Diy Conservatory Building Base

Conservatory Foundation and Base
Most but not all conservatories will sit quite happily on a simple concrete slab base that will serve both as a foundation and floor structure. If the ground is covered by a deep layer of topsoil it will be better to dig trench foundations down into the natural subsoil. Even with foundations under your walls you will still need to support the floor slab on a sound base and the construction of a conservatory is no different in this. Ground-bearing conservatory bases are usually in the region of 100mm thick and are cast over a level bed of hardcore. For the concrete base to function properly it must be carefully formed and there are several key elements to its design and construction.

Conservatory base levels
Ensure that all the vegetation is dug out and removed along with any topsoil and begin from your calculated formation level on the subsoil. If you need to match up with the finished floor level between your home and the addition, it is vital to correctly calculate the depth of construction. Mark the level of your floor finishes on the existing external wall and measure down from this the layers of your base construction before filling with the oversite material. This should make sure that you don’t overfill and end up with too little thickness of concrete or under fill and end up needing too much.

Oversite material
Casting the concrete directly on the ground is rarely possible. So that the quality of the preparation beneath it can determine how long it lasts, the hardcore material should be no larger than 100 mm units.

Depth of oversite
The oversite material must be at least 150 mm thick and can’t simply be dusted over the ground, but where it becomes a greater problem is when it’s too deep. The limit for ground-bearing bases is 600 mm since it is likely to settle and cause cracking if it is much deeper. If part of it exceeds 600 mm deep, it is best to use lean-mix concrete in layers with the hardcore to make up the difference or a type of stone fill, such as 40 mm pebble stone that cannot settle.

Compaction Ensure that the hardcore is compacted in layers no more than 225 mm thick, ideally mechanically compacted with a plate compactor.

Blinding
The hardcore should be finished off with a blinding of fine material such as sand to a maximum thickness of 20 mm.

Do not lay the blinding material too thick. It is only intended to protect the polythene damp-proof membrane from being punctured by sharp edges or hardcore.
Conservatory Damp-proofing
Most damp-proofing is done beneath the concrete slab by laying a polythene damp-proof membrane (DPM). If it isn’t in one piece, it should be tapped down and where separate foundations have been formed, ensure that it is dressed up the outside walls so it can be trimmed back after concreting. The DPM can help to protect the concrete slab from sulphate attack but since sulphates may be contained in some fill material, such as brick rubble, it’s a good idea to use polythene beneath concrete generally.

Conservatory Insulation
To keep some of the cherished heat in, the base should be also insulated and it’s important to use the correct type of insulation graded for floors and not walls.

Polyurethane foam board is often used in 50 mm thickness or more. If you have separate foundations for the dwarf walls, thinner strips (25 mm) can be cut to stand it up to the sides of walls to the slab depth when insulating below the concrete slab so that it is cast fully encased.

While floor-grade can be laid beneath the concrete, there are advantages in it being placed later on top of the concrete beneath the finishings and if you don’t have a simple slab base with no separate foundations, this is your only option. It reduces the time taken to warm up the floor structure if you aren’t trying to heat 100 mm of concrete slab as well. The advantage of having it beneath the concrete is that the base will store heat through the day and release it at night, but only where it is fully encased and separate foundations are used.

Insulation suppliers

Avoiding cracks
Make sure you are not forming the hardcore on backfilled or contaminated ground, or indeed settlement, as clay shrinkage or heave will cause problems. Even on good subsoil the addition of lightweight fabric reinforcement such as A142 is beneficial in avoiding settlement cracks. In very large bases, movement joints such as fibrous material strips should be used to allow some expansion to occur without damaging the slab.

There are sound environmental reasons for refusing demolition hardcore in oversite preparations, but it is essential that it is handpicked and “clean”.

Note that sulphates in hardcore or in subsoil can attack concrete. Where this is likely, you should use sulphate-resisting cement in lieu of ordinary Portland cement (OPC). Polythene DPMs will also help to resist sulphate attack and should always be included beneath any concrete base to resist damp. The industry standard grade for polythene in this use is 1200 gauge. Polythene is available in much thinner gauges for all manner of different uses and sometimes unscrupulous builders may take advantage of this an install a sub-standard grade.

Conservatory Base finishing
The concrete of the base is normally tamped with a timber beam to a level finish than can be screeded over later unless you require a smooth finish either to be left exposed or covered by a floating finishing.

A floating floor finish is any finishing layer that isn’t bonded or fixed down to the concrete base or structure. Laminate and timber floors are often just laid over the insulation without any fixings but to achieve this the base must be finished to a higher standard. Most conservatories can be supported on a simple slab of concrete, which is fine if they are built as separated additions over an external door. But if it is designed and built to be integral with your home, or with a view to last as long, you should look towards a more robust foundation that reflects that of the rest of the building.

