Flat Roof advice

Hi All,
I need some advice on building a roof for a kitchen extension which I am considering.
The main problem is that I am caught for height, as for economical reasons,
I want to make a 'lean-to' extension on to the existing external wall and the height of the
soffit here is only about 8 feet from the ground. I know that flat roofs are often made
from 9" timber but this would be too big.
Can I do something like get a 12'x 8' frame (the roof size required) made from 50mm box iron,
fix plywood to both sides - and 50mm insulation sandwiched in between - and use this as a roof?
Would this suggestion work? Would it be insulated properly or could there be problems with
condensation? Any comments most welcome..... Thanks.....

I stongly advise against doing this sort of thing yourself without taking competent professional advice from someone experienced in this kind of work and/or without employing a competent and experienced contractor.

Depending on how the construction is detailed, you could cause problems with interstitial condensation leading to dry rot in timbers which can in turn migrate through the fabric and affect adjoining properties.

Poor weathering detailing can lead to accumulation of water/moisture and subsequent degradation of the building fabric through wet rot or spalling through frost damage, particularly if its an older structure.

The issues of ventilation of inner rooms, construction of extensions, heating and space separation of conservatory-style extensions needs to be assessed

You may also find you're running foul of both the building regulations and planning and development regulations.

Carrying out development that may be unauthorised or non-compliant is likely to reduce the sale value of your building and may result in action being taken against you by third parties and/or the local authority.

I find it very difficult to understand how you are THIS caught for height.

If this is a small inner city terraced property, you may not be able to fully avail of the floor area allowed for in the exempted development schedule as there are limits to how much site you can cover with an extension.

If this is part of a listed building, you are unlikely to be able to extend without permission.

You may also cause problems for your neighbours [party walls, reduction in light and amenity, damage to services] which could lead to other legal entanglments.

In short, stick to what you're good at, and employ competent people to deal with the rest.