Fen Drayton, Cambridgeshire. Rising Damp

Rising damp is a common problem in older buildings.  Moisture rises up the wall by capillary action, depositing salts and damaging the wall finishes.  However, rising damp is less common than often thought and dampness at the base of a wall may be due to other problems such as penetrating damp and condensation.  More often than not the cause is due to bridging of an existing damp proof course by high ground levels.

Electronic moisture meters are commonly used to diagnose rising damp but these can give false high readings with certain paint and wall finishes.  They may also fail to diagnose condensation and hygroscopic salts (from an historic problem).

If a moisture meter gives high readings then further investigation should be carried out.  Mortar sampling of the masonry is the only accurate way of quantifying the absolute moisture content and hygroscopic salt levels in the masonry.

EBS carries out a visual inspection of problem areas backed up by electronic moisture meters.  Mortar sampling of masonry is carried out where necessary, to determine accurately the moisture levels and hygroscopic salt levels.  The solutions to these problems are varied and depend on individual circumstances, for instance ground levels may need to be reduced, new lime plaster may be required on internal walls, dry lining systems may be necessary or physical damp proof courses may be recommended.  Chemically injected damp proof courses are usually not advised.

Recently EBS carried out an inspection of a listed cottage in Cambridge. The owner was concerned that the bases of the walls were showing signs of damp and discolouration. The building was built in 1850 before slate damp proof courses were generally used and none were found.  The original timber suspended floors had been removed due to decay and replaced with solid floors of unknown construction in the 1940s.  An injected damp proof course was installed in the early 2000s with sand and cement render on the internal walls up to 1m from floor level.  This indicated that there has been a history of dampness at the base of the walls. In this case the addition of a higher level extension at the back of the building also pushed moisture into the rear wall.

However, the fundamental problem in this case was the removal of the original suspended floors and their replacement with solid floors.  Prior to the solid floors being installed, the bases of the walls were able to breathe under the timber floor.  Moisture naturally tends to rise up the wall but provided there is sufficient ventilation under the floor, the moisture will generally not rise above the level of the suspended floor.  Once a solid floor is installed the bases of the walls have no ventilation and moisture will be able to rise to a greater height.  Combined with this is the migration of moisture under the floor slab to the perimeter of the room, particularly if the floor has a damp proof membrane.  This increases the height to which moisture will rise even further.

In our experience, injecting a damp proof course does little to prevent rising damp in most situations, and the installers generally insist that the walls are re-plastered at the base, with a waterproof render to hide the dampness 'whilst the wall dries out'.

The chosen solution in this case was to remove the sand and cement render at the base of the walls to allow the walls to breathe, and then dry line the walls with a ventilated dry lining system.  This provides a blemish free wall finish whilst still allowing the walls to breathe.

Another option would have been to remove the solid floors, reinstate the suspended floors, and re-plaster the base of the walls with a breathable lime plaster. This would have been expensive to carry out and would not have dealt with the additional dampness from the high ground levels of rear extension at the back of the property.

Conclusion

Think carefully before replacing suspended floors with solid floors as this can cause serious problems.

Injected damp proof courses are a quick fix and do not generally solve the damp problem.

Check if the building has an existing DPC and reduce the ground levels to at least 150mm below this wherever possible.  More often than not this will solve a damp problem at the base of the wall.