Better bases
Conservatories, given their plastic roofs and glazed walls, are lightweight structures and being lightweight they shouldn’t require a foundation or should they? It is not only the weight of the building that can cause it to settle or subside, the ground that it sits on can be capable of doing that by itself. A conservatory that sits on a simple slab base, perhaps only 100 mm thick over clay, is likely to be at risk and the first to suffer from the soil’s seasonal movement. Because it is formed on the surface end and without reinforcement, the concrete can fall and rise with the subsoil beneath it and if you already have concrete paths that have cracked with movement over the years, you can assume that the same will happen to your base if you don’t take proper measures to improve it.

Reinforced slab bases or raft foundations
Rather than digging deep foundations to avoid the effects of ground movement, you might choose to adopt a slab base that is reinforced with steel mesh or bars. Given the right amount of reinforcement and the right position for it in the slab, it can stiffen the base and allow it to “float” rigidly over some ground movement without suffering damage. This design, called a raft foundation, can be the most appropriate form for a conservatory on volatile soil. There are basically two types of raft: the plane type and the edge beam type. We are blessed in the UK with a rich variety if soils: silts, sands, rocks, chalks, gravels and clays. Some materials such as chalk are excellent for building on and simple slabs beneath conservatories are ideal here. Other, such as sand and clay, may have good load-bearing characteristics but have a tendency to move and cause subsidence. Clay, prevalent in the south and southeast regions, is a shrinkable material that can change in volume dramatically: in the summer it can dry out and shrink and in the winter it can become waterlogged and heave. A minimum depth of 1 m is usually considered safe in clay soil without trees present, but clay is even more susceptible to change when trees and vegetation exist.

The effects are worst at the surface, which is where bases are formed, but with depth they decrease. Ordinary foundations need to be extremely deep, but they soon become uneconomical to make, and a reinforced raft base may be a more realistic option for a conservatory. In the case of hedgerows, it would be appropriate to make a decision based on the worst case species to be found in the hedgerow. Additions that are controlled under the Building Regulations will need to be approved in respect of the foundation design before work starts. All rafts and bases in these situations should be formed over a generous and compacted depth of stone fill.

Tree roots
Cutting through a live root of 50 mm diameter or bigger is not considered to be a good idea. Instead your base foundation should be designed to bridge over roots, allowing enough space for future growth without the roots’ structure exerting pressure on them. Tree roots in turn can exert considerable pressure on ultra-lightweight structures like conservatories, and during growth they may cause structural damage. If you are building close to a tree, seek the advice of a registered arboriculturist.

Removing trees and clay heave
A large, thirsty tree can suck hundreds of litres of water from the ground on a hot day, and it will soon become desiccated and shrink dramatically in volume: however, removing a mature tree in the garden could also have a bad effect. You’ll need to consider the age of the tree and its size, because this is a measure of its moisture uptake. In the great carbon cycle of life, trees can’t just grow by absorbing the carbon dioxide from the air, they have to add water and minerals to it and they get those from the ground. On hot, sunny days when the process is working hard, trees have to search harder for water and some species are very good at it. Oaks, willows and some poplars to name a few, but also some fashionable garden trees like the Blue Gum eucalyptus. If you have a small one of these in the garden, dig it up now, place it in a large pot and replant it – either that or make sure you prune it every month.

With a large mature tree removed, the ground can become waterlogged and left to expand in volume. Clay can take up water like a sponge, and this process is known as heave. Beneath a lightweight structure like a conservatory, the resulting heave can lift it up out of the ground and cause even more damage than shrinkage. There really is only one way to deal with the prospect of clay heave, and that is to use a sacrificial former beneath the raft base of the conservatory. The product is castellated and weak enough to absorb the pressure of heave. Its job is to act as a cushion between your conservatory and the subsoil beneath like the crumple zones on a car. A former can be anything from 50 mm to 300 mm thick, depending on the expected heave. With foundations, it is possible to build in heave precautions at a relatively low cost. Lining the walls of foundation trenches with heavy gauge polythene will create a slip plane that can help to prevent heaving clay from exerting pressure on the sides of the foundations.

Plane rafts
The plane raft requires very little structural calculation and normally is made up of a thicker slab, perhaps between 150 mm and 250 mm in depth. It is the less rigid of the two designs, but will still contain two layers of reinforcement positioned, one near to the top and one near to the bottom. These rafts are square-edged bases and are cast over a prepared oversite base of hardcore material, so at least some of the concrete edge can appear above ground level.

Edge-beam rafts
This is the type most commonly used in house-building where soft ground is present. In principle it is the same reinforced slab with two layers of reinforcement in the slab, top and bottom, but where it differs is at the edges (beneath the walls). Here, the slab is thicker and a cage of reinforcing bars runs the length of the perimeter. The slab thickness in the middle is still conventionally between 150 mm and 250 mm deep, but the edge beam is more likely to be between 450 and 600 mm. The width of the edge beam is always greater than the depth, normally between 600 and 900 mm. There are several methods of design for an edge beam raft but the usual one allows the engineer to spread the weight of the frame and roof carried by the walls through the area of the slab as well. Reinforcement for slabs comes in ready-made sheets of steel bars welded into a grid, which can be lapped to cover a larger area or cut to a smaller one. With two grades of steel and several thicknesses of bar, there exists a choice of a dozen grades of mesh fabrics, as they are known, each identified by a code. The higher the number the thicker the bars and the stronger the reinforcement.

