Presented By

AHSAN RABBANI

Civil Engg. Dept.

REC Raipur

Presented By

AHSAN RABBANI

Civil Engg. Dept.

REC Raipur

DAMP PROOFING

INTRODUCTIONINTRODUCTIONDamp is generally defined as unwanted water or moisture.Damp is generally defined as unwanted water or moisture.

Dampness is the access and penetration of moisture content into building through Dampness is the access and penetration of moisture content into building through its wall, floor, roof etc.its wall, floor, roof etc.

The existence of dampness in buildings is one of the most damaging failures that The existence of dampness in buildings is one of the most damaging failures that can occur in buildings.can occur in buildings.

Every building should be damp proof.Every building should be damp proof.

The main idea of damp-proofing is to protect buildings against the damaging action The main idea of damp-proofing is to protect buildings against the damaging action of water and humidity that act negatively upon constructions. of water and humidity that act negatively upon constructions.

Neither humans nor buildings can tolerate too little or too much of water.Neither humans nor buildings can tolerate too little or too much of water.

Excess water in buildings is called Dampness. The root of all evil is water.Excess water in buildings is called Dampness. The root of all evil is water.

In construction, a building or structure is waterproofed with the use of membranes In construction, a building or structure is waterproofed with the use of membranes and coatings.and coatings.

Oil-based waterproofing is generally used for roof leakage after construction and Oil-based waterproofing is generally used for roof leakage after construction and use only on wall like primer.use only on wall like primer.

Moisture can also affect the health of occupants typically through the potential for Moisture can also affect the health of occupants typically through the potential for breeding harmful organisms.breeding harmful organisms.

On the other hand, moisture reduces the shrinking cracks of wood and furniture, and On the other hand, moisture reduces the shrinking cracks of wood and furniture, and up to a point, is necessary to avoid respiratory discomfort.up to a point, is necessary to avoid respiratory discomfort.

Thus, moisture is both a necessary constituency of our built environment and a Thus, moisture is both a necessary constituency of our built environment and a potential liability. potential liability.

The issue, then, is not to eliminate moisture from our buildings, but to control it and The issue, then, is not to eliminate moisture from our buildings, but to control it and its movements. its movements.

CAUSES OF DAMPNESS IN BUILDINGCAUSES OF DAMPNESS IN BUILDING

1. 1. Rising of moisture from the ground:-Rising of moisture from the ground:-

All the structures are founded on soils, and the substructure is embedded into it.All the structures are founded on soils, and the substructure is embedded into it.

If the soil is pervious, moisture constantly travels through it.If the soil is pervious, moisture constantly travels through it.

This moisture may rise up into the wall and the floor through capillary action.This moisture may rise up into the wall and the floor through capillary action.

Ground water rise will also result in moisture entry into the building through walls Ground water rise will also result in moisture entry into the building through walls and floor.and floor.

2. Condensation:-2. Condensation:-

Process of condensation takes place when warm humid air is cooled.Process of condensation takes place when warm humid air is cooled.

Due to condensation of atmospheric moisture water is deposited on the walls, floors, Due to condensation of atmospheric moisture water is deposited on the walls, floors, ceilings.ceilings.

This moisture may cause dampness.This moisture may cause dampness.

3. Sources of Leakages or Dampness:-3. Sources of Leakages or Dampness:- 3.1 Leakages from roofs3.1 Leakages from roofs 3.1.1 3.1.1 Faulty roof slopesFaulty roof slopes

Leakage has direct relation to the Intensity of Rainfall in the Region & Type of Roof Leakage has direct relation to the Intensity of Rainfall in the Region & Type of Roof adoptedadopted..

Minimum Slope of Roof: 1 in 100Minimum Slope of Roof: 1 in 100

Recommended Slope: 1 in 40 for Mud phuska Roofing 1 in 60 for Concrete RoofingRecommended Slope: 1 in 40 for Mud phuska Roofing 1 in 60 for Concrete Roofing..

3.1.2 3.1.2 Absence of roof slab projectionAbsence of roof slab projection

In absence of slab projection, the rain water travels along the walls & enters through In absence of slab projection, the rain water travels along the walls & enters through crack at junction of Roof Slab and Wall.crack at junction of Roof Slab and Wall.

Roof slab to project beyond wall by 50 mm (minimum) so that waterfalls Roof slab to project beyond wall by 50 mm (minimum) so that waterfalls clear of the wall.clear of the wall.

