membrane

  • Walls Resistance to weather and ground moisture

    A requirement of the Building Regulations is that walls should adequately resist the passage of moisture to the inside of the building. Moisture includes water vapour and liquid water. Moisture may penetrate a wall by absorption of water from the ground that is in contact with foundation walls or through rain falling on the wall.

    To prevent water, which is absorbed from the ground by foundation walls, rising in a wall to a level where it might affect the inside of a building it is necessary to form a continuous, horizontal layer of some impermeable material in the wall. This impermeable layer, the damp-proof course, is built in, some 150 mm above ground level, to all foundation walls in contact with the ground and is joined to the damp-proof membrane in solid ground floors as described and illustrated in Chapter 1.

    The ability of a wall to resist the passage of water to its inside face depends on its exposure to wind driven rain and the construction of the wall. The exposure of a wall is determined by its location and the extent to which it is protected by surrounding higher ground, or sheltered by surrounding buildings or trees, from rain driven by the prevailing winds. In Great Britain the prevailing, warm westerly winds from the Atlantic Ocean cause more severe exposure to driving rain along the west coast of the country than do the cooler easterly winds on the east coast.

    British Standard 5628: Part 3 defines five categories of exposure as: very severe; moderate/severe; sheltered/moderate; sheltered; and very sheltered. A map of Great Britain, published by the Building Research Establishment, shows contours of the variations of exposure across the country. The contour lines, indicating the areas of the categories of exposure, are determined from an analysis of the most severe likely spells of wind driven rain, occurring on average every 3 years, plotted on a 10 km grid. The analysis is based on the ‘worst case’ for each geographical area, where a wall faces open country and the prevailing wind, such as a gable end wall on the edge of a suburban site facing the prevailing wind or a wall of a tall building on an urban site rising above the surrounding buildings and facing the prevailing wind.

    Where a wall is sheltered from the prevailing winds by adjacent high ground or surrounding buildings or trees the exposure can be reduced by one category in sheltered areas of the country and two in very severe exposure areas of the country. The small-scale and large-scale maps showing categories of exposure to driving rain provide an overall picture of the likely severity of exposure over the country. To estimate the likely severity of exposure to driving rain, of the walls of a building on a particular site, it is wise to take account of the categories of exposure shown on the maps, make due allowance for the overlap of categories around contour lines and obtain local knowledge of conditions from adjacent buildings and make allowance for shelter from high ground, trees and surrounding buildings.

    The behaviour of a wall in excluding wind and rain will depend on the nature of the materials used in the construction of the wall and how they are put together. A wall of facing bricks laid in mortar will absorb an appreciable amount of the rain driven on to it so that the wall must be designed so that the rain is not absorbed to the inside face of the wall. This may be effected by making the wall of sufficient thickness, by applying an external facing of say rendering or slate hanging, or by building the wall as a cavity wall of two skins or leaves with a separating cavity.

    A curtain wall of glass (see Volume 4) on the other hand will not absorb water through the impermeable sheets of glass so that driving rain will pour down the face of the glass and penetrate the joints between the sheets of glass and the supporting frame of metal or wood, so that close attention has to be made to the design of these joints that at once have to be sufficiently resilient to accommodate thermal movement and at the same time compact enough to exclude wind and rain.

    It is generally accepted practice today to construct walls of brick, stone or blocks as a cavity wall with an outer and inner leaf or skin separated by a cavity of at least 50 mm. The outer leaf will either be sufficiently thick to exclude rain or be protected by an outer skin of rendering or cladding of slate or tile and the inner leaf will be constructed of brick or block to support the weight of floors and roofs with either the inner leaf providing insulation against transfer of heat or the cavity filled with some thermal insulating material.

  • Bitumen sheet

    Sheets of bitumen with hessian, fibre or mineral fibre base are spread on the concrete oversite or on a blinding of stiff concrete below the concrete, in a single layer with the joints between adjacent sheets lapped 75 mm. The joints are then sealed with a gas torch which melts the bitumen in the overlap of the sheets sufficient to bond them together. Alternatively the lap is made with hot bitumen spread between the overlap of the sheets which are then pressed together to make a damp-proof joint. The bonded sheets may be carried across adjacent walls as a dpc, or up against the walls and then across as dpc where the membrane and dpc are at different levels.

    The polythene or polyester film and self-adhesive rubber/bitumen compound sheets, described in Volume 4 under ‘Tanking’, can also be used as damp-proof membranes, with the purpose cut, shaped cloaks and gussets for upstand edges and angles. This type of membrane is particularly useful where the membrane is below the level of the dpc in walls.

