Cavity Wall, Advantages, Construction details, Restrictions

1. CAVITY WALL
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Presented by
Ar. Krishna Prakash
CEng, AMIE, M.Tech (IIT Kgp),
B.Arch (NIT Jaipur), B.A. (Hons.)

2. CAVITY WALL
● A cavity wall (or hollow wall) is a double wall consisting of two
separate leaves, each leaf being built of masonry units and separated
by a cavity/gap.
● These two leaves (or skins) should be tied together with metal ties
or bonding units to ensure that two leaves act as one structural unit.
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4. CAVITY WALL
● The outer and inner leaves of a cavity wall may be of the same
material or of different materials, such as brickwork and concrete,
etc.
● The thicker leaf shall be arranged on the inside.
● The space between the leaves being either left as continuous cavity
or filled with a non-load bearing insulating and waterproofing
material.
● This cavity intercepts movement of moisture from outer leaf to the
inner leaf and also improves the thermal insulation of the wall.
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INSULATION

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8. ADVANTAGES
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▪ Prevention of Dampness
▪ Heat Insulation
▪ Sound Insulation
▪ Load on Foundation
▪ Efflorescence
▪ Economical

9. ADVANTAGES
● As there is no intimate contact between the two leaves except at the
wall ties (which are of impervious material), there is no possibility of
the moisture travelling from the outer leaf to the inner.
● The layer of air in the cavity being non-conductor of heat reduces
the transmission of heat from the external face to the internal one
and as such cavity walls are best suitable for a tropical country like
India. Tests have revealed that cavity walls have 25% greater
insulating value than solid walls.
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10. ADVANTAGES
● They have good sound insulation property.
● Due to less solid thickness of wall, the loads on foundation are
considerable reduced.
● The construction of a cavity wall results in the reduction of nuisance
of efflorescence to a great extent.
● They are economical. It is found that the construction cost of a
cavity wall is about 20% less than the construction cost of a
corresponding solid wall.
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11. VIDEOS
● V 0a
● V 0b
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12. CAVITY
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POSITION OF

13. POSITION OF CAVITY
● Foundation of the cavity wall should be solidly constructed like
ordinary wall foundation up to 15 cm below DPC.
● The cavity should start near ground level and terminate near eaves
level in case of sloping roof or near coping in case of flat roof with
parapet wall.
● The cavity should preferably start 15 cm. below the damp-proof
course level. This has the advantage of draining any condensed
moisture below the level of damp-proof course.
● Similarly, to prevent infiltration of moisture, the bottom of cavity
should lie at least 15 cm. above the out side ground level.
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17. POSITION OF CAVITY
● The damp-proof course for the two leaves is laid separately,
although at the same level.
● This is necessary to continue the cavity below damp-proof course.
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18. WALL TIES
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19. WALL TIES
● The two walls are tied together with metallic or terra-cotta ties to
obtain structural stability. They are spaced 90 cm. apart horizontally
and 30 to 45 cm. apart vertically in staggered positions.
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Metal ties
• The function of the wall ties is to bind together the outer and inner
leaves of a cavity wall.
• These may be of galvanized iron, wrought iron, mild steel (flat iron or
round bars), gun metal, brass, copper, stainless steel or any such
corrosion resistant metal, made of flats 20 x 5 mm cranked or
twisted at their mid point with ends split and fish tailed.
• The ties shall be built into horizontal bed joints during erection,
placed sloping towards the exterior side to prevent water from
flowing along it from outer to inner leaf side.

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Bonding Units
• These shall be preferably precast R.C.C. units having cross-section as
per Fig. No (h).
• Length of the Bonding units will be sum of thickness of both leaves
plus width of cavity if the leaves are 75 mm or 115 mm. If the leaves
are more than 115 mm thick, then the length of a unit will be 2 × 115
+ width of cavity as shown in Fig. (h).
• Precast RCC units shall be provided with 2 no., 6 mm mild steel
reinforcement bars tied with 2 no. 3 mm. dia. M.S. wire/hard drawn
wire cross bars placed in the centre of units.
• Cement concrete used in the bonding units shall not be leaner than
1:3:6 (1 cement : 3 coarse sand : 6 graded stone aggregate 20 mm
nominal size).

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Spacing
• Metal ties/bonding units shall be spaced not more than 90 cm apart
horizontally and 45 cm vertically and staggered in each course.
• Additional ties shall be used near openings.

