External solid plastering: overview

Transcription

1 External solid plastering: overview Render gives a weatherproof surface that protects the building and provides a decorative finish. Traditional render is applied by hand; modern render can also be applied by machine. 18th-century render was made of breathable materials but took a long time to set. 20th-century mortars had a quicker setting time essential in housebuilding booms. Modern renders are better in quality, uniform in colour and have a consistent strength. Period constructions still use traditional lime render. There are several issues to consider when choosing a render type.

2 Materials used for external rendering Sand Sand is used as an aggregate. The types of sand used are: pit sand (quarried inland; red in colour) dredged sand (from river or sea beds; dark yellow) artificially created sand (di erent colours). Sand should be clean and not contain more than 10% of silt. Silt test To conduct a silt test you need: a sand sample water salt a glass jar or measuring cylinder a tape measure.

3 Silt test NVQ and Technical Certificate Level 2 Plastering

4 Storage of sand Sand should be: stored in a clean bay covered with tarpaulin to reduce contamination from animals, leaves and other aggregates.

5 Cement Two types of cement are used with external renders: Ordinary Portland cement 75% limestone + 25% clay, producing a grey colour White Portland cement 75% limestone + 25% white china clay, producing a white colour. Setting time for cement The setting of cement is a chemical reaction, a ected by temperature. Cement will set quicker in warmer climates. The initial set takes 45 minutes. The final set takes under 10 hours.

6 Lime Used as a plasticiser in external rendering. Helps to reduce cracking, which also reduces loss of adhesion.

7 Additives Commonly used in cement-based render mixes. They are liquid or powder ingredients. They should be carefully measured before adding to the mix. Di erent types of additive for external cement-based render Plasticiser improves workability and ease of application. Waterproofer improves workability, forms waterproof barrier and equalises suction. Retarder added in warm conditions to slow down setting. Accelerator added in cold conditions to speed up setting.

8 Preparation of backgrounds Type of background Hack o old work Rake out joints Brush and dampen Apply slurry adhesive New brickwork Old brickwork New blockwork Old stonework Concrete surface

9 Bonding adhesives for sand, cement and lime mixes Slurry coat or spatterdash coat Cementitious polymermodified is a modern form of a slurry coat Slurry coats can be applied by a roller, trowel, brush or Tyrolean machine SBR latex-based material

10 Mixing and applying render It is essential to gauge all mix materials correctly with a bucket measure. This will ensure that mix will be of correct strength and colour throughout work.

11 Ratios of the render mix: why use them? Ratios are designed to help the plasterer know the right quantity of each material when mixing. Mixing the render too strong may cause subsequent stress resulting in cracking and blowing. Mixing the render too weak may cause no initial set and subsequent crumble.

12 Ratios for dubbing-out mixes 5 sand, 1 cement, 1 lime, plasticiser additive or 4 sand, 1 cement, lime, plasticiser additive Ratios for scratch coat 1 sand, 4 cement, waterproof additive Ratio for topcoats 5 or 6 sand, 1 cement, 1 hydrated lime, plasticiser additive Ratios for hydraulic lime renders Coat Hydraulic lime Sharp sand Dubbing-out 1 1 Scratch coat 1 2 Topcoat 1 2

13 Two-coat work Two-coat work comprises: the scratch coat the topcoat. Scratch coat Used to straighten out background and is backing coat for render finish. Includes a waterproofer. Thickness is 9 12 mm. Should be ruled out as flat as possible. Keyed with a comb scratcher.

14 Methods of forming a scratch coat to angles Method 1 1. Hold timber rule plumb on one side of wall to thickness mm. 2. Apply render up to timber rule. 3. Slide rule upwards, away from angle. 4. Hold rule on previously laid render to approx mm. 5. Apply render up to rule. 6. Slide rule upwards, away from angle. Method 2 1. Apply render to both sides of angle, approx mm. 2. Hold timber rule on one side, plumb and apply trowel to face timber. 3. Slide rule up, away from angle. 4. Hold rule plumb on opposite angle; repeat process with trowel. 5. Slide rule up, away from angle.

15 Bellcast Bellcast is formed by fixing metal trims above window and door openings, and at DPC level. This is also known as a drip or weathering. Bellcasts are also formed by fixing timber battens, which are removed when material is set. Types of metal trim for external render Bell bead forms drip above windows and doors; stops bridging to DPC level. Stop bead fixed to facing brick or timber frames to form panels. Angle bead gives straightness and strength on external corners. Expansion bead allows movement when two walls meet as straight joint.

16 Topcoat This is applied to scratch coat to form finished render. But first, the reveals must be completed. 1. Run trowel over scratched surface to remove any hard sand or cement. 2. Fix/hold timber rule on main face of wall, plumbing or lining to window or doorframe to mm. 3. Apply topcoat to reveals. 4. When it begins to set, consolidate surface with plastic or wooden float. 5. Sponge or scour over surface to close face, if necessary.

17 Plain face/scoured finish Thickness of topcoat should be as tight as possible, usually mm. Rule out without screeds to a flat smooth finish. Consolidate the face of the render coat. Use plastic float (a sponge or sponge-faced float can be used). Same finish used on Plinths and quoin stones, etc.

18 Three-coat work Three-coat work can be listed as scratch coat, topcoat, decorative coat. It can also be used on EML. Dubbing-out coat can also be applied to an uneven background. Cement-based materials should be left for a minimum of 24 hours between coats. Principle of never applying a strong mix on top of a weak mix.