What We Do

Construction

Brickwork

In most cases, it is important to ensure the extension seamlessly blends inwith the existing building. Matching the masonry is an obvious, but often overlooked, way of doing this.

Boring it may sound, but brick libraries do exist! We have a number of connections with brick merchants and brick matchers to ensure we get the best available bricks for the job. In addition to matching the main brick, we would normally match any feature or detail bricks.

Reclaimed Bricks

Where appropriate we will use reclaimed bricks, from Yellow, Red or MultiStocks, to Plain or Multi Gualts, Arlesey Whites and Luton Greys. These are sourced form a variety of local stockists as local buildings tend to be built from local bricks. Unfortunately battered old second hand bricks cost a lot more than brand new ones, but the end result always justifies the cost difference.

Brickwork and Pointing

Matching the bricks is only part of the job, they also need to be laid and pointed appropriately. For modern houses this is a Stretcher Bond, bricks laid lengthways with a cavity wall.

On older properties a Flemish Bond was often used, bricks laid in alternate directions so that the inner an outer leaves of brickwork were bonded together. This is not appropriate form modern cavity walls, but using“snapped headers” can replicate this effect whilst providing the insulation and damp-proofing properties of a modern cavity wall.

Cavity Wall Insulation

Insulation is fitted in both the floor and walls. The exact specification will vary between jobs; but typically there is 100mm of Isowool type insulation the cavity walls.

For the inner leave, thermal blocks are used which when combined with the insulation and plaster give an overall u-value of around 0.27 W/m 2K

Floor Construction

Either a concrete floor slab is laid or a suspended concrete “beam and block” floor is used. A concrete slab is laid on compacted and blinded hardcore, with a damp-proof membrane. The top of this will typically be 185mm below the finished floor level, the difference is made up with 100mm of insulation and 70mm of screed to allow both new and old floor levels to match.

Floor Insulation

The standard specification is for 100mm of Kingspan/Celotex type insulation to be fitted below the final screed. Unfortunately many firms use cheaper methods, but this detail is paramount if underfloor heating is to be used and requisite if the structure is subject to building control. This type of floor construction will have a typical u-value of 0.17 W/m 2K.

Screed

Using a screed on top of insulation allows us to fine-tune the final floor level to perfectly match the existing, essential when new opening to the house are being created and desirable in most other cases. It also makes it convenient to bury cables and pipework, either to provide services into the extension, and/or as underfloor-heating.

The downside of a screed is that it needs to dry out before a final floor covering can be laid. We normally aim to get the screed down as soon as possible, then work on finishing other aspects of the build whilst the screed dries out.

Foundations

The foundations are arguably the most important part of any building. Proper attention needs to be given to the type of foundations used.

For jobs where building regulations are required, the type and depth of foundations are determined using the NHBC’s foundations depth calculator and have to be inspected and approved by the building control.

In cases where building control approval is not necessary, we dig footings to a minimum of one metre depth, but deeper if required. Neither us nor the client want the expense of digging unnecessarily deep, however it is in both parties interests to ensure there are no future problems.

The ground conditions and presence of trees usually dictate the type of foundations required. The ground in the local area varies between chalk and clay, with “Brownfield” sites requiring special consideration. The subject is far more complicated than can be dealt with here, but a brief summary can be found on the extension foundations section below.

Extension Foundations

Extension Piled Foundations

Using piles for the foundations may seem excessive to some, but in an increasing number of cases it is the most efficient, economical and the best solution. If the ground conditions dictate deep foundations and/or there is no access for a sufficiently sized excavator, then piles can be the best way forward.

Hand-Dig or Mini-Digger?

The answer really depends on access and the size of the conservatory. For a large conservatory with good access, a mechanical excavator is the quickest and most economical way of digging foundations. For sites with restricted access, there is no choice but to dig by hand! However for smaller extensions, this can be more economical and cause less disruption.

Extension Foundations in Chalk

Chalk is relatively firm and drains water away well. Nearby trees have no significant effect. Standard strip footings are most suitable as long as they are deep enough to be in solid chalk.

