Excavation

• Foundation trenches should be straight and even with horizontal bottoms and vertical sides.
• They should be compact and reasonably dry. Rebottoming will be required if the trenches are allowed to crack or become water-filled.
• The excavations should be taken below visible roots (especially in clay soils) and any loose material must be removed before the concrete is poured.

Trench Fill Foundations

• The width of a trench foundation can be reduced to 450mm if ground conditions permit, although the bricklayer may struggle to lay the bricks and blocks in a narrow trench. However, there should always be 50mm projection from the brick outer face to the edge of the foundation concrete.
• Trench foundations can be dug deeper than strip, which means they are particularly practical where the water table is high, where soil is loose and unstable, and in areas with heavy clay soils.
• The trench sides may need to be lined with a slip membrane unless the soil is firm.
• The thickness of any trench filled foundation should be not less than 500mm and the foundation concrete should finish about 150mm to 100mm below ground level.
• NOTE: Trench foundation excavations in excess of 2.5m deep they should be designed by an engineer.

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Steps

• Steps in foundations can be used on sloping sites to minimise the amount of excavation and materials required by accommodating the change in levels.
• The height of a step should not exceed the thickness of the foundation (see below). In clay soils near trees, steps should not exceed 0.5m.

And for Trench Fill:

Concrete Mix

Refer to NHBC Chapter 2.1 ‘Concrete and its Reinforcement’.

• The standards designated for ready-mixed concrete for strip and trench foundations are known as GEN. A typical mix for non-aggressive soils would be a GEN1 or BS 8500.
• A site mixed ‘standard prescribed’ concrete (BS 8500), for non-aggressive soils would be an ST2 mix, given in table below.
• If the foundation is reinforced or sulphates are present in the ground or there is a ground water problem, these mixes are not adequate and a stronger mix will be necessary.

Standard Concrete Mixes

St2 mix for strip footings*

• An St2 mix to give 1 m³ of concrete having 100-150mm slump:

• 285 kg Portland Cement
• 735 kg Concreting Sand
• 1105 kg Aggregate

St2 mix for trench fill foundations*

• An St2 mix to give 1 m³ of concrete having 160-210mm slump:

• 300 kg Portland Cement
• 725 kg Concreting Sand
• 1080 kg Coarse Aggregate

* Recommendations for 20mm maximum aggregate size (assumes cement of standard strength class 32.5).

'Standard Concrete Mixes' Table: BRE Good Building Guide GBG53, 'Foundations for low-rise building extensions'.

Ground Floors

The construction of the ground floor can begin when the foundations have been laid, and all the trenches have been back-filled with properly compacted material and the loadbearing walls have been built up to the DPC.

A ground bearing solid concrete floor is by far the most common form of floor construction for extensions and small domestic works. However, the ground must be assessed in order to confirm that is suitable to support the floor and any other loads.

If the ground is formed of fill over 600mm, a suspended form of ground floor should be used.

A survey should be undertaken to ascertain whether sulphates or other hazardous materials are present in the ground. If so, special floor slab mixes, mortar, bricks, blocks and DPMs should be used which would require consultation with a specialist.

Hardcore

• If the fill depth exceeds 600mm a suspended floor will be required.
• To provide suitable material for the floor slab, a layer of clean hardcore at least 150mm thick but no more than 600mm is provided over the prepared oversite ground.
• The fill material used to make up the hardcore should contain nothing larger than about 100mm and be well graded inert fill without hazardous materials. It should contain a range of particles so that it can be firmly compacted, such as clean broken bricks, roof tiles, concrete or crushed stone, or ready-made loose granular material such as ‘type 1’ hardcore can be used.
• Fill should be mechanically compacted using a small vibrating plate or roller, in layers no thicker than 225mm so that no air pockets are present and settlement is avoided.
• A layer of a minimum 20mm (but can be up to 50mm) of sand blinding should be provided over the hardcore before laying the concrete or DPM and will be essential to prevent sharp stones from puncturing the sheet DPM.

