Although overall weight is an important consideration, it is of significant importance to assess how stable the sub-soil is where your new conservatory will be sited. If it is good load-bearing soil and well drained then a standard foundation will probably be sufficient. However, if the ground is clay, made up or is otherwise unstable, extra depth foundations, steel reinforcing or a specially designed steel raft construction would generally be necessary.
In all cases, however secure the ground may be, should the foundations not be constructed to an appropriate standard, the resultant ‘movement’ which might result could be horrendously expensive to correct.
Your Local Authority Building Control department have good local knowledge of ground conditions and details of drainage, etc, and they will be pleased to advise you even if the works are exempt from their control.
The majority of new conservatories are constructed on to a dwarf cavity wall of some 600mm (2ft) height. You may choose stone, brick, render or whatever is appropriate, and it is usual for this to match the existing property.
However, the internal wall does not necessarily have to be the same, eg, a thermal block providing good insulation with a smoother rendered finish, or plaster which can be decorated with an emulsion colour of your choice. The dwarf wall can be internally closed off with a window cill and will make your new extension feel very much more a full dwelling room and the external appearance will look part of the original building.
The ideal construction would be a 25mm (1”) bed of sand onto which 50mm (2”) of insulation is laid, followed by a substantial damproof membrane. Next comes the 100mm (4”) concrete oversight followed by a 50mm (2”) sharp sand and cement screed which is ready to receive carpet, tiling or wooden flooring.
Underfloor insulation will substantially prevent cold penetrating from below or internal warmth from being conducted through a cold concrete floor. The special foam insulation will turn your concrete floor slab into the equivalent of a giant storage heater and is particularly noticeable in the evening, not just after a warm sunny day, but even in cloudy weather.
This is essential to take away water from the conservatory roof and guttering system thereby preventing the ground near your conservatory and existing home foundations from becoming saturated, with the consequent instability which is a main cause of subsidence. Also, special consideration must be made for existing pipes and facilities beneath the new base which have to be appropriately protected, not just while work is being carried out, but thereafter.
Party Wall Act 1996
Many people find it very hard to interpret the correct way to proceed. The best way to be confident is obtain the explanatory booklet issued by the Office of the Deputy Prime Minister and which is available at most local planning departments. It is best to seek further guidance from Building Control, who you should find most helpful and your best source of reliable information. For most of our clients we are able to undertake this exercise on your behalf, following a visit to your home and a full assessment of the proposed building works.