THE CONSTRUCTION PRINCIPLES OF
The Domestic House
The information below gives a concise description of a domestic building construction and an overview of the ways in which property owners can carry out preventative maintenance of damp damp related problems. It also gives property owners an insight as to what Building Surveyors look for when checking the water tightness of the external fabric.
Traditional pitched Roofs
A pitched roof is usually a simple inclined beam structure, on a timber frame. The structure supports loads imposed on the roof from the weight of the materials and external elements such as wind and snow.
These loads are transferred to the support point on the load bearing walls, usually the inside leaf. Walls are typically conventional load bearing masonry which transfer loads to the foundations.
With cavity wall construction most of the load is carried by the internal leaf of the brickwork or block work. The external leaf provides stability to the load bearing inner leaf by increasing its overall thickness, usually tied together with metal cavity ties, and also provides weather-proofing.
Solid walls rely on the thickness of the material to prevent weather penetration. The principal is that weather hitting the wall will be soaked up by the masonry. Provided that the wall is not too exposed and that there is sufficient heat and air movement, the water will evaporate away before it penetrates completely through to the wall. If the walls are particularly exposed or inadequately maintained penetrating dampness may occur. Thin walls are more vulnerable to penetrating dampness.
Where there are openings in the walls, either brick arches or beams/lintels transfer the weight from above and around the openings to the support point. The thrust created at the support point is resisted by the weight of the masonry on each side of the opening.
Dependent upon the orientation of the elevations, different parts of the building can be more prone to external factors. For example warm and wet winds typically come from the west and south-west, which are likely to create the potential for weathering and penetrating dampness and rot.
North and north-eastern elevations tend to be more cold and relatively dry, although can be more prone to the weathering effect from frost damage or condensation.
Moss build-up on roofs, which can wash off into gutters is also likely to be more pronounced on north and north-eastern elevations. South and south-westerly elevations are generally more exposed to high temperatures during the day and weathering, such as expansion or cracking in masonry, or paint finishes is a possibility.
Timber Framed Buildings.
The general principals of a timber frame construction are framed timber sections pre formed off site and clad in sheet timber material. The site is prepared including drainage, foundations and concrete slab in readiness for the timber frame. The timber frame is designed to take the loading of the floors and roof and are transferred to the concrete slab through to the foundations.
The exterior brickwork is still built in the conventional way, on a section of the concrete slab around the perimeter of the timber frame, In this way you enjoy the benefits of timber frame inside the home, without compromising on the strength afforded by conventional block walls, the external skin provides weather proofing and support for the windows and doors.
The timber frame is heavily insulated and a cavity is provided between the timber frame and the outside skin.
Where there are openings in the walls, either brick arches or beams/lintels transfer the weight from above and around the openings to the support point. The thrust created at the support point is resisted by the weight of the masonry on each side of the opening.
GENERAL RECOMMENDATIONS
All new and refurbishment detailing should be assessed for its effect on environmental and structural health. General principles are set out below. Special care is required when introducing new materials, moisture sources or heating and ventilation systems, for example air conditioning
ROOF AND SURFACE DRAINAGE
Maintenance
All guttering, hopper heads and outlets should be regularly checked and cleared to keep them free of debris, especially during the autumn months
Protection
Hopper heads, gutter outlets and ground gullies should be protected with metal mesh cages so as to prevent blockage and overflow. These should extend higher than the expected water level to reduce the tendency to block and should be easily removable to allow cleaning and maintenance.
VENTILATION
Structural voids
All structural voids within the building should be provided with adequate through ventilation so as to prevent moisture build-up, such as, air bricks in external walls ventilate below timber floors, roof vents in eaves and roof surfaces to ventilate loft areas. This must be done with regard to the applicable fire regulations
Chimneys
All chimneys not in use should be capped so as to minimise water ingress but so as to also allow maximum ventilation of the flues. Flues should be cleared and cleaned to remove blockages. Fireplaces and chimney breasts should be opened or vented to allow through-ventilation of the flues. This prevents moisture build-up in the flues and helps interior ventilation by the stack effect, (this is the process where air rises and will go up ventilated chimney breasts and out through the chimney pots, thus drawing air into the property at lower levels helping to ventilate the rooms)
Bathrooms and kitchens
All bathrooms and kitchens should be provided with adequate ventilation to the exterior, by way of extractor fan systems and means of ventilation to provide replacement air during cooking and washing. These should run on for at least fifteen minutes after occupancy
Roof spaces
All roof spaces, including flat roof areas and gutter soles, should be provided with adequate through ventilation. This may occur via the gaps between slates in un-sarked pitched roofs. However, flat roofs and pitched roofs with sarking or insulation will require the installation of vents through the roof surfaces or at the eaves and ridges. Insulation material in roof spaces should be kept clear of external walls, gutter soles or timbers in contact with damp or potentially damp masonry
Windows
Windows should be refurbished so as to allow easy and convenient opening and closing by occupants in order to encourage proper ventilation of the building. This is important both for environmental and structural health of the building. Windows should be fitted with security locks so as to allow secure locking in a partially opened position.
STRUCTURAL DETAILING
New timbers
New timbers should be isolated from any damp or potentially damp masonry with a damp proof material or ventilated air gap
Timber repairs
Structurally decayed timbers should be removed or cut back to sound timber unless required for aesthetic reasons. Timbers should then be partnered or spliced. If steel plates or hangers are used, they should be detailed so as to allow sufficient ventilated air gaps and drainage to prevent moisture build-up due to condensation. No timber preservation or remedial treatments should be required
Paint finishes and wall coverings
Moisture vapour permeable or ‘micro porous’ finishes should be preferred for internal and external surfaces. This is especially important on window timbers. To take advantage of the properties of such paints, the complete removal of old alkyd paint systems is recommended. Health and Safety: Special precautions should be taken during surface preparation of pre 1960’s paint surfaces as they may contain harmful lead