3.1.3 3.1.3 Junction of Parapet walls & Roof Slab Junction of Parapet walls & Roof Slab

Junction of Parapet walls & Roof Slab is a source of leakage & requires to Junction of Parapet walls & Roof Slab is a source of leakage & requires to be made water proofs.be made water proofs.

To stop leakage, the roof Treatment provided on Roof slab be carried out To stop leakage, the roof Treatment provided on Roof slab be carried out to the Parapet Wall through the Junction of Parapet walls & Roof Slab.to the Parapet Wall through the Junction of Parapet walls & Roof Slab.

A chase must be cut in the Parapet Wall at a Height of 20 cm above the A chase must be cut in the Parapet Wall at a Height of 20 cm above the roof and the water proofing Treatment should be finished inside the chase roof and the water proofing Treatment should be finished inside the chase (Junction of Parapet walls & Roof Slab).(Junction of Parapet walls & Roof Slab).

3.1.4 3.1.4 In adequate number and size of drainage outletsIn adequate number and size of drainage outlets

The number and Size of Drainage outlets are adequate to drain off water without The number and Size of Drainage outlets are adequate to drain off water without stagnation. stagnation.

Min. size not less than 75mm.Min. size not less than 75mm.

No. of drain pipes depend upon shape of terrace, location of drainage points and No. of drain pipes depend upon shape of terrace, location of drainage points and spacing of drain pipe.spacing of drain pipe.

The spacing between outlet pipes should not be more than 6m.The spacing between outlet pipes should not be more than 6m.

CONTD…….CONTD…….

CONTD…….CONTD…….

3.1.5 3.1.5 Spreading of water on roofs due to overflowing from water tanksSpreading of water on roofs due to overflowing from water tanks

This may happen due to missing of Float Valves.This may happen due to missing of Float Valves.

Defective or broken float valves, which result overflowing of water tanks.Defective or broken float valves, which result overflowing of water tanks.

3.1.6 3.1.6 Bad Quality of Concrete used in RCC Roof SlabBad Quality of Concrete used in RCC Roof Slab

Most of leakages can be eliminated if the quality of concrete of Roof Slab Most of leakages can be eliminated if the quality of concrete of Roof Slab is ensured.is ensured.

A well compacted and cured roof slab is itself Water Proof.A well compacted and cured roof slab is itself Water Proof.

Close supervision at the time of construction can eliminate leakages due Close supervision at the time of construction can eliminate leakages due to faulty construction.to faulty construction.

RCC Roof Slabs exposed to Sun lead to large number of hair cracks. RCC Roof Slabs exposed to Sun lead to large number of hair cracks. These hair cracks are the sources of leakages.These hair cracks are the sources of leakages.

3.1.7 3.1.7 Stagnation of waterStagnation of water

Stagnation at the outlets is either due to wrong slopes of the roof or due Stagnation at the outlets is either due to wrong slopes of the roof or due to choking of rain water pipes. to choking of rain water pipes.

Stagnation also takes place due to depressions on the roof or level of Stagnation also takes place due to depressions on the roof or level of inlet.inlet.

Terrace area is required to be sloped towards the outlets so that water Terrace area is required to be sloped towards the outlets so that water flows freely.flows freely.

Rain water outlet pipes should be provided with gratings at the mouth Rain water outlet pipes should be provided with gratings at the mouth and they should be regularly cleaned along with cleaning of roof to and they should be regularly cleaned along with cleaning of roof to prevent from chocking.prevent from chocking.

3.1.8 3.1.8 Poor Quality or Improper Methods adopted in Laying Lime Concrete roofingPoor Quality or Improper Methods adopted in Laying Lime Concrete roofing

Reason:Reason:

Non availability of skilled labor.Non availability of skilled labor.

Time consuming.Time consuming.

High labor cost.High labor cost.

inexperienced contractors & supervisors.inexperienced contractors & supervisors.

3.1.9 3.1.9 Leakages from External WallsLeakages from External Walls

Reason:Reason:

Loosely filled brick joint.Loosely filled brick joint.

Use of poor quality Bricks.Use of poor quality Bricks.

Improper Curing, poor Workmanship.Improper Curing, poor Workmanship.

Hairline cracks on surface.Hairline cracks on surface.

