Construction Technology and Surveying
Main functional requirements of floors
PH2A – Stability
A floor is designed and constructed to serve as a horizontal surface to support occupants, furniture, equipment or machinery. The floor should have adequate stiffness to remains stable and horizontal under the dead load of the floor structure, such partitions and other fixtures its supports along with the anticipated static and live loads that the floor is designed to support. The floor structure should also support and accommodate services either in its depth, void or below the floor without affecting the stability. Solid ground and basement floors are often built off the ground from which they derive support. The stability of such floors therefore depends on the characteristics of the concrete or other material under the floor. Upper or suspended floors are suspended by walls or beams and should have adequate stiffness to minimize deflection under load. Under load the floor will deflect and bend, this deflection or bending should be limited to avoid cracking of rigid finishes for example plaster board which is attached to the ceiling below.
PH2A - Ground Supported Concrete Slab
It is general practise first to build the external and internal load bearing walls from the concrete foundation up to the level of DPC. The hardcore bed and the concrete oversight slab are spread and levelled within the area created by the walls. If the hardcore is spread and levelled over the whole area of the ground floor, into and around excavations used to construct foundations and where soft ground has been removed. There should be very little settlement of the ground that supports the floor slab. Settlement cracking in ground floor slabs is often due to inadequate hardcore beds, poor filling of excavations, the trenches or ground movements due to moisture changes (swelling or shrinkable clay soils).
PH2A – Durability and Maintenance
All floors should be durable for the expected life of the building and should generally require little maintenance or repair. The durability and freedom from maintenance of floor finishes will depend on the nature of the materials used and the wear to which they are subject.
PH2A - Fire Safety
Suspended upper floor should be so constructed as to provide resistance to fire for a period adequate for the escape of the occupants from the building. The level of resistance to fire varies from half an hour to four hours depending on the size, type and use of the building. The requirements generally are set out in the building regulations approved documents B.
PH2A – Resistance to Moisture
Requirements of the building regulations for the resistance of the passage of moisture through ground floors to the inside of buildings are generally dealt within approved document C of the building regulations.
PH2A – Resistance to the Passage of Heat
The floor should provide resistance to the transfer of heat where there is normally a significant air temperature difference on opposite sides of the floor. This would include any building which is heated but would not include certain external buildings such as garages etc. (approved document L in each addition)
Construction Technology and Surveying
Main Functional Requirements of Roofs
PH3A - Cold Roof
Within a pitched a roof the most logical and convenient place for insulation to go is either at the top of or between the ceiling joists. This is called a ‘cold roof’. With cold roof insulation the materials that are typically used are mineral wool, rolls of fibreglass or thermal boards. The fibreglass and thermal boards are spread across and between the joists. This is done so that the layer of insulation even over the whole roof space. The hatch to the loft should be insulated and sealed to prevent drafts. When service pipes penetrate the ceiling finishes there needs to be an effective way to form a draft sill. All water services that carry items such as service pipes, water systems, or tanks in cold roof spaces must also be protected with an insulating material. This is to prevent any damage occurring; e.g. water freezing.
PH3A - Warm roof
An alternative place for the insulation within pitched roofs is between, above or below the roof rafters. This method tends to be more expensive as the area is greater than just covering the ceiling trusses. This method is used when the roof space has been converted into another room (a loft conversion). The major advantage of this way is the roof space will be warm due to heat rising from the heated space below and consequently be comparatively warm and dry with insulation on the sloping roof; hence the name, a ‘warm roof’. When constructing this roof you must consider the Building Regulations Approved Document L.
PH3A - Ventilation
When considering ventilating the roof space you have to take note of the Building Regulations Approved Document C and the National House Building Council 2006 standards. These provide guidelines to prevent excessive condensation occurring in the roof space. Cold roofs should be ventilated to the outside air. They should have vents on each side of the roof to allow for cross ventilation. Warm roofs need ventilation over the insulation as well. The suppliers of tiles and slates also have special fittings to provide ventilation through eaves, ridge and skin roof slopes.
PH3A - Vapour control layers
Vapour control layers help control the movement of warm moist air from inside a building to the cold side of the insulation layer. It is an impermeable material (e.g. polythene sheeting) which is placed on the under side of the insulation. Water vapour is not a common problem with regards to the warm side of the insulation because there are no cold surfaces for condensation to occur. Breathable sarking felt is a very good vapour control barrier as it allows moisture escape from the building but also prevents moisture and wind entering the building.
Construction Technology and Surveying
Structure, Walls and Components
VB1 – Structure
The structure of a building is basically the skeleton. They carry all the loads (the forces which act upon buildings). In load bearing constructions the loads are carried down the walls to the foundations. The foundations then spread the load into the ground. Within framed structures (e.g. steel, concrete or timber) the loads are carried down through a framework to the foundations. The walls are only there to divide space and to provide protection from the weather. An exception is within modern timber framed structures. The frame members are positioned reasonably close together and act more like a load bearing structure than a framed structure.
