How to specify sand and cement render
What a cement-based render clause needs to say about the background, the mix, each coat and the details around it, and how to write it in CAWS and in Uniclass.
Photo: Bill Harrison, CC BY-SA 2.0, via Wikimedia Commons. Resized and converted to WebP.
How do you specify sand and cement renders?
To specify sand and cement render, start from the background and the exposure, then set a mix that is no stronger than the background and no stronger in each coat than the one beneath it, either as a site mix designation from the UK National Annex to BS EN 13914-1 or as a factory-made mortar declared to BS EN 998-1. State each coat's thickness, the finish, the beads and the movement joints, and leave elevations and areas to the drawings.
Cement render fails slowly and expensively. A mix that is too strong for a lightweight block shrinks, cracks and lets water in behind it; a render carried over a dpc or across a junction between block and concrete cracks along the line within a winter or two. Nearly all of this is decided in the specification, before anyone mixes a bag.
This guide covers traditional two-coat and three-coat cement-based render on masonry, classified in Uniclass as Pr_35_31_64_12 Cementitious renders. Lime renders have their own code, Pr_35_31_64_48, and thin-coat silicone or acrylic finishes on insulation are a different system again, covered in the silicone render guide.
What does a sand and cement render clause need to cover?
Some properties are specific to sand and cement renders. Others belong in every product clause. A clause that leaves either out gives the contractor room to substitute.
Properties of sand and cement renders
- PrimerSpatterdash or bonding agent on dense or smooth backgrounds.
- Sample approvalSample panel fixes texture and colour before the main work.
- Movement jointsFollow structural joints; never render across them.
- FinishIntegral colour or decoration; pigment batch consistency.
- MaterialCement, lime or plasticiser and sand to the relevant standards.
- Mix proportionsMix designation, or CS and W classes for premix.
- ThicknessPer coat; final coat thinner than the undercoat.
- TolerancesLine and flatness under a straightedge, especially for smooth finishes.
- AccessoriesStainless steel beads, stops and bellcast drips.
- ReinforcementLath over dissimilar backgrounds and at opening corners.
- Product typeSite-mixed or factory-made; multicoat or one-coat.
- Coat build-upScratch, float and finish coats with curing between.
- Reaction to fire classificationNever guessA1 only if organic content is low enough.
- Surface textureFloated, scraped, roughcast or dry dash.
What every product clause needs
- Manufacturer and product referenceNamed premix, or a site mix with named cement and sand sources.
- Product standardBS EN 998-1 for factory-made render; constituent standards for site mixes.
- Installation methodBS EN 13914-1 and the premix maker's instructions.
- Technical submittalsDoP, sand grading and mix proportions before work starts.
- Operation and maintenance informationRepair method and any decoration regime.
- SubstrateBackground type and strength set the mix designation.
- Application conditionsNo rendering in frost, heavy rain or strong drying wind.
Uniclass Pr_35_31_64_12 Sand and cement render. Not every property applies to every project; where one does not, say so rather than leaving it blank.
Fields marked Never guess (reaction to fire classification) depend on the project and should never be filled from a default or a previous job. If the information is not available yet, the clause should say so with a TBC and a reason.
Which decisions does the clause depend on?
Most properties follow from the product you choose. These depend on the project, so they need a decision from the designer before the clause is written.
- 01
Mix strength against the background
MixThe rule every render text repeats, and many specifications ignore, is that render should be no stronger than the background and each coat no stronger than the one before. A rich cement mix on aircrete or lightweight block shrinks more than the block can restrain and cracks in a map pattern that lets rain in and keeps it there. The UK National Annex to BS EN 13914-1 gives site mixes as designations (i) to (v), from strong to weak, and suggests which suits which background. Factory-made renders are declared instead by compressive strength category, CS I to CS IV, to BS EN 998-1.
How to decide
- Dense concrete or engineering brick: a stronger designation is acceptable, but check the finish coat still steps down.
- Lightweight aggregate block and most common brick: designation (iii), cement:lime:sand around 1:1:6.
- Aircrete and soft brick: designation (iv) or a premix the block maker approves.
- Premix: ask for the CS category and match it to the background as you would a site mix.
Write it asMix: Designation (iii) to the UK National Annex to BS EN 13914-1, cement:lime:sand 1:1:6 by volume, both coats. - 02
Coats, thicknesses and curing
CoatsTwo-coat work is normal on block; three coats are used where the background is uneven or the exposure is severe. The undercoat takes out irregularities and evens suction, and the final coat is thinner so it shrinks less than the coat it sits on. Each coat needs time to cure and shrink before the next goes on, which is where programme pressure does its damage. Trade guidance from the mortar industry puts a typical undercoat at about 9 to 13 mm with a thinner final coat, so a clause that asks for a single 20 mm coat on block is asking for cracking.