Extension Foundations in Clay

Clay holds water and is susceptible to shrinkage as it dries and heave as it gains moisture. Nearby trees can draw up moisture causing shrinkage leading to structural problems.

Extension Foundations in Brownfield sites

Many new houses are built on sites that were once used for another purpose, hospitals, airfields, factories or even schools. Generally these get levelled by the developers, houses built then the garden back-filled with rubble and covered in top soil.

Some areas of the site may be on virgin ground suitable for a strip footing, other gardens may contain a substantial depth of rubble.

Extension Piled Foundations

If the foundations need to be more than around two metres deep and the site is not suitable for a reasonable size digger, it becomes more economical to use piled foundations. Piles are very quick to install and save a lot of the mess and other potential problems associated with deep strip footings, such as trench collapse and water ingress.

We can use a highly portable rig in restricted access situations that will drive steel-cased piles deep into the ground, until they stop up at the required resistance. This is typically around seven metres, though occasionally further. Depending on the difference between outside ground and inside floor levels, either a reinforced concrete slab or a reinforced concrete ground beam will sit on top of the piles. Where a ground beam is required, we would fit a beam and block suspended concrete floor.

For further information, visit extension piled foundations.

Pile Foundations

Extension Piled Foundations

Using piles for the foundations may seem excessive to some, but in an increasing number of cases it is the most efficient, economical and bestsolution. If the ground conditions dictate deep foundations and/or there is no access for a sufficiently sized excavator, then piles can be the best way forward.

Piles vs Deep Trench Foundations

With trench foundations, the deeper we go the exponentially harder, more dangerous and expensive it becomes. The additional volume of concrete and soil disposal costs are fairly linear, but the digging gets progressively more time consuming and expensive as the depth increases. The trenches can and do collapse, fill with water or both. Additional shuttering is required to sure up the sides and ensure the safety of the operatives.

Depending on access, this work can be time-consuming and messy. If the footings have to be dug deeper than expected due to unforeseen conditionsor at the behest of a building inspector, all of the above problems are exacerbated.

The main advantage of piling is that the certainty; with a trench foundation there is always the possibility that we have to go forever deeper incurring extra expense and time, with piles this is never the case. A typical piled system is completed in a matter of days, a deep trench dig can take far longer if access for machinery is poor.

Design

We conduct an initial site survey, assess the species of any trees and measure their distance from the new structure. Ground levels, drainage runs and nearby buildings are also noted. We then consult the Local Authority's Building Control Department and the British Geological Survey on the type of ground we are likely to encounter in that area. A trial hole may be dug and a decision to use either strip foundations or piles made.

Where piles are to be used, a structural engineer will design the foundations with either a piled raft or piled groundbeam. The pile spacings, groundbeam/raft spans and specification are designed in accordance with the loads imposed by that specific structure. Though the single storey orangery and extensions that we build these loads are relatively light.

Each pile will provide a Safe Working Load of 100kN with a safety factor of2, or 67kN with a safety factor of 3. The piles will be driven until they meet the required resistance, or “set”. This effectively tests each pile as it is placed, so a lower safety factor is often justified. However the limitation of the span between piles of groundbeam or raft will mean that a greater number of piles are often called for. This means in practice that each pile will often only be required to bear a load of 50kN or less.

Preparation

The first step is to excavate for the groundbeam or raft. This is normally done manually as the decision to use piles is often influenced by the impracticality of using a mechanical excavator. The position of any drainage is verified, this is exposed or altered if required. Should this conflict with the designed pile locations, it is referred back to the structural engineer who will modify the design accordingly. The pile placements are marked and the piling can begin.

The piles are made from steel tube with an external diameter of 150mm, we generally use sections 2 metres in length. The first section, or starterpile, has a closed end, this has been crimped and welded to a point. Some ballast and dry cement are poured into the end of the pile to a fill depth of 500mm, this forms a plug into which the piling hammer will drive.

All personnel will be wearing Personal Protective Equipment and will have passed the Certificated Training Course on the Correct and Safe Operation on the Piling Equipment.