Remedy:Remedy:

Use of good quality bricks with joints properly filled up.Use of good quality bricks with joints properly filled up.

use of 15mm thick plaster in 1:6. use of 15mm thick plaster in 1:6.

Where quality of bricks is poor, use cement mortar 1:4 in two layers.Where quality of bricks is poor, use cement mortar 1:4 in two layers.

Use coarse sand in external plaster.Use coarse sand in external plaster.

A number of hair cracks. These hair cracks are the sources of leakages.A number of hair cracks. These hair cracks are the sources of leakages.

CONTD…….CONTD…….

3.1.10 3.1.10 Dampness due to Leaking Waste Water & Soil PipesDampness due to Leaking Waste Water & Soil Pipes

Reason:Reason:

Cracks in pipes.Cracks in pipes.

Leaking Joints. Leaking Joints.

Remedy:Remedy:

The inspection of pipes for signs of cracks before fixing.The inspection of pipes for signs of cracks before fixing.

Openings around the pipes in walls are filled up with PCC 1:3:6.Openings around the pipes in walls are filled up with PCC 1:3:6.

Plumbing services be planned in such a way that can be maintained easily. Plumbing services be planned in such a way that can be maintained easily.

3.1.11 3.1.11 Chajja Wall Junctions Chajja Wall Junctions

Reason:Reason:

A possible source of dampness in a building is the junction between the A possible source of dampness in a building is the junction between the wall and chajja .wall and chajja .

This is due to wrong slope of chajja.This is due to wrong slope of chajja.

This dampness leads to growth of Fungus at the chajja wall junction.This dampness leads to growth of Fungus at the chajja wall junction.

Remedy:Remedy:

Chajja should be given a slope of 1 in 30 with a drip course at the end so Chajja should be given a slope of 1 in 30 with a drip course at the end so that water may not travel back.that water may not travel back.

CONTD…….CONTD…….

3.1.12 3.1.12 BalconiesBalconies

Balconies should be given a slope of 1 in 100 so that rain water is drained Balconies should be given a slope of 1 in 100 so that rain water is drained out.out.

The minimum size of the outlets should not be less than 50mm dia.The minimum size of the outlets should not be less than 50mm dia.

Step of 20 mm at the junction of Balcony with room be provided to stop Step of 20 mm at the junction of Balcony with room be provided to stop back traveling of rain water.back traveling of rain water.

All outlets are planned in such a way so as to connect them with Rain All outlets are planned in such a way so as to connect them with Rain Water Pipes.Water Pipes.

3.1.13 3.1.13 Floor TrapsFloor Traps

For preventing passage of foul gas, traps with water seal are invariably For preventing passage of foul gas, traps with water seal are invariably used at every fixture.used at every fixture.

For effective drainage, the slope should not be less than 1 in 100.For effective drainage, the slope should not be less than 1 in 100.

Junction of the trap and the waste pipe which is embedded in the floor is Junction of the trap and the waste pipe which is embedded in the floor is a possible source of leakages.a possible source of leakages.

The joints should be thoroughly tested for leakages before doing the The joints should be thoroughly tested for leakages before doing the flooring.flooring.

  

CONTD…….CONTD…….

Following are the ill effects of entry of dampness.

Dampness gives rise to breeding of mosquitoes and create unhealthy living conditions.

The wall decoration( i.e. painting, plastering etc.) is damaged and removed, which is very difficult and costly to repair.

Continuous presence of moisture in the walls may cause efflorescence resulting in disintegration of bricks, stones, tiles etc. and consequent reduction in strength.

Timber fittings such as doors, windows, almirahs etc coming in contact with damp walls get deteriorated.

Electrical fittings get deteriorated, giving rise to leakage of electricity and consequent danger of short circuiting.

Moisture causes rusting and corrosion of metal fittings attached to walls, floors and ceilings.

Reduce the life of structures

Causes spots on the floors and walls.   

EFFECTS OF DAMPNESS IN BUILDING EFFECTS OF DAMPNESS IN BUILDING

WATER PROOFING TYPES WATER PROOFING TYPES

1. Cementitious waterproofing

It is often used in the internal wet areas such as toilets.

Most of the time the type of cementitious waterproofing is semi-flexible.

Advantages of cement based repair materials are cost effectiveness, ease of application, generally non toxic, can be applied in wet condition, physical and chemical properties of cement based materials are almost similar to concrete.