How to decide
- Set a thickness per coat, excluding dubbing out, and say which coat is scratched or combed for key.
- Require each coat to be cured and to have completed most of its drying shrinkage before the next.
- Severe or very severe exposure: consider three coats or a premix with a declared water absorption class.
- Point to the M20 curing clause rather than restating it in every type clause.
Write it asUndercoat: 12 mm, scratched for key. Final coat: 8 mm. Curing: As clause 880. - 03
Background, preparation and dissimilar materials
BackgroundMost render cracks start in the background. Strong smooth concrete gives no key and needs a spatterdash or bonding coat. Very absorbent block pulls water out of the undercoat before it can cure. Where block meets a concrete column or a steel frame, the two move differently and the render cracks along the line unless the junction is bridged with lath over an isolating layer or detailed as a movement joint. In persistently wet conditions, sulfates from clay brick or the surroundings can attack Portland cement render, which is why sulfate-resisting cement is used on salt-bearing brickwork.
How to decide
- Name the background in the type clause, not just 'masonry'.
- Dense, smooth backgrounds: spatterdash or the bonding agent the premix maker recommends.
- Junctions of dissimilar backgrounds in the same plane: stainless lath over an isolating layer, as the crack-control clause.
- Salt-bearing or wet brickwork: sulfate-resisting cement and protection from saturation.
Write it asBackground: Lightweight aggregate concrete blockwork, raked joints. Crack control at junctions with concrete: As clause 646. - 04
Reaction to fire
Never guessA sand, cement and lime render with very little organic content can be classed A1 without testing; the threshold the rules use is 1% homogeneously distributed organic material. Polymer-modified premixes, renders with organic fibres and many coloured finishes can exceed that and then need a tested class to BS EN 13501-1. On a relevant building under regulation 7(2), the render is a finish on the external wall and has to be A2-s1,d0 or A1, and the same applies to anything behind it that forms part of the wall. Do not let a polymer-modified substitute arrive on the strength of the word 'cement'.
How to decide
- Site mixes of cement, lime and sand: A1 by classification without further testing.
- Premixes: ask for the declared class on the DoP, not the brochure.
- Relevant buildings and residential buildings with a storey at 11 m or more: check the whole build-up with the fire engineer.
- Any substitution: compare the substitute's class with the clause before approving.
Write it asReaction to fire: Class A1 to BS EN 13501-1. Premixed products: submit declaration of performance. - 05
Movement joints, beads and the dpc
DetailingRender cannot take movement that the wall behind it is designed to take. Where the masonry has a movement joint, the render needs one in the same place, formed with a proprietary bead rather than a trowel cut. The other recurring defect is render carried down past the dpc, which bridges it and draws ground moisture up into the wall. A stop bead or bellcast bead above the dpc stops this and gives a drip. External beads should be stainless steel or a plastic the render system holder accepts; galvanised beads rust through thin final coats.
How to decide
- Copy every masonry movement joint into the render and show them on the elevations.
- Bellcast or stop bead at least at dpc level, with the render stopped above it.
- Stainless steel external beads to BS EN 13658-2, or plastic beads the premix maker approves.
- Long uninterrupted elevations: ask for the joint layout as a submittal if the drawings do not fix it.
Write it asMovement joints: To match masonry movement joints, formed with stainless steel movement beads, positions as elevations.
The values under "Write it as" are examples. The right value depends on the location, the use and the fire strategy.
Which standards should the clause cite?
Cite the product standard the declaration of performance refers to, the code of practice for installation, and the test method behind each value you set. A value without its test method cannot be checked against a submittal.
- BS EN 998-1
- Specification for mortar for masonry. Rendering and plastering mortar
The product standard for factory-made rendering mortar. It sets the mortar types (general purpose, lightweight, coloured, one-coat, renovation and thermal insulating), compressive strength categories CS I to CS IV and capillary water absorption classes W0 to W2.
In the clause Cite it for premixes with the classes you need: "Factory-made render to BS EN 998-1, type GP, CS II, W2".
- BS EN 13914-1
- Design, preparation and application of external rendering and internal plastering. External rendering
Design and workmanship for external render: backgrounds, preparation, coats, curing and detailing. The UK National Annex gives the site mix designations and an exposure classification based on driving rain.
In the clause Use it for workmanship and the mix: "Mix designation (iii) to the UK National Annex to BS EN 13914-1".
- BS EN 197-1
- Cement. Composition, specifications and conformity criteria for common cements
Common cements by type and strength class, including the sulfate-resisting variants used where render is applied to salt-bearing brickwork.