Piling Equipment

The piling hammer is powered by air from a compressor, this is usually a road-towable type and is left at the front of the property. The two hoses which run from it can pass through an existing building if no other access is available, to the area where the piling is to take place. One hose connects to a lubricating unit which adds a small amount of oil to the air, then both hoses merge through a Y-branch and connect to the piling hammer by means of a reinforced hose.

We use a Grundomat 130P pilling hammer, this weighs 117kg and will be lifted by mechanical hoist into the starter pile. A typical hoist arrangement is a 3 metre high tripod (shearlegs), a lifting chain and a chainblock pulley, all of which are rated to lift at least 500kg. The lifting chain is secured to the back of the piling hammer, this remains attached through the procedure.

One advantage of this bottom-driven piling system is the low impact on the local environment, both in terms of sound and vibration. The loudest sound is usually the compressor, which is similar to the diesel engine of a motorvehicle. The energy is focused directly on the bottom of the pile in the ground below, which means surface vibrations are minimal and reduce as the pile goes deeper.

Piling Method

Once the Grundomat is lowered into the pile, it will rest on the ballast and cement plug. The power is gradually applied and the Grundomat will crush and compact the plug so it forms around the head of the Grundomat providing a tight connection between the pile and the hammer. This allows the force to be distributed evenly to the pile and reduces the possibility of the hammer driving out through the bottom of the pile.

As the hammer sets into the plug, the sound will change and the pile will start driving downwards and the power can be increased to full. It is important at this stage to ensure the pile is perfectly vertical by use of a spirit level. The first two metre pile will continue downwards through the loose soil near the surface until the back end is only half a metre above ground, then the power is cut. It is now necessary to to weld a second section, or “follower” onto the rear of the starter.

It is not desirable to remove the piling hammer, it will spoil the plug and is a waste of time, so the hammer is left in the bottom of the first pile with the air hose and lifting chain connected to the the back of it. We have to disconnect the air hose from the Y-branch, thread a new piling tube section onto both the hose and the lifting chain, then attach the second tube to the first.

The rear of each section of piling tube is splayed slightly to form a socket into which the follower is inserted, these are then arc-welded together. Once the new follower is attached, the power is re-applied and the pile continues downwards. The rate at which the pile descends normally decreases as it meets firmer, more compacted ground. Further followers are attached in the method described above until the required set rate is reached.

In clay soils the piles typically reach the required set at a depth of around 6 metres. The requirement for this is for the pile to penetrate a depth of 10mm or less over a period of 10 seconds. This should be checked over a penetration of 100mm. Once this set is achieved the hammer can be extracted. A log is kept of the set times and depths of each pile.

The hammer is put in reverse by a quarter turn of the air hose, this allows it to free itself from the plug. It is then hoisted out by the chainblock and placed ready for the next pile. The end of the pile is unlikely to be protruding the required distance from the ground to make the connection to the steel reinforcing, so may need to be cut down. Depending on ground conditions a void former may be required beneath the groundbeam or raft to allow for clay heave.

Steel reinforcing bar is placed into the centre of the pile, this is then connected to the steel mesh used to form the groundbeam or raft. The steel reinforcing is pre-cut and bent to our specification, then joined on site either by tying or welding. Once in place the piles and steelwork is ready for inspection by building control. The depth of the piles can be verified by tape measure. Having been approved, the piles and groundbeam/raft are ready to be concreted in one process. C35 concrete is normally specified, it is important this is consistently compacted to the bottom of the pile.

Summary

Other than the initial excavation, the entire process will take around five days or so for a typical domestic extension. The end result is a groundbeamor slab ready to accept masonry and capable of withstanding loads well in excess of those that it is likely to receive. Even though the bearing capabilities of the piles are often superfluous in the context in which they are used, the speed, safety, tidiness and certainty of this method mean it is the most practical and economical choice in many situations.

This method is also more environmentally friendly than trench-fill foundations both on a local level in terms of noise and disruption, and a global one due to a lower level of carbon emissions as a result of less concrete being used.

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