2. Liquid waterproofing membrane

It is a thin coating which consists of usually a primer coat and two coats of top coats.

It offer more flexibility than the cementitious types of waterproofing.

3. Bituminous coating

It is made of bitumen based materials and it is not suitable for expose to sunlight.

It become very brittle and fragile when long exposure to the sunlight.

The flexibility of the finished products always depend on the solid content of the polymer added to the bitumen.

Polyurethane liquid membrane is also use for the flat roof area and exposed to weathering but it is expensive.

It can offer higher flexibility.

Polyurethane is very sensitive to moisture content present, therefore before application, one has to be very careful evaluating the moisture content of the concrete slab.

  

WATER PROOFING COMPOUNDS WATER PROOFING COMPOUNDS

1. Bitumen and bituminous material

These are still considered as very effective systems for Waterproofing.

These are used either as hot applied or cold applied.

Generally provided waterproofing treatment for old roofs showing sign of leakage.

Primer coat is applied with brush over dry clean surface and allowed to dry for 24 hours. Second coat is applied next day and final coat is applied third day. It is finally covered with 2 cm thick coarse sand or gravel.

The main advantages of these systems are low cost, Labor intensive, Easy application and fast degradation under exposed conditions.

2. Integral waterproofing compoundUsed to reduce the water absorption and the porosity/permeability.

Also increase cohesion and surface finish.

Basically they are Permeability reducers, Water repellents or Polymer modifiers.3. Sealants

Used to seal the joints provided. New generation sealants are based on poly-sulphides, polyurethane, and

silicon.These are required to provide a seal against ingress or seepage in

addition to other properties like movement accommodation etc.The main advantages of these systems are Low cost, Cheap Labor, Easy

application in structures like expansion, construction etc.

  

4.POLYMER MODIFIED CEMENTITIOUS MEMBRANES

Water soluble polymer in conjunction with cement makes good water proofing systems.

They have good adhesion, low water permeability, and compatibility with respect to concrete.

They have very good bond with concrete surfaces and are breathable systems.

It can offer higher flexibility.

In addition these systems are fairly flexible and can bridge small cracks.

5.SILICON COATINGS

These are colorless water repellent solutions, which are mainly used to protect masonry structures.

These coatings are mainly used as damp proofing masonry and concrete structures.

The advantages of these coatings are: - Low cost, Easy and fast rate of application, can be applied to a wide verity of sub states (masonry, tiles, concrete etc).

The disadvantages of these coatings are: - poor crack bridging characteristics & incapable of withstanding hydrostatic pressure.

6.CRYSTALLINE CAPILLARY WATERPROOFING SYSTEM

These system compose of Portland cement, quartz aggregates & special chemicals, which in the presence of moisture, chemical react with free lime to produce insoluble crystals.

This crystalline growth reduces porosity by blocking capillaries & filling hairline, non structural cracks (upto 0.25 mm) caused by shrinkage or expansion.

Membrane waterproofing system form a surface barrier but these systems in presence of water continue to produce crystals & a lasting impervious to water.

Advantages: Penetrates concrete, seals capillary tracts, and contains no chloride, Single part system hence easy application.

Suitable for external & internal applications, Resists negative water pressure.

RECENT DEVELOPMENTS IN WATERPROOFING COMPOUNDS RECENT DEVELOPMENTS IN WATERPROOFING COMPOUNDS

A number of new products with different combination have come to the market in the recent past, to overcome the disadvantages of normal bitumen based products.

Salient features and recommended areas of applications are given below:

  SL NO.

PRODUCT SALIENT FEATURES RECOMMENDED AREA OF APPLICATION

1. Polymer modified bitumen coatings

i) More flexible and elastic.ii) Their elastic recovering is high and are best suited for cooler climates.iii) Better resistance to fracture over a wide range of temperature and moisture variations. iv) Cracks under weathering action. v) Black in colour and therefore not pleasing in appearance

i) Water proofing of roof slabs, foundation, basements, floors, and underground structures, storage tanks, pipelines etc.ii) Heavy duty roof coating new or old construction. iii) Waterproofing of domes shells.

2. Polymer Modified bitumen felts.

i) Treated or reinforced felt sandwiched between polymer modified bitumen layers. ii) Behaves like a continuous membrane

Roofs and basements require protection on top with grit/sand from ultraviolet rays of sun.