In the clause "Cement: To BS EN 197-1, CEM I or CEM II, strength class 42.5".
- BS EN 13139
- Aggregates for mortar
Sand for mortar and render, including grading and fines categories. Poorly graded or clayey sand is a common cause of shrinkage cracking.
In the clause "Sand: To BS EN 13139, grading 0/4, from one source".
- BS EN 459-1
- Building lime. Definitions, specifications and conformity criteria
Air limes and hydraulic limes. In a cement:lime:sand render the lime is usually a hydrated calcium lime, which improves workability and reduces the stiffness of the set render.
In the clause "Lime: Hydrated lime to BS EN 459-1, type CL 90-S".
- BS EN 13658-2
- Metal lath and beads. Definitions, requirements and test methods. External rendering
Metal lath and beads for external render, including corrosion protection requirements.
In the clause "Beads and stops: Stainless steel, to BS EN 13658-2".
- BS EN 13501-1
- Fire classification of construction products and building elements. Classification using data from reaction to fire tests
Euroclasses A1 to F. Cement and lime renders with low organic content are A1 without testing; polymer-modified products need a declared class.
In the clause "Reaction to fire: Class A1 to BS EN 13501-1".
- NHBC Standards, chapter 6.11
- Render
The warranty provider's technical requirements for render on new homes, including backgrounds, dpc details and junctions with roofs.
In the clause On NHBC-registered housing, cite it alongside BS EN 13914-1: "Workmanship: To BS EN 13914-1 and NHBC Standards chapter 6.11".
Confirm the current edition of each standard at every issue. Avoice's State of UK Specifications 2026 found a withdrawn standard cited in 40% of specifications at contract issue.
For the regulations behind these choices, see Approved Document B explained, or search the Avoice Regulations library.
How do you write it in CAWS?
In CAWS, cement render is written in M20, alongside plaster and roughcast. Each render is a type clause in the 100s that reads as a recipe for one complete build-up: the background and its preparation, the mortar, the mix and thickness of each coat, the finish and the accessories. Real M20 specifications on public tender sites use titles such as "Cement:sand render to external blockwork" for these clauses, one per background or finish.
The type clause should not repeat workmanship. Materials, cold weather, suitability of substrates, beads, crack control, application, undercoats, finishes and curing are written once in the later M20 clauses and called up by number. That makes the cross-references part of the specification: a type clause that says "As clause 646" for crack control is only as good as clause 646.
- Background
- Lightweight aggregate concrete blockwork, joints raked
- Preparation
- As clause 510; dampen to even out suction
- Cement
- To BS EN 197-1, as clause 438
- Lime
- Hydrated lime to BS EN 459-1
- Sand
- To BS EN 13139, as clause 440
- Undercoat mix
- Designation (iii) to the UK National Annex to BS EN 13914-1, 1:1:6
- Undercoat thickness
- 12 mm, excluding dubbing out, scratched for key
- Final coat mix
- As undercoat
- Final coat thickness
- 8 mm
- Finish
- TBC Floated or scraped finish not yet chosen; depends on the planning condition on materials. Resolve before the sample panel.
- Beads and stops
- Stainless steel to BS EN 13658-2, as clause 636
- Bellcast bead
- At dpc level, render stopped above dpc
- Movement joints
- Stainless steel movement beads at masonry movement joints, positions as elevations
- Crack control
- At junctions with concrete, as clause 646
- Curing
- As clause 880
- Reaction to fire
- Class A1 to BS EN 13501-1
- Decoration
- Masonry paint, as section M60
Illustrative, not from a real project. One property is left as a TBC with a reason so it can be tracked and closed before issue.
Clauses the type clause relies on
- M20/497
- Cold weather No frozen materials or frost-bound backgrounds; temperature limits for external work.
- M20/510
- Suitability of substrates Soundness, cleanliness and tolerances of the background before rendering.
- M20/636
- Beads/ stops for external use Bead material for external work, usually stainless steel.
- M20/646
- Crack control at junctions between dissimilar solid substrates Isolating layer and lathing widths where block meets concrete.
- M20/810
- Application generally Continuous application between angles and joints, and control of drying.
- M20/880
- Curing and drying Keeping each coat damp and letting shrinkage happen before the next.
Clause numbers vary between NBS versions and practice masters; the general clause numbers here are taken from published project M20 specifications, so check each one exists in your M20.
Photo: Elisabeth Carrera (WMNO), CC BY-SA 4.0, via Wikimedia Commons. Resized and converted to WebP.
How do you write it in Uniclass?
In Uniclass the rendered finish is a system, Ss_25_45_70_55 Multicoat render systems, under render and roughcast coating systems. The system clause carries the background, its preparation, the coats with their thicknesses, the beads and movement joints, curing and the finished appearance.
