Indy Total Construction

An important consideration when building an extension is the position of any drainage pipe run — either an existing one from the house or any new waste pipes from fittings in the new extension. You must sort out the route the pipes will take before the walls are built since they will pass through them below floor level, and openings must be left in the walls as they are built.

Lintels will need to be incorporated to support the wall above the openings. If the extension is to have trench-fill foundations, ducts should be made in the concrete to allow the passage of pipes. A simple method is to set slightly larger pipes in the concrete as it is poured and then run the pipes through these round openings later.

The positions of the inner and outer leaves of the walls should be marked centrally on the concrete of the foundations with chalk. The center lines of the wall and foundations being within lin of each other.

As the walls are built, stringlines are stretched between the corners to make sure each course of bricks or blocks is laid in a straight line.

Although you can use brick for both inner and outer leaves of the wall, in practice it makes more sense to use lightweight concrete blocks for the former since these will provide a certain amount of insulation — a requirement of the Building Regulations.

With this type of construction, the inner leaf is the load-bearing part of the wall, carrying the weight of any floors and ceilings so lintels must be fitted across doorways and windows. Steel boot lintels are best since they are relatively lightweight and their shape ensures that any water that penetrates the outer leaf of the wall is prevented from reaching the inner leaf and is channeled out over the toe of the boot.

The two leaves of the wall should be constructed simultaneously, laying a few courses of each at a time. As construction proceeds, the two leaves must be linked together with metal or plastic wall ties to prevent them leaning away from each other.

Ties are designed to prevent water running across them to the inner leaf but they must still be set in the mortar joints so that they slope downwards slightly towards the outer leaf. Ties should be set about 18in apart vertically and 3ft apart horizontally, the positions in each horizontal row being staggered with those above and below. At door and window openings, ties should be set one above the other at 12in intervals.

Water penetration must also be prevented from below and this is achieved by inserting a flexible bitumen damp-proof course (DPC) in a horizontal mortar joint around the base of each leaf, at least two courses of bricks above ground level.

When the floor is laid, a damp-proof membrane (DPM) is taken up the walls and tucked under the DPC. Strips of DPC must be fitted in the vertical mortar joints where the inner leaf is turned to close off the cavity at windows and door openings, and below the threshold of the door, linking to the DPC in the outer leaf.

The walls must be toothed into the existing house walls at alternate courses to ensure permanent stability.

Although a wood-framed partition is easy to build, it does not provide the most effective sound insulation and it will need extra strengthening if it is to carry shelves or cabinets. In situations such as this, a partition built from lightweight concrete blocks is much more suitable. However, you cannot build such a partition on an upper floor, since even a reinforced concrete floor is unlikely to be strong enough to carry the load of a concrete block partition. A concrete first floor makes an ideal foundation and even a suspended wood first floor will do if a full-width wood sole plate is put down first, but check with your local Code.

Before starting work, the floor, walls and ceiling should be stripped of all coverings and any coving and base cut away with a chisel to clear the blocks. The easiest way to mark the position of the partition is

with a chalked plumbline, snapping it against the floor to leave two parallel chalk lines the width of the blocks. Continue these lines up the walls and across the ceiling making sure they are vertical.

For strength, it is best to tie the partition to adjacent walls by cutting recesses in them to accept the end blocks of alternate courses or similarly by using galvanized metal ties screwed to the walls and buried in the blockwork mortar joints. Nailing a guide batten to the wall against one of the chalk lines is also a good idea to help with the alignment.

Trowel a 6in wide layer of mortar (1 part masonry cement: 6 parts soft sand) across the floor to span the chalk lines on it, leveling it out to about 1/2 an inch thick. Then scribe a guide line through the mortar in line with the chalk marks on the end walls, using the point of your trowel and a long straight-edged plank.

There are many different types of concrete block to choose from, but the best types for building an internal partition are known as aerated blocks (A).

These are light in weight, so they are easy to handle — an important quality since they are twice the size of a normal brick. This fact also means that you can build a full-height partition relatively quickly. You can drill them, knock nails into them or, using a general-purpose saw, cut channels in them (B) to conceal electric cables and pipe work. Sound will not pass through them as easily as it would a woodframed partition, nor will heat.

Aerated blocks should be laid in the same manner as bricks in a “stretcher” bond pattern with mortar joints. Their normal size is 17 x 81/2 x 4in. For finishing, you can either plaster them directly or nail on battens and fix a gypsum board cladding to the battens.