Floor Slab

• A typical concrete mix for a ground bearing slab is 1:2:4 ‘GEN 3’ mix. However, where there is a risk of sulphates, or other harmful chemicals are in the ground, a special concrete mix may be required.
• The floor slab is usually placed over the DPM
• The floor slab should not be less than 100mm thick.
• Ensure all the services and ducts running under the floor are installed and tested before pouring the slab.
• If temperatures are likely to go below zero, the concrete should not be poured.
• In cold conditions, hessian should be used to protect the concrete once poured.
• In hot weather, newly poured concrete will need protecting with polythene to prevent it drying too quickly.
• If required, the concrete slab may be reinforced with a layer of steel mesh, typically A142 mesh.
• Once the concrete slab has been poured, it can be temped with a heavy beam to remove air and surplus water and ensure a level surface.
• The concrete slab should be left to dry out for around two to three days or as required in BS 8203:1996.

Floor Screeds

A 65mm deep sand/cement screed should be poured over the concrete slab or insulation boards and VLC.

• A typical screed mix is one part cement to three or four parts course sand.
• To avoid possible shrinkage, lay the mix fairly dry.

Board finishes

• If providing a board finish, this can be laid onto the insulation, providing there is a separating laying of polythene (VCL) over the insulation boards
• VCL should have 150mm lap joints and be continued up 100mm at room perimeters behind the skirting boards to minimise the risk of condensation forming at the insulation/slab interface, to prevent the screed penetrating the joints and to prevent moisture from the drying floor damaging the floor boards.
• Any chipboard used must be 18mm tongued and grooved flooring grade type C4 to BS 5669.
• Joints should be glued with a woodworking adhesive and laid staggered. These can then be sanded and stained, tiled or carpeted. Ensure a 10-12mm gap all around the edges of the floor to allow for expansion.
• In all wet rooms, for example kitchens, utility rooms and bathrooms, the flooring board must be a minimum of 20mm moisture resistant grade in accordance with BS7331:1990.
• Identification marks on the boards must be laid uppermost to allow easy identification.

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Walls below Ground

• The concrete foundations should be allowed to cure for at least a couple of days before the external ground walls are constructed to DPC level.
• The walls, usually brick or block-work, should be built up in the centre of the strip foundations (It is possible to build off-centre when using trench fill, as the concrete is considerably thicker). At least 150mm projection of the concrete foundation is required at either side of the wall.
• Check that the bricks or blocks are suitable for underground use. The blocks used below the DPC should be specified to comply with BS 5628 part 3.
• Ensure mortar below DPC is suitable for underground use.
• The cavity in the walls below ground should be filled with a weak concrete mix (stopped 225mm below horizontal DPC in the walls or cavity tray provided), to prevent the leaves being pushed together when the trenches are back filled.

In Cold Weather:

• Do not lay brickwork or blockwork when the air temperature is 2°C and falling.
• If the air temperature should drop below 2°c once built, walls should be protected from frost.

Wall Ties

The two skins of a cavity wall must be tied together at regular intervals with wall ties to ensure that the wall is structurally stable and strong.

• All wall ties should be stainless steel or non-ferrous and in accordance with BS EN 845.
• Wall ties should be long enough to ensure they can be embedded a minimum of 50mm in each masonry leaf.

Tie Types

There are a number of ties on the market, to suit particular cavity widths and wall thicknesses.

• Double triangle wall ties have taken over from the butterfly type as the most common in modern construction.
• Double triangle and butterfly type ties to BS 1243 are suitable for cavities up to 75mm.
• Vertical twist ties to BS DD 140 are suitable for wider cavities. Longer 250mm or 275mm ties can be used where cavities are over 100mm wide.

Tie Spacing

To provide structural stability, the wall ties should be spaced at regular intervals and staggered where possible.

• Ties should be positioned in the wall at 750mm or 900mm apart horizontally and 450mm apart verically. This will ensure a spacing of at least 2.5 ties per square metre.
• Provide a row of ties every sixth brick course. In blockwork, this will be every second course.
• There must be additional ties at window and door openings and at either side of movement joints. They should be positioned within 225mm of the side of the opening, at not more 300mm centres vertically ensuring a tie every block course or every fourth brick course.

Wall ties and cavity insulation

• In a partial fill cavity wall, wall ties can be spaced more closely to match insulation board height.
• Ties should be spaced at 600mm centres horizontally using 2 ties to support the insulation to coincide with the horizontal joints of the 1200mm long boards. They do not need to be staggered vertically.
• Retaining devices clipped to the ties should be used to hold the partial cavity insulation against the inner leaf.
• Make a clean cut in the insulation at reveals where wall ties need to be closely spaced.
• To ensure the wall ties do not cause cracking in the event of thermal movement, ties should not be placed within 450mm of returns in a masonry wall.

Tie installation

Moisture can travel along the tie towards the inner skin if the wall ties are not correctly installed.

