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Back Boiler

A small boiler often set behind a fireplace or room heater to provide hot water/central heating.

Back Gutter

A small gutter formed between a chimney and an adjoining roof slope, usually lined with lead or a similar supported roofing material.

Balanced Flue

Combined intake and outlet flues, usually arranged concentrically; one drawing combustion air inward and the other emitting exhaust fumes. Sometimes fan assisted.


A projecting platform from a wall, often supported by posts, brackets or cantilevered. A balcony may be enclosed by rails, a balustrade or parapet walls.


Unscreened mixture of sand, gravel and small stones. Sometimes used to make concrete.

Ball Valve

A valve operated by a float to control the level of water entering a water storage tank or cistern.


A vertical post used in a series to form a balustrade.


A protective guard rail at the edge of a platform, landing or stairs to prevent people falling. The gap beneath the rail is often filled with balusters.

Band Course

A decorative projecting course of stone or bricks in an elevation, often forming a drip or a continuation of window sills or floor detail.

Barge Board

A fascia fixed to the verge of a gable roof, most commonly formed in timber but may be uPVC.

Bark-boring Insect

A common form of wood boring insect that is found in the bark and outer layers of timber. Generally cause superficial damage only. Please see Wood-boring insect.


Please see Cellar.


Lengths of thin timber to which sheet material, tiles or slates are fixed.

Bay Window

A window formed in a wall projecting from the main elevation and supported by its own foundations. If rounded, it is called a 'bow' window.


Sloping cement or concrete at the base of an inspection chamber to form a deep channel either side of the drainage run. Sometimes referred to as 'Haunching'.


A tie used to fix a number of framing components together, usually within a roof; for example a ceiling joist binder. May also refer to a binding agent within a mix of different materials.

Birdsmouth Joint

A 'V' shaped cut in a timber to allow its connection to another, often found at the foot of a rafter where it fits over the wall plate.


A smooth black derivative of petroleum, similar to tar or pitch. Bitumen becomes a liquid when heated and is often used for bonding compounds, waterproof binders, sealants, vapour or water barriers and roofing materials including various felts. Bitumen oxidises when exposed to sunlight and air, and therefore requires a degree of solar protection.


Pre-cast masonry blocks, usually concrete. Usually quicker and cheaper than traditional brickwork. Whilst it has better thermal insulating qualities it is often more prone to movement and moisture retention.


Refers to the connection of different materials via a mechanical key or through chemical adhesion or possibly a combination of both. Bond also is commonly used to refer to the layout of courses of bricks. Most modern walls are laid to a 'stretcher' bond. Older walls may be of 'English, Flemish, Garden-Wall, Header or Rat-trap Bonds'.

Borrowed Light

An interior window positioned within a partition wall to transfer natural light and help illuminate darker areas within a building.

Box Gutter

A roof gutter formed of wooden boxing and lined with lead, bitumen or a similar impervious material. They are usually positioned below the roof line and flow to an outlet. Box gutters are common in older properties and are prone to blockage and leakage.

Breather membrane

A microporous membrane designed to allow ventilation and the escape of water vapour but to help resist driven moisture. Used on the 'cold' side of construction/insulation as opposed to a 'vapour varrier'.


A large lintel formed over an opening to carry a wall above, traditionally formed of timber but now more commonly formed of reinforced concrete or steel.

Building Regulations

Statutory control over the layout and materials used in buildings to ensure their strength, durability and the health and safety of the occupants. Technical guidance contained within the 'Approved Documents' and implemented by Local Authorities' Building Control.

Butt Joint

A joint between two square ended pieces of material that does not overlap.