The first document in Building Control Approved Documents series, Approved Document A, is concerned with the structure of a building and ensuring that it is safe, stable and will not collapse.
The document itself is in place to make sure that walls are constructed to a certain specification (e.g. are thick enough and supported by the correct foundation), cover a certain floor area, timbers and supporting beams are correct for the load they will be carrying, walls and chimneys are a certain height, wall coverings such as render stays on the wall and roofs can support the weights they are intended to carry.
In light of this, Approved Document A is broken down in to the following areas:
- Loading on areas of the building or structure
- Movement of ground
- Disproportionate or unreasonable collapse
Loading on Areas of the Building or Structure
In terms of the load imposed on a building or structure, the document states that any building should be constructed to make sure that all the combined loads that a building or structure will be subjected to (imposed load such as the weight of the roof and also wind load, snow load etc….) should be suitably transfered down to the ground to a stable and solid foundation.
It also states that the transfered load should be done safely and also avoid any damage to the rest of the building or damage to any surrounding structures.
The above paragraph mentions "imposed load" and this needs to be clarified. Imposed load covers several things but one of the best examples is from snow. A heavy snowfall can add a great deal of additional weight to a roof and although the area you live in may only get one or two snowfalls a year, the roof still needs to be designed to carry this additional load.
With this in mind, the amount of additional or implied load that a roof will need to carry that will be stated by your local authority building control office will vary depending on the area of the country in which you live and the likelyhood of heavy snowfall e.g. implied loads will most probably be greater in Scotland than they will be in Cornwall.
Another type of load mentioned is wind load. If you live in a particularly exposed and windy location (e.g. on the coast of the Isle of Skye) then the force that the wind subjects a bulding to will be much greater than one located in teh centre of a large city and due to this the structure will need to be designed so that it can withstand these additional forces.
In terms of ensuring that a property is stable and that the design and construction meet with the rules stated in Approved Document A, the following guidelines need to be fullfilled:
- The actual size of the building a proportions in terms of width and height are inline with the guidelines stated for particular construction types
- Both internal and external walls are constructed to form a 3D box-type structure with the number of rooms within restricted to those stated by the guidelines
- Internal and external walls should also be suitably connected together
- Intermediate floors (if present) and the roof should be both constructed and connected to the walls structures in such a manner that they give support and also aid in dealing with wind load
Within this section of the document it also states the requirements for sizes of timbers for particular areas of the building (e.g. roofing timbers, floor joists etc….), required thicknesses of walls, maximum height of the building (including wind speed map), maximum floor area, required heights of chimneys above roof surfaces, foundation requirements and the required strength of any masonry. Due to the complexity of many of these guidelines, please refer to teh following sections of Document A as follows:
- Timber Size Requirements: The specifications and guidelines for timber sizes (except for roofs that incorporate trussed rafters) can be found in the publication by TRADA or by refering to BS EN 1995-1-1:2004, also in the BSI published document PD 6693-1:2012 and finally by also viewing BS 8103-3:2009
- Required Wall Thickness: When dealing with "small buildings" such as residential dwellings less than 3 storeys high, small single storey buildings and annex-type buildings that join residential buildings such as a garage, please refer to section 2C of Approved Document A
- Building and Wall Heights: For height details for buildings you will need to refer to paragraph sections 2C4, 2C16, diagrams 1 and 2 and diagram 7 table C within this section
- Floor Area: The general guidelines here are that no floor space should be over 70m squared where it has structural walls surrounding all four sides and no more than 36m squared if it has a structural wall covering three of its sides. Full details can be found in Section 2 sub-section 2C14, Diagrams 3 and 5 and Section 2C38
- Chimney Height: For rules on the height of chimney that you can have and the width it can be in relation to the height and construction material requirements please see section 2D
- Foundation Requirements: For information and requirements for foundations (using plain concrete) for both normal foundation situations and areas where the ground may be softer you will need to refer to section 2E and section 2E4 including Table 10
- Masonry Strength: This section is concerned with the fact that any masonry used within a construction is such that it is strong enough to cope with any weight, load or force applied to it. Full details on this can be found in Section 2C20 of Document A, 2C21 and also Tables 6, 7 and Diagram 9
- Roofing: It must be ensured that any material used to cover a roof area is capable of coping with any concentrated imposed load upon the roof. For more information concerning this please refer to Section 4 of Approved Document A and also BS EN 1991-1-1-1:2002
- Wall Cladding: When it comes to wall cladding all forms of cladding are included e.g. render, non-structural coverings such as curtain walling etc…. it must be ensured that all forms of load are suitably transfered onto the support structure, the cladding itself is securly fixed to the structure, that it moves with the structure and that all forms of fixings are durable and intended to last at least as long as the cladding itself. For further detailed information please refer to Section 3
Please also pay attension to Section 1 that contains all the relevant Euro codes, British Standards and references that govern many areas covered within the document. It is important that you read these in full as they contain further details and specifications beyond those stated in Approved Document A.
