Foundation Problems Under Conservatories – due to inadequate Foundations.
At DB Building Warrington we have attended countless properties, and found foundation problems, due to the inadequate foundations on new and old conservatories, ending in expensive repair work (underpinning). This is what you need to know about conservatory foundations.
Conservatories are lightweight structures and therefore the foundation loads are generally low. This often leads clients and some builders to believe that shallow foundations will be satisfactory, and there foundation problems under conservatories. occur. This would be true in so far as settlement of the foundations is concerned, because adequate allowable ground bearing pressures for conservatory loads can usually be achieved with shallow foundations. But the problem is that shallow foundations are more susceptible to subsidence – the effects of seasonal variations in the ground and nearby trees acting on clay soils being the most prevalent causes of subsidence.
Typically most conservatories are built with foundations which are shallower than the Building Regulations would otherwise allow and are not inspected by a Building Control Officer or other knowledgeable person. Therefore it is not surprising that problems arise.
The Building Regulations say:
A2. The building shall be constructed so that ground movement caused by :-
(a) Swelling, shrinkage or freezing of the subsoil; or
(b) land-slip or subsidence (other than subsidence arising from shrinkage), in so far as the risk can be reasonably foreseen, will not impair the stability of any part of the building
But this is further clarified by the Approved Document A:
Minimum Depth of Strip Foundations.
2E4 Except where strip foundations are founded on rock, the strip foundations should have a minimum depth of 0.45m to their underside to avoid the action of frost. This depth however, will commonly need to be increased in areas subject to long periods of frost or in order to transfer the loading onto satisfactory ground. In clay soils subject to volume change on drying (‘shrinkable clays’, with Plasticity Index greater than or equal to 10%), strip foundations should be taken to a depth where anticipated ground movements will not impair the stability of any part of the building taking due consideration of the influence of vegetation and trees on the ground. The depth to the underside of foundations on clay soils should not be less than 0.75m although this depth will commonly need to be increased in order to transfer the loading onto satisfactory ground.
And the NHBC Standards give the best and most detailed advice with regard to foundation depths in shrinkable clays and where trees are close by. Soil samples have to be collected at various depths and tested to determine their Plasticity Index, this gives an indication of their Shrinkage Potential. Trees have to be identified and their distances from the foundations measured. The geographical location of the building has to be taken into account. These factors are fed into the equations, tables and graphs, and the result is a minimum foundation depth required to avoid subsidence. The minimum foundation depth may be as much as 3 metres or more.
Where a conservatory roof butts up to an existing outside wall, a cavity tray should be installed to ensure there is no water penetration.
In practise a cavity tray is rarely, if ever, installed for a conservatory, and, perhaps surprisingly, this rarely gives a problem. This is because, for a cavity tray to be of use requires significant water to penetrate the outer leaf and run down the cavity. Such water penetration can only normally occur in walls exposed to high driving rain, and usually these have been given additional protection in their own right, such as tile hanging or render finishes.
What is seen with most of the ‘quality’ conservatories is lead flashings tucked into the bed joints where the conservatory roof abuts the house wall. This looks good and certainly adds to the ‘quality’ appearance, as well as stopping rain running down the external face of the wall entering into the conservatory.
To ensure you don’t have foundation problems with your conservatory, please contact DB Building today on 01925 801134 or 07525124324 or send us an enquiry and we’ll get back to as soon as possible.
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