Most traditional structures have been load bearing. A good example of this is the old cathedrals. You can tell by looking at the picture to the right that the loads are carried down huge columns towards the foundations. They are then spread into the ground. Most modern domestic buildings are load bearing structures. Some of the walls carry the dead loads of the building, some carry imposed loads and some act as bracing against wind loads. The roofs in domestic load bearing structures sit on top of the walls (similar to the cathedral pictured to the right). The loads from the roof are then carried down the timber frame, through the load bearing walls and to the foundations. A good way of telling is a wall is load bearing or not is to tap it; if it sounds hollow it is made up of plasterboard and studs. These are not load bearing walls. Another consideration when constructing a building is the wind loads. Masonry construction is very strong and can resist wind loads well. It also helps the fact that the buildings are often relatively small and have a lot of internal walls.
Timber framed structures are becoming more and more popular with the modern domestic market. Historically they consisted of hardwood (typically oak) which formed the primary structure. This then carries the loads through columns and beams to the foundations. The secondary infill’s carry their own weight and simply add protect from the weather/privacy. Modern timber framed structures are completely different to historical structures. Computers are used to calculate all the loads on the inner leaf, internal walls, floors and roofs. They then work out the sizes for each component. The loads are carried through studs which are spaced at a maximum of 600mm; because of this the studs can be relatively small. A typically stud size is just 100 x 50mm. The roof trusses sit on top of the walls which also carries the loads from the floors down to the foundations. Bracing is also incorporated within modern timber design. It is provided by a layer of oriented strand board (OSB); the trade name for this is “Stramit”. There are also diagonal members attached to the trusses to prevent movement from the roof. Often the inner walls have a bracing function that adds to the strength of the structure. The simple way of testing if a wall is load bearing within masonry structures does not work for timber framed structures. This is because the loads are carried though the studs. It is too hard to tell how the structure is supported by simple methods.
A very brief construction comparison is that the masonry walls have two leafs. The inner leaf carries the loads from the floors and roof. The outer leaf is there to primarily to protect from the weather and add privacy. Within a domestic timber framed structure is it basically the same although the loads are carried through studs evenly spaced out throughout the construction. There is also a wider range of cladding options for timber framed buildings.
VB2 – Walls
There are many functions of walls within a building:
- Hold up the upper floors and roof,
- Keep out the rain,
- Stop rising damp,
- Reduce heat loss,
- Control solar heat,
- Divide space,
- Add privacy (internal and externally),
Historically walls used to be solid. They would either be single skinned (half brick wall), double skinned (brick wall) or rubble filled. These would not have any insulation, damp proofing or and real structural calculations. Traditional type walls are made up of bricks and blocks. They are bonded together with mortar. This is basically a glue that holds them together. They can also be laid in different ways; some add strength to the structure. Below are some examples of brick patterns.
Stretcher Bond (Also known as Running Bond)
English Bond
Flemish Bond
The strength of a wall comes from two main factors, these are the thickness and the restraints. A half brick wall can stand up to 750mm without any restraint. Any higher than this and it is easy to push over. If you restrain the wall it is possible to build a full storey high wall from a single skin. Restraints can come from the ends, head, loading or piers and buttressing walls. External walls gain their strength from having two skins. Historically they would use two skins side by side (bonded together). Nowadays there are two skins with a gap which forms a cavity. The outer leaf is exposed to the elements, the inner leaf supports the loads and internal finishes where as the cavity acts as a place to install insulation. The cavity wall has many advantages over historical walls. They help prevent driving rain from getting in, allows the possibility for a wider range of materials both inside and outside for strength or aesthetic purposes and it also allows a place for the insulation to go. As both skins are thin and therefore weak it is essential that you link the two together using wall ties. There is a British Standard for wall tie spacing and design. This sets out the typical spacing in a large stretch of wall and tells you where ties are needed around weak areas such as openings. This can be found in the Building Regulations Part A Section 1C. Apart from structural difficulties there are many other issues that need covering when building a wall. An important issue is to stop the rising damp. Because brick and blocks are porous they can absorb water up from the ground by capillary action. Because the bricks / blocks are built so close together water can rise up to 1m. The simple way of stopping this is by using a damp proof course (DPC). This is basically a layer of material that is waterproof placed at the bottom of a wall to block rising water. Another important issue is the thermal control. Today’s’ building regulations are getting tighter on insulation. This can be found in Part L . As we already know the insulation usually goes in the cavity. There are two types of fill; either a full fill or a partial fill. The insulation is held in place by the wall ties. Older structures can also have insulation upgrades. These can go internally, externally or even in a cavity fill.
A different construction method is to use timber. Currently masonry is most commonly used but timber is becoming more and more popular. Historically timber constructed buildings were made from hardwood; typically oak. This used to form the primary structure of the building. The loads would be carried through large columns and beams to the ground. The infill’s would be there for insulation and protection from the elements. They would only carry their own weight. Today computers can calculate the loads on the walls, floors and roofs. Within the walls the vertical members have a maximum spacing 600mm; therefore they transmit loads evenly to the foundations. The insulation is placed in between these studs (see diagram to the right/below).
References
Wall ties diagram - http://www.wincro.com
Timber house image - http://www.wilkinsonbuild.co.uk
Cavity Information - http://www.haringey.gov
