Replacement concrete floor and DPM

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Cheshire
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I am about to start renovating a 300 yr old cottage, neadless to say the existing floors and wall have no DPC and suffers from damp, the exterior ground level is approx 150mm above the existing concrete slab on one side of the house and above level on the other.

1st of all i plan to dig a trench to foundation level around the property to allow the walls to start and dry out, this will be back filled to a lower level once a french drain is installed. Is this the correct thing to do?

2nd i plan to remove the existing concrete floor and re-lay with insulation and DPM, i am unsure on where to tie in the DPM from the floor once layed as there is no existing DPC in the brick work. Is it acceptable to terminate the DPM at the edges of the floor? Im guessing not.

Is it possible to install a retrospective DPC (not chemical) into the brickwork?

Any advise will be much appreciated.

Rob
 
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Hi Rob,

I'll answer each part of your questions in turn...

Digging the trench is fine as is the French drain. Make sure that you finished external floor level is a minimum of 200mm below the internal finished floor level and this will go some way to solving your damp problems.

Remove the existing concrete floor; this will be retrofit anyway and is not really compatible for a building of this age. Ideally you should forget about replacing the concrete slab and install a timber floor with a sub floor void. Its difficult to say what was there originally in a building of this age but it may have originally been a rammed earth floor. Either way a timber floor is compatible with the age of the building; concrete isn't and you'll continue to have rising damp problems due to the weight of the floor slab forcing moisture up the walls under hydrostatic pressure.

If you do opt for concrete then the dpm should be lapped into the wall which is a difficult detail to get right retrospectively. However this course of action only makes sense if theres an existing physical dpc in place which there won't be in a building of this age. You can install a retrofit physical dpc using the DeBeers method which basically entails cutting a 1m slot in the mortar bed, inserting the dpc then grouting the slot to make good. You can't cut all the way round the building at once for obvious reasons hence the process of working in 1m bays. This would be expensive and not my preferred option. Use compatible building materials which allow the building to breathe, get the floor levels and construction right and your damp problems will be cured for good rather than simply managed.
 

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