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32. VIDEOS
● V 1a
● V 1b
● V 1c
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33. CONSTRUCTION DETAILS
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34. CAVITY WALL
● A cavity wall can be either load bearing or non-load bearing.
● Generally speaking, both the leaves of masonry are half brick thick, built
in stretcher bond. If for architectural effect, the outer leaf has to be in
“Flemish bond” it is necessary to make use of purpose made snap header,
since headers made by cutting normal full length bricks do not prove to be
satisfactory.
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35. ● Thickness of each leaf of a cavity wall shall not be less than 7.5 cm.
● The width of the cavity shall not be less than 40 mm and not more
than 115 mm.
● Where the outer leaf is half masonry unit in thickness, the
uninterrupted height and length of this leaf shall be limited so as to
avoid undue loosening of ties due to differential movements between
two leaves.
● The outer leaf shall, therefore, be supported at least at every third
storey or at every 10 m of height, whichever is less, and at every 10
m or less along the length.
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Fig 1(a),(b) & (c) : Brick work in Cavity Walls

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Fig 1(d) : Brick work in Cavity Walls

38. ● In cavity wall either one leaf (inner) or both leaves could be load
bearing.
● It has been observed from tests that a cavity wail is 30 percent
weaker than a solid wall of the same thickness as the combined
thickness of two leaves of the cavity wall, because bonding action of
ties cannot be as good as that of normal bond in a solid wall.
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39. ● It is necessary to provide enough ventilation to the hollow space of
the cavity wall.
● This is achieved by providing air bricks at top and bottom of the the
external wall.
● Air bricks are provided immediately above damp-proof course.
● The air bricks should be kept 90 cm. apart.
● Similarly, air bricks are provided near the top of the cavity.
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VENTILLATION

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42. ● In the plan, the cavity is discontinued at the openings such as doors,
windows, etc. The jambs of openings for doors and windows are
constructed solid either in brickwork or with layers of slates or tiles.
If metal windows are to be used, specially prepared metal frames can
be used for this purpose.
● Fig. 12 (a) shows the details at head of opening when a common
R.C.C. lintel is provided. An inclined flexible D P.C. is provided to act
as a bridge over the cavity.
● The D.P.C. should extend lengthwise beyond the frame for a
distance of about 150 mm on either side. Fig. 12(b) shows the details
at the window sill.
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CONSTRUCTION AT OPENINGS

43. 43Fig 11 : Details of Damp-proof Course at jamb for openings in Cavity wall

44. 44Fig 11 : Details of Damp-proof Course at jamb for openings in Cavity wall

45. 45Fig 12 : Details of Damp-proofing at lintel head level for openings in cavity walls

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47. VIDEOS
● V 8a
● V 8b
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48. ● It is necessary to take adequate steps at top to prevent the entry of
dampness to the inside portion of the wall. The cavity may be
constructed upto the coping of the parapet wall or alternatively, it
may be closed at the bottom of parapet wall by a damp-proofing
course.
● In case of a pitched roof, the tops of two portions are connected by
solid brickwork to support the roof truss and damp-proofing course
is inserted immediately below this solid portion.
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CONSTRUCTION AT THE TOP

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EFFECTIVE THICKNESS

51. ● During construction of a cavity wall, it should be seen that mortar, bats,
etc., do not fall in the hollow space. The presence of such materials in the
hollow space seriously affects the working of a cavity wall.
● For this purpose, a wooden strip of width slightly less than that of the
hollow space, is supported on ties and it is raised as the work proceeds.
Also, some bricks at the bottom are left out and bats, etc. falling in the
cavity are removed from these holes.
● When the work is completed, this bottom portion is sealed by filling it
with bricks.
● It should also be seen that the vermins or mosquitoes do not find access
in the cavity.
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DROPPING OF MORTAR, BATS, ETC.

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55. RESTRICTIONS
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Restrictions
• Cavity walls shall not normally be built more than 7.5 metres in
height and 9 metres in length.
• Where large lengths and heights are desired, the wall shall be divided
into panels with strengthening measures such as pillars etc.
• Cavity shall be covered at the top with at least two courses of
masonry unit and/or a coping over it.
• Adoption of cavity walls is not recommended when heavy
concentrated load from beam etc. are to be supported by walls.

57. REFERENCES
▪ S.P. Arora, S.P.Bindra, The text book of Building Construction, Dhanpat Rai Publication
▪ Sushil Kumar, Building Constructipon, Standard Publishers Distributors
▪ P.N. Khanna, Indian Practical Civil Engineer’s Handbook, USB Publishers’ Distributors Pvt. Ltd.
▪ W.B.Mckay, Building Counstruction (volume 1), Pearson
▪ Handbook on Masonry Design and Construction, Bureau of Indian Standards
▪ National Building Code of India 2016, Volume 1
▪ CPWD Specifications, Volume 1
▪ IS 1905 : 1987
▪ IS 2212 : 1991 (Brick Works - Code of Practice)
▪ https://www.nicee.org/
▪ http://www.bmtpc.org/
▪ Maity, Damodar. (2014). Experimental investigation on chemically treated bamboo reinforced
concrete beams and columns. Construction and Building Materials. 71. 610-617.
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58. THANK YOU
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Presented by
Krishna Prakash
Contact no. - 9265516718
krishna2007prakash@gmail.com