Building an Extension

Filed Under: Do it yourself, Home repair, Remodeling    by: ITC

A ground floor extension can be purpose-designed and built to suit your needs exactly, or it can be constructed from a number of standard prefabricated components purchased from an extension manufacturer. Which type you choose depends on what you will use it for. The former is ideal for bathrooms, kitchens, bedrooms, living rooms; the latter is more suited to laundry rooms, sun rooms, children’s play rooms, work-shops and so on, and includes simple metal-framed, full-glazed conservatories.

The most important parts of the structure of your new extension are the foundations, which support the walls and spread the load evenly across the ground. Consequently, their design is quite critical and should be carried out after consulting your local Building Code which will specify the type of foundations required for the job and the depth to which they must be dug, based on local ground conditions.

To be effective, foundations must lie on firm, stable sub-soil, and depending on the soil type this may mean digging to a depth of 3ft or more. The type of soil will also dictate the type of foundations needed, as will the method of construction of the extension.

For a purpose-built extension with brick or block walls, it is usual to lay concrete in a trench and build the walls on top but for lighter constructions, such as prefabricated buildings, a slab of concrete known as a “raft” is more common.

The most common form of foundation is the “strip” type. With these a layer of concrete at least 6in thick is spread along the bottom of the trench, leveled off, then the walls built on top. Normally, a width of 18in is quite adequate, but at depths below 3ft or on certain types of weak soil a width of 30in or more is preferable — often with steel reinforcement added.

The trench-fill foundation is filled with concrete to within 6in of the ground level and the walls begun.

The concrete for this type of foundation should be at least 20in deep and about 6in wider than the width of the wall. The sides of the trench must be vertical to prevent any possibility of the load above causing the foundations to topple.

The walls of a habitable extension to your house must be of cavity construction; that is comprising an outer leaf of bricks and an inner leaf of bricks or. more usually, concrete insulating blocks with a 2in Pit- gap in between giving a wall thickness of 1 lin, although the cavity may be 3in wide to accommodate polystyrene slab insulation and still leave an air gap.

Even if the main part of your house has solid outer walls, the Building Code specifies that your extension must be of cavity wall construction.

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