    Bitumen sheets, which may be damaged on building sites, should be covered for protection as soon as possible by the screed or site concrete.

  • Hot pitch or bitumen

    A continuous layer of hot applied coal-tar pitch or soft bitumen is poured on the surface and spread to a thickness of not less than 3 mm. In dry weather a concrete blinding layer is ready for the membrane 3 days after placing. The surface of the concrete should be brushed to remove dust and primed with a solution of coal-tar pitch or bitumen solution or emulsion. The pitch is heated to 35°C to 45°C and bitumen to 50°C to 55°C.

    Properly applied pitch or bitumen layers serve as an effective damp-proof membrane both horizontally and spread up inside wall faces to unite with dpcs in walls and require less patient application than plastic sheet materials.

  • Materials for damp-proof membrane

    The materials used as damp-proof membrane must be impermeable to water both in liquid and vapour form and sufficiently robust to withstand damage by later building operations.

  • Damp-proof membrane below a floor screed

    The oversite concrete is laid during the early stages of the erection of buildings. It is practice to lay floor finishes to solid ground floors after the roof is on and wet trades such as plastering are completed to avoid damage to floor finishes. By this time the site concrete will have thoroughly dried out. A layer of fine grained material such as sand and cement is usually spread and levelled over the surface of the dry concrete to provide a true level surface for a floor finish. As the wet finishing layer, called a screed, will not strongly adhere to dry concrete it is made at least 65 mm thick so that it does not dry too quickly and crack. Electric conduits and water service pipes are commonly run in the underside of the screed.

    As an alternative to under concrete or surface damp-proof membranes a damp-proof membrane may be sandwiched between the site concrete and the floor screed, as illustrated in Fig. 28. At the junction of wall and floor the membrane overlaps the damp-proof course in the wall.

    Damp proof membrane below floor screed

  • Surface damp-proof membrane

    Floor finishes such as pitch mastic and mastic asphalt that are impermeable to water can serve as a combined damp-proof membrane and floor finish. These floor finishes should be laid to overlap the damp-proof course in the wall as illustrated in Fig. 27 to seal the joint between the concrete and the wall.
    Surface damp proof membrane
    Where hot soft bitumen or coal tar pitch are used as an adhesive for wood block floor finishes the continuous layer of the impervious adhesive can serve as a waterproof membrane.

    The disadvantage of impervious floor finishes and impervious adhesives for floor finishes as a damp-proof membrane are that the concrete under the floor finish and the floor finish itself will be cold underfoot and make calls on the heating system and if the old floor finish is replaced with another there may be no damp-proof membrane.

  • Damp-proof membrane below site concrete

    The obvious place to use a continuous damp-proof membrane is under the oversite concrete. The membrane is spread on a layer of comparatively dry concrete, clinker or ash which is spread and levelled over the hardcore as illustrated in Fig. 26. The edges of the membrane are turned up the face of external and internal walls ready for concrete laying so that it may unite and overlap the dpc in walls.
    damp proof membrane
    The membrane should be spread with some care to ensure that thin membranes are not punctured by sharp, upstanding particles in the blinding and that the edge upstands are kept in place as the concrete is laid.
    The advantage of a damp-proof membrane under the site concrete is that it will be protected from damage during subsequent building operations. A disadvantage is that the membrane will delay the drying out of the oversite concrete that can only lose moisture by upwards evaporation to air.
    Where underfloor heating is used the membrane should be under the concrete.

  • Damp-proof Membrane

    Concrete is spread oversite as a solid base for floors and as a barrier to moisture rising from the ground. Concrete is to some degree permeable to water and will absorb moisture from the ground; a damp oversite concrete slab will be cold and draw appreciable heat from rooms.

    A requirement of the Building Regulations is that floors shall adequately resist the passage of moisture to the inside of the building. As concrete is permeable to moisture, it is generally necessary to use a damp-proof membrane under, in or on top of ground supported floor slabs as an effective barrier to moisture rising from the ground. The membrane should be continuous with the damp-proof course in walls, as a barrier to moisture rising between the edges of the concrete slab and walls.

    A damp-proof membrane should be impermeable to water in either liquid or vapour form and be tough enough to withstand possible damage during the laying of screeds, concrete or floor finishes. The damp-proof membrane may be on top, sandwiched in or under the concrete slab.
    Being impermeable to water the membrane will delay the drying out of wet concrete to ground if it is under the concrete, and of screeds to concrete if it is on top of the concrete.

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