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A guide to cavity trays

The external masonry of modern buildings, especially those built after the 1930’s are cavity walls. The outer leaf and inner leaf of the walls should have a minimum of 50mm gap, be filled with insulation and tied with a wall tie. Cavity trays are included in cavity wall construction where there are penetrations across the cavity. Such as;

– At an abutment  with a roof

– Above openings such as windows and doors

– Where extensions are constructed against existing walls

– Above a concrete slab or beams

– Above airbricks, ducts and pipes

-At the bottom of a wall if the cavity does not extend 225mm below dpc.

Cavity trays prevent moisture, travelling downwards, being carried to the inner leaf. Whereas a dpc tends to protect a build from rising damp.

The diagram above shows a typical cavity tray detail. Cavity trays should be provided to ensure water drains outwards;

– Where the downward flow will be interrupted by an obstruction such as a lintel.

– Under openings, unless there is a sill & the sill and its joints form a complete barrier.

– At abutments between walls and roofs.

Cavity trays can be formed using a pliable material such as lead but more commonly they are preformed and come in a range of different shapes and sizes. Cavity trays must always be bedded onto fresh mortar.  Most damp ingress occurs around either window/door openings or roof abutments.

When using roll dpc material, the profile must rise a minimum of 140mm within the cavity and should be shaped to promote the easy removal of mortar droppings. The tray should be positioned directly above the opening and not several courses above. Trays formed this way, using roll dpc material must have stop ends formed by folding the ends into the ‘perp’ joint of the outer leaf. The tray must extend beyond the lintel ends.  Weep holes should be provided at 900mm max centres and at each end of the horizontal cavity tray.

 

 

 

 

 

 

 

 

Plasterboard

Plasterboard consists of gypsum plaster sandwiched between x 2 layers of thick paper. It is usually fixed to walls and ceilings. Plasterboard can be plastered over or painted. Using plasterboard as a wall finish can be much cheaper than wet plastering. The most common thicknesses for plasterboard are 9.5mm and 12.5mm. Larger sheets of plasterboard come in 8ft x 4f sheets. They can come in 15 & 19mm thicknesses also and have a variety of specified uses.

Common types of plasterboard

Wall board is suitable for most applications where normal sound levels, structural properties and fire specifications are found. It can be fixed at a double thickness to give greater assistance with fire and sound levels.

Vapour barrier board has a thin metallic film on the reverse sheet. This film stops warm vapours passing through the board which could lead to condensation when passed through to a colder surface.

Acoustic board has a higher density core providing a good level of sound insulation. It is effective on thin walls and ceilings making it useful in blocks of flats. It is most often used in conjunction with mineral wool and various sound proofing fixing methods such as resilient bars which keep the surface of the board slightly away from the surface. It is usually a blue colour.

Fire board has glass fibre and other additives in the core to protect against fire in most domestic situations. Usually fire resistant plasterboard comes with a pink face which can be plastered or painted.

Water resistant board has water repellent additives in the core making them suitable for a base for tiling some wet areas. This type of plasterboard usually comes with a green face and is not usually plasterboard. They are good for high humidity areas but, as all gypsum plasters soak up moisture quickly, it would be counter-productive to plaster them.

Impact board which has a very dense core is designed to resist impact and day to day knocks. This board is usually yellow.

4-in-1 Plasterboard With recent developments in technology some manufacturers now produce plasterboard that features not only the ability to be used as a normal wall board, but are also water proof, fire resistant and impact proof.

Lintels

A lintel is essentially a structural support that spans an opening in a wall. The use of lintels in architecture dates back many centuries, from the highly decorative masonry creations used in ancient Greece, to the timber lintels seen in Tudor times. These days lintels are mainly constructed in concrete or steel.

Steel lintels are lighter than concrete and are easier to manually handle than concrete. Steel lintels are popular because they don’t interrupt the look of brick work by being hidden during their construction process. Most steel lintels are made from pre-galvanised steel which is cut into shape.

When selecting a steel lintel you have to consider; the type of wall under construction (cavity, solid, timber etc) and the lintel length which is worked out by calculating the total width of the structural opening plus 150mm end bearings at each end. Another important calculation to consider is the dead and imposed loads the lintel will be under.

