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Part C Site Preparation and Resistance to Moisture

Section 3: Ground Moisture and Resistance to Moisture

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It is important to ensure that moisture ingress that would cause damage or deterioration of structural elements or materials, reducing their performance or posing health and safety concerns to the occupants of a building, should be protected against.

Floors

Floors should be designed and constructed to ensure that water from the ground does not cause it damage and to ensure moisture does not reach the upper surface of the floor.

Diagram HC8 - Movement of moisture - floors - Extract from TGD C
Diagram HC8 - Movement of moisture - floors - Extract from TGD C

Ground Supported Floors

Except in the case of ground floors that will be subject to ground water pressure the construction of a ground supported floor should comprise a hardcore bed with a dense concrete laid on top incorporating a damp-proof membrane. The following guidance should be followed for non-complex buildings where normal design and construction methods are used:

Hardcore – The bed of hardcore should be a minimum of 150mm thick which should comply with the requirements of IS EN 13242: 2002 +A1: 2016 and meet the specifications in Annex E of SR 21: 2014 +A1: 2016. It is important to ensure the hardcore layer is free from any material that would cause damage to the concrete, is well compacted and clean.

A binding layer is to be provided in accordance with the details provided in Annex E of SR 21: 2014 +A1: 2016. The binding layer should provide a even surface without any sharp elements that may cause a tear in the damp proof membrane, it should be of sufficient depth to fill and surface voids.

Concrete – The structural design of the building will dictate the required depth of concrete however it should be no less than 150mm thick and should be made with cement complying with IS EN 197 – 1: 2011 and aggregate in accordance with IS EN 12620: 2013: Aggregates for Concrete.

Concrete that has not been reinforced should have a maximum 0.85 water to cement ratio with a minimum cement content of 200kg/m^3^ achieving a 28day cube strength of 20 N/mm^2^.

Reinforced concrete should achieve a grade in accordance with IS EN 206: 2013 +A1 2016.

Damp proof Membrane – The damp-proof membrane can be provided above or below the concrete and should be continuous with the damp-proof course in the walls. The type of damp-proof membrane to be used will be dependent on where it is to be located:

1. Below Concrete – Damp-proof membrane should be at least 300µm with its joints sealed in accordance with CP 102, the binding material on which it is laid should be free from any material that may damage the membrane.

2. Above Concrete – The damp-proof membrane should be one of the following:

  • 3 no. coats of cold applied bitumen solution or other suitable material that is both moisture and water vapour resisting.

  • Polythene sheet a minimum of least 300µm with its joints sealed in accordance with CP 102.

Regardless of whether the membrane is above or below the concrete it should be covered by a screed floor finish or other similar material used as a floor finish. Where pitchmastic is used for damp proofing a screed floor finish should not be used.

Should a timber floor finish be laid directly on concrete the material which it is bedded in may also serve as its damp-proof membrane. Where the concrete is the fixing for a timber floor finish must be treated with an appropriate preservative. BS 8417: 2011 +A1: 2014 provides guidance on preservative treatments.

general G6
Diagram HC9 - Ground supported floor - Extract from TGD C

Diagram HC10 - Damp proof membrane continuous with damp proof course - Extract from TGD C
Diagram HC10 - Damp proof membrane continuous with damp proof course - Extract from TGD C

Suspended Timber Ground Floors

Where a suspended timber floor is to be provided its construction should be as follows:

  • A well compacted hardcore layer a minimum of 100mm thick comprising of broken stones, bricks or other suitable material while ensuring there is no material that will damage the concrete. On top of this hardcore layer should be a minimum of 100mm concrete of a grade as described for ground supported floors above. It is important to ensure that the concrete does not have a level below the adjoining external ground or paving.

  • A void space should be provided for ventilation which should be a minimum of 75mm from the concrete to the underside of a wall plate and a minimum of 150mm from the concrete to the underside of the suspended timber floor or if provided to the underside of the insulation. Ventilation openings should be provided in each external wall located to ensure the air has a free path between opposite sides and to all parts. The ventilation openings should be a minimum of 1500mm^2^ for every metre run of wall, should pipework be required it should have a minimum diameter of at least 100mm.

  • The damp-proof course should be located to ensure that moisture from the ground cannot reach any timber or other elements that would be damaged by moisture. The damp-proof course should comply with the requirements of IS EN 14967: 2006, BS 743, BS 6398, BS 6515, BS 8215 or CP 102.

Diagram HC11 - Ground supported floor finishes - Extract from TGD C
Diagram HC11 - Ground supported floor finishes - Extract from TGD C

Diagram HC12 - Suspended timber ground floor - Extract from TGD C
Diagram HC12 - Suspended timber ground floor - Extract from TGD C

Suspended Concrete Ground Floors

Suspended Concrete Ground Floor with Ventilated Voids

Suspended concrete ground floors should be constructed in-situ or using precast concrete a minimum of 100mm thick, the structural design however will determine the actual required thickness. A suitable damp-proof membrane should be provided, and the concrete should achieve a grade as described for ground supported floors above.

The ventilated air space to be provided below the suspended floor should be a minimum of 150mm from the ground to the underside of the floor however, this may be reduced if proprietary void formers are used.