• Ties should slope at a slight gradient down towards the outer leaf, so that any moisture can find its way out.
• The drip of the wall tie should point downward and be placed in the centre of the unfilled cavity.
• Ties should be fully bedded at least of 50mm into mortar joints on each leaf of the cavity wall
• They should be pressed down into the mortar bed and should not be pushed into joints.
• Ties must be kept clean from any mortar droppings and debris which may bridge the cavity. A cavity batten can be used to prevent mortar dropping into the cavity or onto the insulation.

Blocks

Introduction

All concrete blocks should comply with BS EN 1996-2. A Standard block is 440mm long x 215mm wide x 100mm deep.

Concrete Blocks come in a variety of grades and densities ranging from 3.6kn to 10kn. Blocks are made from combining cement, sand and crushed gravel and even aggregates such as expanded furnace slag, sintered ash and pumice.

The selection, combination and quality of materials will dictate the compressive strength.

Concrete blocks are cheap, quick to lay and are also good thermal insulators. They can be used as an infill for beam and block flooring, the internal leaf of cavity walls, internal dividing walls, and often for the external leaf, if the external finish is to be cladding or render.

Most concrete blocks can now also be used below the Damp Proof Course. Concrete blocks have excellent fire protection properties providing at least 1 hour fire resistance and Class 'O' surface spread of flame.

Dense Blocks

The average standard block is 3.5N strength which is suitable for the construction of one and two storey dwellings (there may be other factors requiring a stronger block ie, sulphate resistance)

• Any building of 3 storeys or over will require dense blocks (heavyweight blocks) with a high 7.3N/mm2 strength. Their high strength means they are often used for foundations and load bearing walls.
• The high density provides good sound insulation, ideal for use in party walls but also good heat conduction and therefore a low level of insulation.

Lightweight Blocks

• Lightweight blocks can have a compressive strength as low as 2.9N. These blocks are light and easy to handle on site.
• Made from a variety of lightweight aggregates, they are slightly more expensive than ordinary dense blocks but have better thermal insulating properties.
• Lightweight blocks are primarily used for the internal skins of cavity walls although some types are suitable for use in loadbearing walls and below the DPC and even as the infill for block and beams floors.
• Because of their low density, most lightweight blocks will have a low compressive strength.
• Lightweight blocks are generally unsuitable for use in party walls due to their low mass which makes them poor sound insulators. They can be prone to shrinkage cracking during the drying-out process of the plastered internal walls.

Aerated Blocks

• Aerated concrete blocks are light and easy to handle on site making them very popular for domestic buildings.
• Although not particularly strong, aerated blocks are extremely thermally efficient and are widely used for inner leaves and partition walls.
• Aerated blocks are made from cement, lime, sand, pulverised fuel ash (PFA) and aluminium powder and contain up to 80% recycled materials. Mixing aluminium filings with the concrete causes them to react with the lime producing hydrogen, creating tiny bubbles within the block.
• Due to their low mass, aerated blocks are generally unsuitable for party walls and are not usually suitable in situations where there are point loads or where high compressive strengths are required.

Trench Blocks

• Trench Blocks or Foundation blocks are lightweight and can provide a quicker build below ground.
• Commonly used in a range of thicknesses from 255mm upwards, these Blocks are highly resistant to the freeze-thaw conditions likely to occur below DPC level.

Cavity Wall Construction

• Do not mix clay bricks and concrete blocks.
• Brickwork should not be carried out when temperatures fall below 2°C.
• Good workmanship is essential in preventing any water seeping through the outer leaf in the gaps between the bricks.
• Use bucket handle, weathered or struck pointing. Recessed pointing should only be used in sheltered locations.
• Recessed joints should not be used with full fill cavity insulation.
• When constructing a cavity wall, the height difference between the two leaves should never be more than 6 standard block courses.

Movement Joints

Movement joints in the outer leaf of external masonry walls prevent movement from expansion and contraction causing cracks in the brickwork.

• Movement joints are not usually required in the internal blockwork walls as they are regularly interrupted by party and partition walls.
• Movement joints are usually hidden in corners or behind rain water pipes.
• All movement joints provided in the substructure should run the full height of the masonry wall. However, movement joints are not normally required below DPC level because the moisture content and temperature should be relatively constant.
• Wall ties are required either side of the movement joint.