Fibre-reinforced concrete
If you can’t be bothered with all that heavy steel but still need a reinforced concrete base, there is one other alternative. Fibre reinforcement comes in bags and is sprinkled into the mix while it’s being prepared. The fibres are either steel or nylon and individually are quite small and toothy. The proportion to which they are added to the other ingredients is critical, as it always is with cooking and concreting, but the fibres are designed to be randomly placed in the hardened slab. Together they reinforce it in the absence of steel bars quite effectively, so this method is commonly used for extremely large slabs on industrial developments in place of steel reinforcement.

Steel frame bases
In kit form, box-section steel tubing is available to form a grid of steel on which the frame can be fixed and flooring can be anchored down. These steel grids sit on a surface of concrete via adjustable legs to get it level. The external section is 75 mm square, making it possible to sit a single brick skin on, and the internal tubes, which you might think of as floor joists, are 50 mm square and set apart at standard floor joist centres to allow flooring-grade chipboard or boarding to be spanned across them. Undoubtedly they speed up construction, but they shouldn’t be used without a depth of solid concrete to support them. This concept is more or less the same as that employed with deck construction, although the steel box section is much stronger than timber. This system best suits the contemporary style of glass walls, but perhaps not a single brick skin dwarf wall. Brickwork leaks unless it is built very thickly and in cavity wall construction rain can easily be forced by wind pressure through mortar joints to the inside – and having to rely on a single skin raised from a metal tube doesn’t seem like a good idea.

In the contemporary glass box style, however, without brick wall and with the need for a precise level base for the glass frame to be fixed to, this could be the best method yet, although it should be supported with solid concrete foundations that are suited to your ground conditions.

Setting out the base
You can’t afford to be complacent about setting out the base or foundations, because getting it wrong is all too easy. If you happen to get a conservatory base size wrong it will also throw out the superstructure and for many system-build models the tolerances for error are not generous. It does happen from time to time, however, and extending the base for digging extra trenches filled with concrete means extra cost right from the start. Later on, cutting down roofing sheets that are also in standard sizes will result in waste and more cost. It may also be critical to the planning authority that your addition is built to the correct dimensions and levels.

Boundaries and dimensions
The distance to the boundary can not only be important from a planning aspect, given the issues of overlooking and overshading, but it can also have an effect under the Building Regulations with fire spread and the Party Wall, Etc. Act. With any non-combustible elements on your boundary elevation, such as windows, plastic or timber boarding, being limited in area depending on the distance to the boundary, some errors in setting out can be disastrous. On an addition controlled by Building Regulations, up to 1 m away from the boundary, a wall may be allowed with only a small window. Over 1 m and the allowance in these non-combustible areas can be increased by at least five times, which can mean the difference between having your side gable end covered in PVC-u or the wall having an entrance door or not. Check out all the dimensions before you form the base! One other measurement that can be important is the distance the addition is set back from the highway. On corner homes or front additions it can be critical. The physical act of setting out involves a few wooden pegs or stakes, some string lines and chalk spray. Measure and set out corners at exactly 90 degrees.

Concrete
Bases and rafts are usually formed using shuttering, plywood sides that are set up to create a mould into which concrete can be poured. Formwork has to be robust, since the thought of wet concrete collapsing and pouring out is not a pleasant one. The sides need to be convincingly stake-posted into the ground. For strip foundations, along with dimensions for the centre line of the trenches and later for the outside of the walls, you will need some levels. Once the trenches have been dug, you will need to mark the sides with the level of the concrete proposed – you need some pins on the trench walls or bottom to mark that level and rake the concrete to it. It must be levelled properly as the bricklayers will not thank you, with their trade following, if it is not. Ordinary concrete is not self-levelling, but it has the consistency of porridge and needs to be raked level.

A product worth considering is a ready-mix concrete with a plasticizer additive that makes it much more fluid and self-levelling but this isn’t widely used. Ready-mix concrete has the advantage of quality control in the mix, guaranteeing its strength and workability. It also takes a lot of the hard work out of the job, particularly when it can be placed directly from the mixer truck into its position on site. Raking it to the level required is hard work in itself, but you can make the job easier by knocking in plenty of steel pins to mark the finished level you’re after. With bases and rafts, the hard work is in tamping it to the level by using a straight length of timber that will span across the base to the formwork either side from which your level is taken. Tamping the concrete in this way helps to consolidate it in the same way that vibrating it does, and a gentle sawing and tapping motion combined brings the surface to the level and finish needed. Mixes for ready-mix concrete need to be quoted when you order. The code ST relates to a standard mix that can be used in most situations where reinforcement isn’t required and ground conditions aren’t aggressive.

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Mixing on site
In the small quantities needed for the average conservatory addition, site-mixed concrete is the most economic. A mixer, some cement and all-in ballast is all that’s needed and if you have the advantage of being able to mix the concrete close to where it’s going to be used so much the better. The other option is ready-mix, which arrives in 5 or 6 cu m loads. Although it’s priced per metre, you’ll pay a surcharge for any short loads that makes it uneconomical in less than full truckloads.