Movement of Ground
As you would imagine, any building or property needs to be constructed on stable and solid foundations that are capable of coping with the pressure and forces applied to them by the building above.
The surrounding ground that a building is constructed on play a huge part in how successful a building is in remaining upright. As we know, different materials act differently in certain conditions and this is no more evident than in construction.
Masonry and concrete expand and contract due to heat and cold as different rates than timber does and both react in different ways when exposed to moisture and this trend goes for all the common materials used to form a building.
These traits are also true for the ground that surrounds a building and due to this, Section A2 of Document A exists to ensure:
- Ground movement caused through freezing, expansion and contraction does not affect the stability of a building
- Damage through movement caused by ground subsidence (not including shrinkage) or land-slip potential is taken fully into consideration (as far as can be seen) and the property and foundation is constructed to ensure its stability
The stability of ground can be affected by many things and in most cases if you are planning construction in an "at risk" area then the local authority will be aware of this and adise you accordingly.
One of the most common causes for instability comes from mining and the many disused mines that are scattered around the country and if you find that the ground on which you are proposing a construction is liable to subsidence or any other instability issue then it would be a good idea to consult the DOE Planning Policy Guidance Note 14 for development on unstable ground as this will provide further details and suggestions on how to overcome these issues.
As we have established, the foundations of a building play a massive part in its stability and to these ends they must be formed on solid, stable ground that is limited to the effects of any form of movement. With this in mind, foundations should be:
- If foundations are not formed on rock, a strip foundation should be a minimum depth of 0.45m to its underside as this will avoid any affects from frost. This depth may have to be increased in areas that are subject to prolonged frost or where loads need to be transfered to more stable ground
- Clay-type soil can be prone to quite significant shrinkage changes from drying out and due to this the following depths to the underside of teh foundation should be achieved:
- Low Shrinkage: Minimum of 0.75m
- Medium Shrinkage: Minimum of 0.9m
- High Shrinkage: Minimum of 1.0m
It should also be noted that surrounding vegetation growth and trees need to be considered and where a stable base cannot be established within the above stated depths then these depths will need to be increased until one can be found.
Disproportionate or Unreasonable Collapse
The basis of this final section of Document A (Requirement A3, Section 5) is in place to confirm that a building or structure will remain standing in the unfortunate event of an accident, within reason concerning the extent and nature of the accident.
This is obviously quite an extensive area and can cover a great many different buildings that could each be subjected to a different potential accident. Due to this, buildings are split up into four "Consequence Classes" as follows (covered in detail in Document A, Section 5, Table 11):
- Consequence Class 1 Buildings: Constructed inline with Approved Document A guidelines
- Consequence Class 2a Buildings: Low risk properties
- Consequence Class 2b Buildings: Higher risk properties
- Consequence Class 3 Buildings: Buildings that exceed the area or number of stories defined under 2a and 2b
Where a building does not fall into one of the above categories or where the type of construction may cause it to be more susceptible to the risk of collapse and due to this require much more stringent checks, additional recommendations can be saught using the publications outlined under Section 5.4
As this is quite a complex area and requires all construction guidelines to be followed to the letter, please refer to Section 5 of Document A for all approved construction methods.
In light of the nature of this approved document and the subject it covers – namely the foundation and structure of a property, it is imperative when referencing any of these guidelines that you are referring to the most recent and up-to-date guidelines so with this in mind please use the above information as an overview only and always use the official Approved Docuemnt A that can be downloaded from the Planning Portal website on the link at the top of this page.
All project content written and produced by Mike Edwards