SL NO.

PRODUCT SALIENT FEATURES RECOMMENDED AREA OF APPLICATION

3. Prefabricated elastic membrane.

i) Polymeric bitumen reinforced with non woven plastic fabric. ii) Provided with a thin polyurethane film on outer surface. iii) Life 5-15 years. High crack resistant and heat resistant.

Terraces, water tanks swimming pools.

4. Polymer Emulsions/ membranes water based coatings

i) Performance is good. ii) Allows concrete to breathe without allowing diffusion of Oxygen, CO2, rainwater etc. from outside. iii) These are transparent after drying. iv) Excellent adhesion to all types of building materials such as Concrete stone, brick and timber. v) Resistance to high temperature variations, ultra-violet rays etc. vi) Solvent based polymer water proofing compounds applied on dry surface by spray only.

To prevent dampness and efflorescence and for water proofing of terraces, walls, chhajjas and all other concrete surfaces.

CONTD…….CONTD…….

SL NO.

PRODUCT SALIENT FEATURES RECOMMENDED AREA OF APPLICATION

5. Alkali proof synthetic felt

i) Gives water proofing and thermal insulation on walls.ii) Life 40 years.

Terrace garden.

6. Silicon based emulsions

Colourless water repellent, limited life, poor bridging capacity, do not withstand hydrostatic pressure.

Vertical walls, specially stone facades.

7. Epoxy based coatings

i) Very good adhesion and bonding properties. ii) Costly, limited resistance when exposed to sunlight. iii) Cannot be use for external walls. iv) Cannot withstand expansion/ contraction

i) Used as water proofing, protective coatings for internal application. ii) Useful for coating in water tanks, surface floors, swimming pools, treatment plants.

8. Cementitious Polymer Waterproofing compound

i) Available in two packs part-A Cement and chemical additives Part-B Polymer emulsion. ii) Suitable for application on surfaces having macropores also. iii) Can be applied on wet surface. iv) Resistant to aggressive environment and abrasion. v) Can be applied by brush on dry & wet surfaces.

Terraces walls, water tanks effluent, treatment plants, lagoons etc.

CONTD…….CONTD…….

DESCRIPTIONDESCRIPTION

A cement based waterproofing compound that chemically reacts with moist cement based substrates.

Crystal Sealer forms insoluble crystals in the capillary tracts of the substrate permanently waterproofing the surface yet still allowing the surface to breathe.

USESUSES

Waterproofing of cement based surfaces, i.e., cement block, cement render and concrete floors.

Walls and floors of domestic, commercial and civil constructions, foundation walls, structural floors and walls of industrial buildings, concrete block walls in warehouses and domestic constructions, swimming pools, storage tanks, tunnels and dams, waterproof barrier between base floor slabs and concrete toppings.

FEATURES

Crystal Sealer penetrates into and becomes a permanent integral part of the surface, therefore it cannot peel or blister off as do normal coatings.

Crystal Sealer will not tear or puncture and does not have to be protected from back fill.

Crystal Sealer being a cement based material has an expansion and contraction the same as the cement surface to which it is applied.

COVERAGE0.8-1.1 kg per m2/coat (2 coats required)

CRYSTAL SEALER CRYSTAL SEALER

PERFORMANCE DATAPERFORMANCE DATA

Resists chemicals e.g. chlorine, ethylene glycol, petrol and diesel oil.Protects-reinforcing bars from corrosion.Strength–Increases concrete strength by up to 20%.

SPECIFICATIONSPECIFICATION

The waterproofing compound shall be a cement based compound that seals cement based surfaces by crystallization and can withstand up to a 100 meter head of water such as Crystal Sealer manufactured by Construction Chemicals and shall be applied strictly in accordance with the manufacturers instructions.

SURFACE PREPARATION

Apply to Portland cement based surfaces that are free from dirt, dust, grease, oil, efflorescence and other contaminants.

Clean out and patch cracks larger than 0.25 mm thick. Mechanically roughen smooth surfaces.Use caustic soda solution to clean grease and oil contaminated surfaces. Badly contaminated areas may require sand blasting to provide a positive

key.Gloves and eye protection must be used when using acids and cleaning

agents. The concrete/cement surface to be coated must be thoroughly saturated

prior to coating (this will enable crystal growth back into the wall and allow the crystal sealer to hydrate/cure correctly).