Movement Joint Spacing

Movement joints are usually created by providing straight, unbounded, vertical joints in the brickwork at spacings detailed below:

Install ties to each side of movement joints:

• Vertically - 300mm or each block course
• Horizontally - within 150mm of the joint

Movement Joint Filler

Movement joints should be filled with the correct compressible filler. For clay brickwork, use flexible cellular polyethylene, cellular polyurethane or foam rubber faced with a flexible sealant at least 10mm deep to ensure a good bond.

Cavity Trays

• Cavity Trays should be provided above window and door openings and at all interruptions to the cavity such as lintels, roof abutments, air bricks and meter boxes.
• Ensure any water running down the cavity is directed out through weep holes.
• Provide a cavity tray over full fill insulation where the insulation is not taken up to the roof in order to prevent any water dripping from the wall ties higher up in the wall from falling and ponding onto the top of the insulation, leading to damp penetration to the inner leaf.
• Provide cavity trays for lintels which do not have a cavity tray incorporated in their design.
• Cavity trays over lintels should project at least 25mm beyond the cavity closer and cover the ends of the lintel.
• Cavity trays should be installed in one continuous length. If the tray is not continuous, provide a minimum 150mm stop ends to prevent any moisture running off the ends of the tray and back towards the inner leaf.
• The cavity tray should be extended 150mm beyond each side of the opening.
• Cavity trays should have a rise of at least 140mm from the outer leaf up to the inner leaf.
• The rise across the cavity should be at least 100mm.
• Return the upstand of the cavity tray into the inner leaf unless it is rigid enough to stand against the inner leaf without support.

Lintel cavity Tray:

Parapet Walls

Parapet walls are exposed to the elements on their both sides and top. This can lead to premature failure and possible water ingress.

When constructing a parapet wall, only bricks with a high level of frost resistance and low salt content should be used .

Parapet Wall DPCs

• Provide a throated coping or cap to prevent moisture penetration at the top of the wall, with a sealed DPC below.
• The DPC should be supported over the cavity to prevent sagging.
• There must also be a DPC, a minimum of 150mm above the roof surface, to lap with the upstand flashing, ensuring continuity with the roof covering.

Parapet Wall with DPC Support:

Chimney DPCs

Where a masonry chimney penetrates a roof structure, a DPC may be needed to prevent water soaking the masonry inside the building.

Internal Wall DPC

DPCs at the base of partitions built off oversite where there is no integral DPM, should be the full width of the partition.

Cavity Insulation

Energy conservation requirements demand ever thicker levels of insulation. Around a third of all heat loss in an un-insulated home occurs through the walls. Insulation in external walls is normally positioned within the cavity.

Insulation can also be fitted on the outside of the cavity walls requiring an external finish such as render, tile-hung or timber-clad. Alternatively, the insulation may be installed internally as a drylining solution.

The insulation performance is measured as a U-value expressed as W/m2K.

Insulation installed within the cavity can be either total fill or partial fill. This will depend upon the insulation material being used and site exposure.

• The partial fill solution will often use rigid foil-backed polyurethane sheets such as Celotex or Kingspan. It is reasonably expensive but does have around twice the thermal performance of the mineral or rockwools although the wools will provide good standard of protection from sound and noise transmission.
• Fully filled cavities in exposed locations are at risk from penetration of moisture through the external leaf, soaking the insulation and transferring damp through the inner walls. Therefore a fully filled cavity is not acceptable in severe weather locations such as Scotland.
• There are also more eco-friendly insulation products available such as natural cellulose fibre made from recycled newspaper or sheep’s wool.

Partial Fill Insulation

• Partial fill insulation boards should be fixed tight against the inner leaf of the cavity and retained in place with the correct tie retaining clips before the outer brickwork is built up.
• Ensure the wall ties provide suitable structural integrity.
• Butterfly-type ties should not be used with partial fill insulation.
• Insulation boards should start 2 brick courses below the DPC with the first row of boards supported on the wall ties and each board on at least two wall ties per 1200mm board positioned at max 600mm centres horizontally.
• For partial cavity fill, the spacing of the wall ties should coincide with horizontal joints (maximum 450mm centres vertically and 900mm centres horizontally). However, around reveals or movement joints etc, where wall ties need to be more closely spaced, ties can be installed by providing a clean neat cut in the insulation.
• Insulation boards should be tightly butted with staggered joints and no gaps to minimize heat loss and dampness.
• NHBC requires a clear 50mm residual cavity between the insulation boards and the external leaf. However, a cavity width of 25mm is possible in sheltered location, provided the workmanship is of a high standard, to minimise the risk of damp penetration.
• Damp problems can be caused by mortar droppings bridging the cavity. Therefore, during construction, it is essential to place a batten across insulation and cavity to stop mortar dropping into the cavity, and to remove any excess mortar from the wall and the top of the insulation materials.

