Architectural Guidance PLASTERING

Architectural Guidance PLASTERING

Friday, April 11, 2008

PLASTERING

INTERNAL PLASTERING

Plastering is a relatively cheap means of providing a durable hygienic surface to walls and ceilings.

First class plastering is done in three coats

Rendering coat-10mm

Floating coat -6mm

setting coat -3mm

Hence giving a total thickness of 19mm

Now for much general building work, the render coat is omitted, the floating coat is made thicker and the overall thickness is 16mm. Which is sufficient for all but very rough walls.

MATERIALS USED

Formerly, lime plaster was the basic material for this purpose, mixed with sand and more latterly, cement, for certain layers; the constituents are measured by volume.

The lime used was non-hydraulic or fat lime prepared in a pit on the site one month before use by mixing Quicklime, obtained by burning Limestone in a kiln.

This was a lengthy procedure and hence lime plasters have been replaced by Calcium Sulphate or Gypsum plasters.

ADVANTAGES

set within a few hours

produce a harder finish

expand slightly on setting

The addition of lime reduces hardness and in final coats decoration by oil paints cannot proceed until the wall has dried out ; this may take 6 to 12 months.

CALCIUM SULPHATE PLASTERS

This is class A plaster.

Gypsum is the raw material.

When mixed with water it sets within few minutes, so it is unsuitable for general plasterwork but it may be used for patching.

An additive or retarder must be incorporated to delay the set and so produce class B plasters which are softer than the remaining two classes.

Classes C and D are slow in hardening and so an additive is an accelerator to make them suitable for plastering.

These can be used for both under and finishing coats except for one coat on plasterboard or fibreboard due to insufficient adhesion.

The mixing water should be clean and free from impurities.

The sand should be clean and well graded; rounded particles are preferred to the harsher kinds and a clay and silt content; upto a maximum of 5%.

Plaster should be stored in a dry place.

Cement should not be mixed with gypsum plasters.

Class B plasters can be allowed to dry to dry out immediately after application, bit classes C and D require upto 48 hours for adequate hydration and so should not be permitted to dry during this period.

All classes should be applied before they start to stiffen and re-tempering after the commencement of the initial set must not be allowed.

Tools and mixing boards should be thoroughly cleaned after each batch has been used because portions of plaster left on the boards will accelerate the set of the neat mix.

The intermixing of different classes is inadvisable.

Gypsum plasters cannot be used in damp situations and lime or preferably cement plasters are better in such places

Gypsum mixes are best for concrete walls.

For brickwork 1cement: 2lime: 9sand are suitable.

Brick walls must have their joints raked out 10mm.

Smooth concrete surfaces must be roughened by

hacking

the application of thin 1cement: 2sand splatterdash coating or

applying a retarder.

The cracking of plaster frequently occurs where there is a change of background; between the wall and ceiling. This can be prevented by having a cornice or by making a horizontal cut with a trowel at the junction.

For 6mm thick plastering a single coat more than 6mm thick is applied and it is then levelled.

For 25mm thick plaster two coats: first one 18mm and second one 7 mm is applied.

For cement finishing, a coat of pure Portland cement slurry (1.5mm thick) shall be applied to the plastered surface with a trowel while the first coat is still plastic.

FINISHING

When no finish is specified the plastered surface shall be rubbed well to an even plane with a wooden float for external surfaces and finished smooth with a steel trowel for internal surfaces.

PALSTERING TECHNIQUE

After the fixing of door and window frames, skirting plugs etc have been completed, the surfaces to be plastered are cleaned.

Wall surfaces are done first and those that are very porous are dampened if necessary.

Before the undercoat has hardened the surface is well scratched for the next layer.

Screeds or 150mm wide strips of floating coat are then formed vertically at 1.8 to 3m intervals, they are made plumb and in exact alignment.

Intermediate screeds are then made about 1m apart and the spaces between are filled and levelled as before.

The surface is again roughened, the setting coat applied, and this is polished with a steel trowel just before it sets.

Cement and/or lime undercoats must be allowed to dry before further coats are added and unlike gypsum mixes, the surfaces must be sprinkled with water.

Skirting, architraves and other cover moulds should not be fastened until the plastering has set.

PLASTERING FAILURES

Poor adhesion caused by high suction of the backing, too rapid drying out or by moisture being imprisoned in the wall which subsequently emerges through the plaster in the form of blisters. (Due to inadequate key and incorrect choice of plaster).

Cracking due to shrinkage on drying out, it is associated with cement or lime mixes. Movement of the background is also responsible, as drying of timber ceiling joints.

EXTERNAL PLASTERING

EXTERNAL PLASTERING OR RENDERING

Rendered walls are an alternative finish to facing bricks, they can be made in different colours and are used in places where clay bricks would be out of harmony with the surrounding landscape.

Rendering is used extensively as a waterproof finish to no-fines concrete walls, such walls are made from 300mm thickness and upwards and consist of 1 part cement: 8 parts of large aggregate (13mm); sand is not included in the mix.

Gypsum plaster mixes are quite unsuitable for external rendering; much traditional work still exists and this is made of lime mixes protected by paint.

Cement: lime: sand mixes are now adopted and the proportions of these is dependent on the nature of the background and on the degree of exposure.

