Damp proofing and waterproofing part

Introduction

IN last week’s edition, we started on the topic of weatherproofing where we looked at the general requirements for damp proofing; importance of waterproofing; position of damp proof course (d.p.c.). This week we put a lid on the topic as we touch on other few aspects on the topic. A house that is not waterproofed is very uncomfortable to stay in and the water can also inflict a lot of damage to property.

Types of dpc

Damp proof courses are classified as flexible, semi-rigid and rigid:

i. FLexible dpc: are those materials which can be formed into any shape and therefore can be used on curved and stepped structures without any damage. The most common of flexible materials is bituminous felt. Where two sheets meet , they shall be lapped by at least 150mm. Other flexible materials are polythene, plastic/polyvinyl chloride (PVC), copper, zinc and lead sheeting

ii. Semi-rigid: suitable for very thick walls as they are capable of carrying very heavy loads and resist high water pressure. Mastic asphalt applied hot is a typical semi rigid material

iii. Rigid materials: are those which allow least structural movement than flexible and semi rigid materials. They are however capable of resisting very heavy loads and high water pressure. Examples of some rigid materials are:

• Two or more courses of engineering bricks layered in English bond using cement mortar

• Construction using water bond and cement mortar. This is normally done to water tanks.

• Two courses of slates laid in half bond with overlapping joints

• A layer of waterproofed cement mortar. This method was the most commonly used in rural areas a couple of years back

Applications of damp proofing and

waterproofing systems

• Foundation-walls: all foundation walls shall have horizontal dpc set not less than 150mm above the finished ground level and also on the underside of any timber in the ground floor construction which is in contact with such walls, and such timber should be treated with preservatives and properly supported. Walls enclosing a floor in contact with the ground should have dpc placed in a way that doesn’t allow moisture from the floor to move up the wall. For internal walls, the walls should be built on a dpc laid on the floor.

• Cavity walls: Dpc should be provided in such a manner as to prevent moisture from the soil or foundation walls from rising into the wall above the level of the ground floor or reaching any timber in the floors. The dpc should also allow any moisture from the cavity to drain outwards through weep holes or open vertical joints left at intervals in the outer leaf of such walls. In external cavity walls dpcs should be provided at cills and heads of all openings to prevent the passage of moisture from the outer to the inner leaves of the wall

• Parapets and external projections: dpc shall be provided to parapets and other external projections to prevent the passage of moisture into the building

• Floors: solid floors in contact with the ground floor should have a horizontal dpc laid under it to it’s full extent to render them damp proof eg by use of polythene paper. Floors of washrooms and toilets should also be waterproofed by use of tile, terrazzo, granolithic or other impervious materials. Around the walls should also be a skirting of impervious material which protects the walls.

• Basements: basements are some of the most difficult units to manage water and moisture in terms of prevention of moisture penetration in walls and how to dispose of the ground water. This applies to underground rooms and sunken rooms. The walls should be provided with vertical dpc to prevent contact between the ground and the walls. Where ground water is present or is likely to rise above basement floor level, it has to be drained away from the building. The means of draining can be provided by gravity or the water channelled to a sump from where it shall be pumped away to a discharge point

Flashings

These are pieces of metal cut and profiled to a predetermined specification and placed at specified points for the purpose of conveying rain water to gutters and, eventually the ground. Flashings are provided to prevent dampness at points such as valleys and the junction of roofs with chimneys, parapets and other abutting surfaces so as to render the roof weatherproof. Flashings should be installed by a highly skilled workman or they can be a nightmare because they can be a source of high water leakages.

Requirements for flashings

The flashings should comply with the following requirements:

• They should be corossion resistant or rust proof

• Shall be composed of soft copper, zinc, aluminium, hard lead, galvanized metal, asbestos-cement or other approved material having similar corrosion-resistant qualities

• Dissimilar materials which may cause electrolytic corrosion should never be used

• Where that happens corrosion can be prevented by painting the contacting surfaces with bitumen based paint or by any other method which will prevent a galvanic couple being formed

• Zinc flashings should have a mass of not less than 4kg per square metre; aluminium and lead flashings of not less than 15kg per square metre

• The minimum thickness for flashings of copper, galvanized metal or aluminium shall be 0.55mm. asbestos-cement flashings shall be at least 5mm in thickness

• Flashings to the junction of roofs with chimneys, parapets and other abutting surfaces should be double layered or in two pieces, i.e under and cover flashings. The underneath flashing shall be covered with the second flashing which should be properly secured and bonded in mortar, wedged and pointed.

• The material should be damp proof

◆ Innocent Chatikobo is an Engineer by profession with AtroServe Engineering Zimbabwe. He has extensive knowledge and experience in Structural Engineering and Construction. For your comments, views, questions and feedback he can be contacted on Cell: +263 777 950 224; +263 712 376 037; +263 782 502 732

◆ Email: eng.chatyk@gmail.com . Like the Facebook page: AtroServe Engineering Zimbabwe. Follow the Whatsapp Channel: AtroServe Engineering Zimbabwe