Lintels offer structural support and play a part in reducing heat loss in a building. An opening in the building fabric is a possible route for heat to escape which is known as thermal bridging. Thermal brake plates are lintels which have been designed to overcome thermal bridging. There are various designs of lintels which help eliminate thermal bridging.

Fire stopping

Section 10.2 of the building regs states; If a fire-separating element is to be effective, every joint or imperfection of fit, or opening to allow services to pass through the element should be adequately protected by sealing or fire stopping so that the fire resistance of the element is not impaired. In the case of a fire, passive fire protection helps seal gaps and stop the spread.

There are different types of opening that can compromise the integrity  of a fire resistant structure, like, opening for pipes, vent duct, flues etc. Where openings have to be created, they should be kept too a minimum, the smallest size possible and fire stopped.

Intumescent collars – These are designed for use on plastic pipes that pass through masonry floors and walls. The intumescent material expands inwards in a fire situation, to squeeze the collapsing pipe until the opening is completely sealed.

firecollar

Fire batt –  Fire batt is an insulation type material (Rockwool) which acts as an air seal barrier to reinstate the fire resistance and acoustic performances of concrete floors, masonry walls and dry walls systems, following the creation of voids for service passage. A 50-60mm thickness can give you up to 4 hours fire resistance. Where gaps of +10mm are apparent, fire batt is a more suitable solution.

fire-rated-batts-500x500

Intumescent mastic – Fire mastic is a versatile sealing solution to fill gaps and movement joints. When exposed to fire the intumescent sealant expands in volume to fill cavities. It also acts as a smoke seal. It is also suitable for sealing gaps between fire resistant walls, floors, between conduits and structural supports.

Intumescent foam – Where large gaps exist between the door frame and wall construction, conventional intumescent seals may not always be appropriate, but foam will. It can also be used for filling around ducts and other service penetrations, expansion joints and linear joints. Its flexible and will accommodate movement, yet is durable and resistant to extreme environmental variations. On activation the fire resistant foam degrades and the volume is replaced by an expanding mass of intumescent graphite which prevents the passage of smoke, flames and hot gasses.

44-pva_foam

Bricks, spoiled for choice

 

Facing bricks

Facing bricks are the most popular type of brick and have been the facade material of choice in the UK for thousands of years. They are primarily used for external walls of a building and are chosen for their aesthetic qualities. Facing bricks include any brick which is sufficiently hard burned to carry normal loads and is capable of withstanding the effects of rain, wind, soot and frost without breaking up.

Engineering bricks

Engineering bricks have high compressive strength and low water absorption. They are used for their physical characteristics and not their appearance. They are traditionally used in civil engineering projects and are most suitable for applications where strength and resistance to frost attack and water are important. Situations where they are used include, ground-works, manholes, sewers, retaining walls and damp proof courses. The two best known engineering bricks are the red Southwater brick and the blue Staffordshire brick. Both are very hard and dense and do not readily absorb water. The ultimate crushing resistance of engineering bricks is greater than 50N/mm2.

Engineering bricks are rated as class A or B with class A being the strongest. Class B engineering bricks are more common than class A bricks. They are both commonly a smooth red colour although blue engineering bricks are also widely available.

Common bricks

Despite their name, common bricks are actually the least common brick types in the UK. They tend to have lower compression strengths than facing bricks or engineering bricks and are generally lower quality. There is also less focus put into a consistent appearance on common bricks. Common bricks are generally used for internal brickwork only.

London stock brick

London stock bricks are a type of handmade brick used for a majority of building work in London. Its distinctive yellow colour and soft appearance come from yellow clay. London stocks are still made in comparatively small quantities in traditional brick works. Red stock bricks are also quite common, but the yellow or brown coloured bricks are usually known as London stocks. This brick is usually manufactured in Essex and Kent. They are made from clay composed of sand and alumina to which some chalk is added. The manufacturers grade the bricks as 1st hard, 2nd hard, and mild.

Stock bricks

The term stock brick can either indicate the common type of brick stocked in a locality or a handmade brick made using a stock. A stock or stock board is an iron-faced block of wood fixed to the surface of the moulder’s bench. The brick mould fits over the stock; the brick maker fills the mould with prepared clay and cuts it off with a wire level with the top of the mould; before turning out the ‘green’ brick onto a wooden board called a pallet for drying and frying.