaying and Blockwork Mortar Mixes

Rendering Mixes

Pointing Mixes

Floor Screed Mixes

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