I have a problem with damp in a wooden cabin. The cabin is double glazed and insulated. However, the concrete base on which it sits is not quite big enough and I have what I think is a rising damp problem in the two back corners of the cabin. It appears that the damp proof membrane has stripped away under the building and cannot be replaced as it is not possible to get under the building. Can you help with any advice?
This is situation is not unfamiliar to me and I have experienced these problems with similar buildings which originate from two distinct faults. The first problem may arise from the building sitting upon a 4” x 2” timber sole plate laid directly onto your concrete slab that projects beyond the end walls without space for driving rain to run away. This is exacerbated by the lack of guttering despite the overhanging roof.
The floor structure usually consists of light timbers laid without a damp proof course (DPC) onto the slab with polystyrene insulation between and tongue and groove floor boards over. The lack of DPC or a vapour barrier allows capillary creep, driven rain and condensate water to soak the timbers and cause high levels of humidity in the building itself; in one case I saw this caused an outbreak of wet rot in untreated flooring.
The wall timbers are treated to prevent such rot but the finger joints at the corners are prone to the capillary leakage which I noticed in your pictures again exacerbated by driven rain and the poorly fitted guttering. The solutions in most cases are to lift the building by about 25mm corner by corner and insert plastic “folding” packers regularly under the sole plate to break the capillary path and allow air to flow below the floor. This sounds radical, however, these buildings are flexible and being comparatively light are easily raised using an appropriate lever and fulcrum.
Additional packs will be required below the flimsy floor “joists” at adequate centres to prevent sagging. If rot has set in then the floor may need to be replaced; if so better insulation and vapour control should be considered.
Gutters and downpipes should be adjusted to better remove excess water. Corner external joints should be filled with a proprietary waterproof flexible sealer. Make sure this is a paint grade product and the joints are well cleaned out before being inserted. Treat walls and especially the timber ends afterwards with a compatible decorative preservative. A good joiner should be able to carry out this work in two days and depending on the extent of timbers to be replaced the cost might not exceed €1,000. If in doubt, you should consult your local building surveyor.
Fergus Merriman is a chartered building surveyor and a member of the MediKids Building Surveying Professional Group