Full Fill Insulation

• In fully filled cavities, the cavity should be 5mm wider than the full fill insulation batt specified.
• The insulation boards should be supported on the wall ties DPC at 450mm centres horizontally. Subsequent boards should be tightly butted together with staggered joints between the ties.
• The batts should be built into the wall as construction progresses.
• Ensure all mortar joints are completely filled with mortar. Do not use recessed joints in a full filled cavity wall.
• To prevent mortar snots bridging the cavity, leading to possible damp problems, a cavity batten should be placed across the insulation and cavity to prevent mortar dropping into the cavity. Any excess mortar must be removed from the wall and the top of the insulation materials.

Rendering

Rendering the external surface of a wall will improve its air tightness and weather resistance, hopefully preventing any rain penetration.

• The rendered wall should comply with BS EN 13914 ‘Design, preparation and application of external rendering and internal plastering’.
• The specified mix should comply with BS EN 13914 ‘Design, preparation and application of external rendering and internal plastering’. Particular care should be taken when specifying a mix for aerated or lightweight concrete blocks.
• A render mix will comprise of cement, lime to increase workablility, water and sharp sand (grading type A). Admixtures may also be used (air entrainers should not be used with masonry cement.) (Refer to the NHBC Good Building Guide.)
• To prevent the render shrinking and cracking as it dries out, ensure the mix does not contain too much water or cement.

Insulation of rendered walls

• The lack of ventilation in the cavity of a full filled cavity wall may adversely affect the drying out process of the render and a specific render mix, as well as special bricks or blocks, may be required.
• In exposed locations subject to driving rain, full filled cavity insulation is not suitable for a rendered wall.
• A cavity wall which is to have partial fill insulation may be rendered, provided a 50mm residual clear cavity is maintained.

Preparing the surface

• The surface to be rendered should be free from dust, loose particles, efflorescence and organic growth. It must be moderately strong and porous, thus providing an adequate key and ensuring a good bond.
• Dense blocks with a smooth surface are not suitable.

• Textured Blocks

• No preparation required.

• Clay brickwork and dense block

• Provide 15mm recessed joints for a sufficient key (by raking out joints).
• Hack the Surface.

• Smooth Blockwork or Bricks

• Provide a spatterdash coat (strong cement/sand slurry thrown onto the surface).
• Provide a stipple coat (strong cement/sand slurry possibly with a bonding agent brushed on to the surface).
• Provide a suitable adhesive.
• Hack the surface.
• Apply a bonding agent.
• Provide suitable metal lathing (see below).

Refer to NHBC 'Superstructure'.

• Painted Brickwork

• Provide suitable metal lathing (see below).

Metal Lathing

• Metal Lathing should be stainless steel compliance with BS EN 845
• For a good bond, position the metal lathing slightly away from the wall surface so that render can be pushed through the mesh.

Application

• When rendering on masonry cavity walls, two coats of render are usually sufficient, although in exposed areas, on solid wall construction or where a metal lathe is used, two undercoats and one finishing coat are usually required
• Ensure each coat of render is weaker and thinner than the previous coat or than the material it is applied too.

The First Coat

• The first coat (undercoat) should be between 10mm to 15mm thick. It should be adequately levelled and combed to provide a good key for the second coat.
• Allow the first coat to shrink and dry for a minimum of 3 days so the render is hard but not completely dry before applying the next coat.
• Subsequent coats must be thinner and weaker than the first.

The Finishing Coat

• The finishing coat should be between 6mm and 10mm thick and may have a smooth, textured or course aggregate finish. (A severely exposed site would benefit from a rough textured finish)
• Do not use a strong mix for finishing coat.
• Keep the finishing coat damp for at least 3 days. (In very hot, dry weather it may be necessary to spray the finishing coat with water or cover with a polythene sheet)
• Do not apply render in hot temperature or in bright sunshine.
• Do not apply render in wet or frosty conditions of when temperatures reach 2°C and falling.
• Provide suitable details around openings service penetrations, movement joints etc.
• The render should be stopped just above the DPC.

Drainage Foul Water

Flat Roof Types