Concrete three days old will only require surface wetting.

MIXINGMIXING

Mix only the amount that can be applied in 10-15 minutes.Add 5 kg Water Proofing Compound to 1.5 liters of water and mix to a

thick paste. Mix thoroughly and add remaining water to bring the mixture to the

desired consistency. The final powder water ratio will be approximately 5 kg powder to 2 liters

of water. Note the final powder water ratio so that future batches can be made the

same. Discard unused material after 20 minutes and do not add additional

water after the second mix.APPLICATIONAPPLICATION

Crystal Sealer must only be applied to green or thoroughly wet cement based substrates.

Apply with a stiff brush or broom working the mixture well into the voids.To prevent segregation and ensure a uniform mixture stir the crystal

sealer solution frequently during the application and dip brush deep into the mixture so that a thick coat can be applied.

Apply a second coat while the first is still green or tacky OVERCOATINGOVERCOATING

Overcoating is not recommended as the CRYSTAL SEALER will need to breath, but can be render coated by neutralizing the surface by scrubbing with clean water.

Allow to cure 21 days then prime with PRIMAX prior to render application.

SET TIMESET TIME

20-30 minutes depending on temperature. Back fill 24 hours after application.

CURINGCURING

Protect the surface from rain or frost until it has cured. Water cure the treated surface by mist spraying with water 3-4 times

daily for 3 days to hasten crystal growth.PRECAUTIONPRECAUTION

Crystal Sealer is alkaline so gloves must be worn. Wash splashes immediately off skin and eyes with clean water.

FEATURESFEATURES

Crystals can grow up to 0.4 meters into the concrete in the presence of water. Crystals grow approx. 1cm depth/week.

SHELF LIFESHELF LIFE

12 Months.

• Impervious

• Durable, Ideal for the entire building life.

• Steady

• Resist any load safely.

• Flexible

• Dimensionally stable

• Cheap

• Salt free

REQUIREMENTS OF AN IDEAL DAMP PROOFING MATERIALREQUIREMENTS OF AN IDEAL DAMP PROOFING MATERIAL

Damp proofing course may be vertical or horizontal.

Horizontal DPC should cover the full thickness of the wall.

The horizontal DPC should be laid continuously at junctions

and corners.

Mortar bed below DPC should be leveled, even and no

projections to avoid damage of DPC.

Each DPC should be placed in correct relation to other DPC

so as to ensure a complete and continuous barrier to the

passage of water floors, walls or roof.

DPC should not be kept exposed on the wall surface to avoid

damage.

A cement concrete fillet of 75mm radius should be provided

at the junction when the horizontal DPC continues vertically

on the wall.

GENERAL PRINCIPLES OF WATER PROOFING COURSE (OR DPC)GENERAL PRINCIPLES OF WATER PROOFING COURSE (OR DPC)

If the level of the ground floor is at the same level as the ground surface, the damp proofing course and the damp proofing membrane should be continuous and stepped vertically as shown in the figure.It should be flexible and at the junction it should be fillet at 75 mm radius as shown.

Damp Proofing of an external wall

METHODS OF WATER PROOFING METHODS OF WATER PROOFING

If two floors are at different levels, divided by an internal wall, the damp proofing course on the internal wall should be at the level of the lower floor.The basic principal of stepping it with the damp proofing membrane on the floor must also be applied.An additional vertical damp proofing membrane must be applied to prevent moisture from entering through the staggered floor levels.

Damp Proofing of an internal wall

This method is used to prevent moisture from entering the walls through the adjacent ground.A hollow chamber of air is constructed parallel to the external wall. the width may be 200-300 mm.The chamber is closed on the top by RCC or stone top with grated openings at regular intervals. The vertical and horizontal damp proofing courses may be provided as shown.

Damp Proofing through air drain

Provision of coping on top of the wall surface, prevents the entry of damp from the top surface of the wall.Coping may be provided by means of:•stone•PCC•Metal flashing•Molded RCC parapet.All forms of coping must be sloped in order to prevent water stagnating on the top surface.

Damp Proofing of Parapet Walls

Two parallel walls separated by air gap except at wall ties which are of impervious material.The method of damp proofing is similar to normal single leaf-walls, except that the two leafs of the cavity walls have separate damp proof courses.

Damp Proofing by Cavity Wall