The bricks should be well fired and durable and the joints raked out 13mm.

Surfaces should be dampened if they are too dry before plastering starts and strong finishing coats must not be applied over weaker undercoats.

TYPES OF FINISHES

SAND FACED FINISH

Base Coat- It shall be of cement mortar 1:4.

Water proofing compound of approved make like Pudlo, Sika, Accoproof shall be added according to the makers instructions.

Thickness-15mm for brick work and 20mm for rubble masonry.

Base coat shall be dried for minimum 2 days.

Sand faced treatment- The cement mortar shall have washed Kharsalia or Kasaba or similar type of approved sand with slightly larger proportions of coarse material.

The cement to sand proportion shall be 1:4.

Water is added gradually to make the mixture homogeneous.

Thickness of finishing coat- 8mm

Surface to be finished with a wooden float.

Surface to be kept moist for 14 days continuously.

ROUGHCAST FINISH

Base coat- The first coat shall be of cement mortar 1:4.

Finished thickness- 12mm for brick masonry or concrete surfaces

Architectural Guidance PLASTERING

15mm for rubble masonry

Plaster shall be laid by throwing the mortar on the prepared surface, with a trowel in an uniform layer, and pressed to form a good bond.

The surface shall be roughened.

Second coat- Consists of aggregate which may vary in size from 5 to 8 mm and may consist of specially graded mixture mixed with fine sand and cement.

The proportion of cement to sand and aggregate shall be 1:1.5:3.

It should be applied while the first coat is still soft and plastic.

It should be about 12mm thick

PEBBLE DASH FINISH

The mix and procedure is the same as for rough casting except that the thrown-on coat consists of dry pebbles or crushed gravel only; the pebbles tend to drop off any time.

ROUGH CAST CEMENT PLASTER WITH COLOURED FINISH

High grade mineral pigment shall be mixed with ordinary cement to obtain the shade and tint as approved by the engineer.

MACHINE MADE FINISH (Tyrolean)

The undercoat procedure is the same as for the scraped finish.

The final coat is thrown on by the blades of a small hand machine, alternatively it can be sprayed on by a hose delivering the mix by air pressure.

NEERU FINISH

Preparation of surface- The plaster surface shall be combed lightly with wire brushes or nails before it is completely set to form key for neeru.

The undercoat shall only be damped evenly but not soaked before the application of neeru.

Application- Neeru shall be applied to the prepared and partially set but somewhat plastic surface with steel trowel to a thickness slightly exceeding 1.5mm and rubbed down to 1.5 mm thickness and polished to a perfectly smooth and even finish, working from top to bottom.

Moistening shall be commenced as soon as the plaster has hardened sufficiently and is not susceptible to injury.

Soaking of wall shall be avoided and only as much water as can be readily absorbed is used.

The surface shall be kept sprinkled with water for 14 days.

MUD PLASTER

12mm thickness for brick and 20mm for stone surfaces.

Mud mortar- Shall be prepared from none but well tempered clay or brick earth free from vegetation, gravel and other rubbish.

The clay is to be shifted fine and mixed with cow dung equal to 25% of its volume.

The mixture shall be soaked in water for 24 hours and left for a week or two without allowing it to dry.

Application- Mud mortar shall be applied in two coats on the surface to be treated, well pressed and floated with wooden floats.

Before the second coat is applied the first coats must be allowed to set bit not become dry.

After having been floated, the second coat of plaster shall be allowed to dry. The cracks that open out during drying shall be filled with a mixture of cow dung and clay.

Finishing- The plaster shall then receive one coat of moderately liquid mixture of equal parts of cow dung and finely powdered clay well mixed with water.

MODE OF MEASUREMENT AND PAYMENT

For jambs, soffits, sills, etc, for openings. Not exceeding .5 sq.m. each in area, ends of joists, beams, posts, girders, etc. not exceeding 500sq.cm. each in area and opening not exceeding 3sq.m. each, deductions and additions shall be made in the following manner-

No deductions shall be made for ends of joists, beams, posts etc. not exceeding 500sq.cm. and for openings not exceeding .5sq.m. each and no addition shall be made for reveals, jambs, soffits, sills etc, of these openings nor for finishing the plaster around ends of joists, beams, posts etc.

Deduction for openings not exceeding .5sq.m. but not exceeding 3 sq.m. each shall be made as follows and no addition shall be made for reveals, jambs, soffits, sills of these openings-

when only one face is plastered no deductions shall be made

when both faces are plastered, deduction shall be made for one face only for square openings without considering splays, if may.

When two faces of a wall are plastered with different plasters or if one face is plastered and other pointed, deduction shall be made from the plaster or pointing on the side of frames for doors, windows etc.

In case of openings of area above 3 sq.m. each, deductions shall be made for the actual openings, but jambs, soffits and sills shall be measured and paid.

Ceilings with projecting beams, shall be measured with their plastered surfaces and added to the plastering on ceilings when plaster is thicker than 6mm but finishing plaster upto 6mm shall not be paid for separately.

The measurements of lengths of wall plastering shall be taken between walls or partitions and for the top of floor or skirting to the top of wall for height.

Ribs and mouldings shall be measured separately.

Sides of plasters, projections, etc. shall be added to the plaster on walls.


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