building construction 3 - Lecture 5

1. PLASTERING AND DRY LINING
Building Construction III
Spring 2012
Dr.Hikmat Hammad
Lecture 5

2. Preparing of the surfaces for
plastering
• Preparation of the surfaces for plastering is an
essential step towards obtaining a sound and
durable finish.
• A good bond between the plaster mortar and the
background surface should be established.
• The surfaces of dense concrete block walls
provide sufficient key and generally no major
preparation other cleaning is required.
• However, spattering of a cement/ sand slurry
dash coat increases the bond.

3. Preparation works are summarized as
follows:
– Wall surfaces should be brushed and any
mortar droppings to be removed.
– Oil marks and stains from formwork to be
cleaned.
– Brick and smooth surfaces dense concrete
block wall joints to be raked to a depth about
10-12 mm
– Any depression on the surfaces to be filled
with suitable mortar and application should be
delayed until the mortar gains sufficient
strength

4. – Dense concrete surfaces should be hacked to
produced rough surface.
– Metal lath to be fixed along the jointing line of
two dissimilar backgrounds such as between
concrete column and brick wall.
– Smooth surfaces to be treated with bonding
agents such as P.V.A.
– For dense concrete block surfaces spatter-
dash coat of 1 cement 2 sand by volume
should be applied.

5. Making dabs on a wall surface
before plastering

7. Application of plaster on a wall
surface

9. Methods of application
• Plaster is generally applied in either two or
three coats depending on the condition of
background.
• On plaster board or similar smooth
surfaces one coat of premixed plaster
application is also possible
• Three coat application is required on metal
lath and severe uneven backgrounds.

10. Undercoats
• First coat is known as render coat.
• Second coat is known as floating coat
( these two coats in some works can be combined
as one coat and known as “browning coat” or
“under coat”. Total thickness of undercoats can be
up to 20 mm and coarser sand is used in the mix.)

11. The main function of undercoats are:
– To level the uneven background surface
– To prevent rain penetration
– Provide uniform suction and good adhesion
for the finishing coat
Undercoat mixes should be sufficiently
strong to obtain a good key but should
not be stronger than background or
previously applied undercoat, otherwise
cracking and lose of adhesion occurs.
Therefore in undercoats mainly cement/
sand mortars are used.

12. • Mix proportions commonly used for strong
background is 1:3 cement: sand or
1:.5:4cement:lime:sand. Or 1:1:5 cement: lime:
sand or class B type a Browning premixed
plasters. All mix proportions give are by volume.
• Undercoat should be scratched to provide key
for the finishing coats. Each coat should be
allowed to dry properly before the next coat is
applied.
• For undercoat subject to severe exposure of
atmospheric agencies it is recommended to use
water-repellent or water-proofing admixture.

13. Finish coat
• Finishing coat plasters are either applied neat or with
fine sand to give smooth and level surface for further
finishing works
• The usual thickness of finishing coat is 3-5 mm.
• On smooth plasterboards surface 3 mm lightweight
premixed plaster will provide acceptable finish.
• For rendered surfaces over trowel ling with trowels may
create craze due to the evaporation of surface water and
shrinkage of cement.
• It is therefore desirable to have rough surface as an
undercoat.