Help for a conservatory !!!

Hi Mac,
There are a few things to consider in situations of this type but some of the points that might be worth considering generally:

The level of the dpc should be at least 150mm above the external ground to comply with the recommendations of the Building Regulations. The function of the dpc is only to resist moisture rising up through the wall and not to hold moisture back under hydrostatic pressure (in this case penetrating dampness).

So if the dpc is 150mm above the soil and the floor to the conservatory is some way below this and the surrounding ground to match the exisiting house there will need to be a structural waterproofing system to cope with this situation from the level of the dpc down to the floor and also the floor itself must be able to cope with this situation in addition to the floor / wall joint. Such systems can attempt to hold the moisture back externally, within the depth of the wall itself or internally. In addition there are systems that allow the moisture to come in behind a membrane but then drain that moisture away before it reaches surfaces / finishes.

I know you really want to have in your mind a specification for the system but really you need to simply make the contractor aware of the situation and make it clear to them in writing that the structural waterproofing required will be entirely their responsibility as will making good of any failure. Refer them to BS8102 which is a British Standard mainly concerned with true basements but which will apply to this situation since it is a building structure below ground level and request in writing that they fully comply with the requirements of this standard.

There may be some benefit in below ground level drainage adjacent to the proposed structure to lower water tables in this immediate area but this requires careful design. Also be mindful of how this conservatory / structural waterproofing may impact upon the adjoining property if such is present. Nothing worse than finding you have altered things such that next door suddenly has water pouring into their house / their garden floods when before it was dry for which you may be held liable.

Be mindful that with a structural waterproofing system that is of a type that holds back moisture the weakest point is often the floor / wall joint as the new floor / walls will tend to move / settle independent of each other and this may crack / open up the structural waterproofing system at the floor / wall junction resulting in damp / moisture ingress / rot and degradation of plaster / finishes including that to floors. Also with such systems you will have to ensure the system is not punctured by things such as joinery fixings / services etc.

If the house floor is a suspended timber floor then you (and the contractor) also need to ensure that sub floor ventilation to the existing timber floors is not reduced / compromised by the addition of the conservatory. The threshold for decay in timber is around 20-22% for and the contractor needs to ensure that their conservatory works will not raise the moisture content to existing timber floors above this level. Again throw this responsibility back onto the contractor in writing.

Where the conservatory roof abuts the original house wall the works should allow for the insertion of a cavity tray with appropriate weep joints. The function of the cavity tray is to direct moisture within the cvavity be that condensation or penetrating dampness out onto the roof via the weep joints before it can permeate down below to what will become an internal wall / soffit to a lintel that was once an outside wall.

With a project of this type there may be considerable benefit in consulting the Local Authority Building Control Officer for guidance (they do not bite and are there to help) rather than hoping the contractor works to an appropriate standard. Hope this helps, kindest regards David Aldred Independent damp and timber surveyor.