The render itself is a product clause under Pr_35_31_64_12 Cementitious renders, written either as a premix declared to BS EN 998-1 or as the mix of a site-made mortar. Where one render goes on block in one place and on brick in another, two system clauses with different preparation call up the same product clause. A one-coat premix belongs under Ss_25_45_70_60 One-coat render systems instead.
- Description
- Two-coat cement:lime:sand render on external blockwork
- Background
- Lightweight aggregate concrete blockwork, joints raked, suction evened
- Render
- Pr_35_31_64_12 Cementitious renders (R01)
- Coats
- Undercoat 12 mm scratched, final coat 8 mm
- Beads
- Stainless steel stop, angle and bellcast beads
- Movement joints
- At masonry movement joints, positions as elevations
- Curing
- Each coat kept damp and protected from frost and drying wind
- Reaction to fire of finished system
- Class A1 to BS EN 13501-1
- Type
- Site-mixed cement:lime:sand render
- Cement
- To BS EN 197-1
- Lime
- Hydrated lime to BS EN 459-1
- Sand
- To BS EN 13139, grading 0/4, one source
- Mix
- Designation (iii), 1:1:6 by volume
- Admixtures
- Air entraining plasticiser only if approved; no calcium chloride
- Finish
- TBC Floated or scraped finish not yet chosen; depends on the planning condition on materials. Resolve before the sample panel.
- Organic content
- Not more than 1.0%, for Class A1 without testing
- Alternative
- Factory-made render to BS EN 998-1, CS category to suit the background, submit proposals
Illustrative. Performance of the finished system sits in the system clause; properties of the product sit in the product clause.
In NBS Chorus the product clause also carries an NBS reference (for cementitious render, 45-55-60/330); use whichever your masters use, but keep CAWS and Uniclass references out of the same specification.
What goes in the spec, and what goes in the schedule?
The specification defines each type once. The schedule and drawings say where each type goes.
In the specification
What the product is and how it performs and is installed. Written once per type.
In the schedule or drawings
Where each type goes and how much of it. Changes with every plan revision.
Give each render a type mark (R01, R02) and let the elevations show where it goes and where the joints fall. Joint positions belong on drawings, because a clause cannot describe them well enough to price.
What are the common mistakes?
Most problems are a missing property, a property filled with the wrong kind of value, or a decision left silently open.
- A strong mix on a weak block. "1:3 cement:sand" on aircrete or lightweight block cracks. Match the mix designation to the background and step down, not up, in later coats.
- Render carried over the dpc. It bridges the dpc and wicks ground moisture into the wall. Stop the render on a bellcast bead above it.
- No movement joints in the render. The masonry joints are drawn but the render runs straight across them. Every joint in the wall needs a matching bead in the render.
- Galvanised beads outside. They rust through a thin final coat and stain the face. Specify stainless steel or an approved plastic bead.
- One thick coat to save a visit. A single heavy coat shrinks unevenly and cracks. Set the coats and thicknesses and require curing between them.
- A polymer premix accepted as 'cement render'. Some premixes are no longer A1. Ask for the declared reaction to fire class before approving the substitute.
How Avoice helps
Avoice's spec writer drafts render clauses from the elevations and the wall types already on the project, and marks what it cannot settle.
- Drafts from your drawings and schedules
It reads the render types (R01, R02) and their backgrounds from the elevations and wall build-ups, then sets up the M20 type clause or the Uniclass system and product clauses with the coats, beads and finish.
- Leaves a TBC where the information runs out
If the finish or colour is waiting on a planning condition, the clause says TBC with that reason, and the item joins the project TBC register so it is closed before tender.
- Writes in your format and your style
Choose CAWS or Uniclass. Avoice adapts your practice's own M20 master clauses rather than replacing them with generic text.
- Checks the clause against everything else
QA/QC flags an R03 on the elevations that the specification never defines, a type clause pointing at a crack-control clause that has been deleted, or a withdrawn standard in the materials clauses.
- Keeps product data in one place
Premix data sheets and declarations of performance sit in the Product Room linked to the clause, so a proposed substitute can be checked against the CS category and fire class the clause asks for.
Related guides: How to specify silicone render, How to specify lime plaster, How to specify masonry paint.
Software can draft and check. The designer still chooses the product and is responsible for the specification.
Draft your sand and cement render clause in Avoice
Upload your drawings and schedules. Avoice drafts the clause in CAWS or Uniclass, cites the standards and flags anything your information does not settle.
This guide is for general education and is not professional advice. Check current editions of standards and Approved Documents, the manufacturer's data, and the requirements of your project before relying on any clause.