Ventilation openings should be provided in each external wall located to ensure the air has a free path between opposite sides and to all parts. The ventilation openings should be a minimum of 1500mm^2^ for every metre run of wall.

Suspended Concrete Ground Floor cast on the Ground

Floors of this type should be cast in-situ using a concrete having a grade as described for ground supported floors above, having a minimum thickness of 100mm unless the structural design requires a greater depth incorporating an appropriate damp-proof membrane where necessary.

This system is only suitable for use where it is unlikely that a large gap will be formed under the suspended floor due to settlement of the ground as this would create a risk of explosive gases collecting beneath the floor.

Walls

Both internal and external walls should not be damaged by ground water, should stop the passage of moisture into the building and should not carry ground moisture to any part of the building which may be damaged by water. In addition to the above requirements external walls must stop the passage of rain or snow to the inside of the building, should not be damaged by rain or snow and should not carry rain or snow to any part of the building that it may damage.

Internal & External - Moisture from the Ground

Walls should be provided suitable damp-proof course that will stop moisture ingress. The damp-proof course provided to the walls should be a continuation of that provided in the floor. Damp-proof courses in the external walls should be a minimum of 150mm above the adjoining external ground or paving level.

Diagram HC13 - Movement of moisture - walls - Extract from TGD C
Diagram HC13 - Movement of moisture - walls - Extract from TGD C

External Cavity Walls – To ensure that no moisture passes to the inner leaf of the cavity wall a damp-proof tray should be provided or alternatively the cavity should extend a minimum of 150mm below the lowest damp-proof membrane.

Diagram HC14 - External or internal wall - Extract from TGD C
Diagram HC14 - External or internal wall - Extract from TGD C

Further guidance for the selection of damp-proof courses, their design and installation and the ways to stop the entry of ground or surface water into the building are contained within BS 8215: 1991 and BS 8102: 2009 respectively.

External - Moisture from Outside

External cavity walls should be constructed to ensure that moisture cannot pass to the inner leaf, the following guidance is appropriate for use in non-complex building of normal construction. The external cavity wall should be constructed as follows:

Outer Layer – The outer layer should be of masonry construction i.e. blocks, bricks, stone etc.

Cavity – The cavity should be a minimum of 50mm wide and should only be connected to the inner leaf using walls ties or damp-proof trays to ensure moisture does not pass to the inner leaf.

Inner leaf – The inner leaf can be of masonry or framed construction with a suitable lining.

Insulation – It is acceptable to provide insulation in the cavity between an outer and inner leaf of masonry construction only if the following has been met:

1) Any insulation to be provided in the cavity must have been tested and shown to prevents the passage of moisture to the inner leaf.

2) In situations where the cavity is to be only partly filled the remaining cavity should be a minimum of 40mm wide.

Guidance on insulation should be obtained from Technical Guidance Document L which can be found in the Building Regulations Requirements Section of this app.

Cladding

Cladding - External Walls and Roofs

External walls and roofs should be so constructed to ensure that they will not be damaged by rain or snow, will stop the passage of rain or snow into the building and will protect any elements that may be damaged by rain or snow.

There are many different materials that can be used to clad a building some of which have been listed below:

Impervious – Impervious cladding materials include metal, glass, plastic or bituminous materials.

Weather Resisting – Weather resisting cladding materials include cement-based products, natural stone or slate, fired clay and wood.

Moisture Resisting – Moisture resisting cladding materials include bituminous and plastic products which should be lapped if they are to be used as a sheeting material.

There are two ways in which cladding can prevent the passage of rain or snow into the building, either by stopping it at the face of the cladding (impervious cladding) or by stopping the passage of moisture at the back of the cladding (Weather resisting cladding backed by a moisture resisting layer).

Diagram HC15 - Cladding - walls and roofs - Extract from TGD C
Diagram HC15 - Cladding - walls and roofs - Extract from TGD C

It is important to ensure that the exterior of the building is suitably weather resistant. Cladding can be considered suitable for use as part of an external wall or roof if:

1) It is impervious to moisture and is either jointless or has sealed joints.

2) The cladding is either impervious to moisture or is weather resisting and backed by a material that will stop the passage of rain or snow and will direct it towards the outside face of the building and is installed with overlapping dry joints.

Important Notes

It is important to ensure that where a cladding is being used it is of a suitable material that will not rapidly deteriorate.

Unless there is a ventilated space being provided behind the cladding it should be permeable to water vapor. Reference should be made to Technical Guidance Document B to determine whether cavity barriers and/or fire stopping is required where a cavity is to be provided behind the cladding.

Where the cladding is jointless or where sealed joints are to be used there should be suitable allowances made for both structural and thermal movement.

Should dry joints be used they should be so designed to ensure that rain and snow will not pass through them. Alternatively, the joints should be designed to ensure that any rain or snow that does pass through the joints is directed to the front face of the cladding and will not penetrate the back layer of the cladding. The decision on whether dry joints are suitable should be made by a suitable trained/qualified person who can make the determination based on the cladding to be used, the design of the joint and the severity of the conditions in which the cladding will be located.

All elements of cladding should be securely fixed in place as per the manufacturers specification and any other relevant standard or code.

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