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January 2019 Blog: Damp Bridging and how to avoid it!


Regrettably, the party is over and it's time to turn our minds towards some of those irritating damp problems around the home. Hopefully, this blog will at least bring the promise of a possible solution!

Damp Bridging

There are many reasons that dampness may be transmitted through buildings from one material to another. Capillary Action is usually the root cause, but stopping the spread of transmitted damp can often be quite simple. Although, as in all things, it is usually a lot simpler if thought about before or during installation, rather than retrospectively.

Things to try and avoid where possible:

1) Attaching timber to the outside of buildings
- Quite often we attach timber to enhance a building aesthetically, like for instance, trellis-work to allow climbing plants, etc. Unfortunately the timber, where it touches the masonry, can lock moisture into the building's fabric. This can make drying a much more prolonged process and the effects can be exacerbated over time. It can also, possibly, lead to moisture penetration, thermal bridging or even frost damage. This is not to say that timber cannot ever be applied to masonry, but some thought should be given to create a barrier between the two wherever possible. For example, if timber cladding is installed to refresh the looks of your building or to enhance weatherproofing, it can be advisable to install a vertical DPC (damp proof course) or DPM (damp proof membrane) between masonry and timber.

2) Training creeping plants to the sides of buildings - This can have a very similar effect to point one, above. The moisture gets locked into the masonry and normal drying from wind and sun is dramatically reduced. Plants can also have other deleterious effects on buildings too, but they are not covered by this particular blog.

Damp Bridging, Penetrating Damp & Rising Damp remedies - Shrewsbury, Chester, Mid Wales

3) Applying external render all the way to the ground - When render goes to the ground, it sucks water up the wall (capillary action again) and this can get locked into the masonry; particularly if the masonry is finished with a waterproof, non-breathable finish. It can also bridge a damp proof course by carrying moisture around it, and introducing dampness into the wall above DPC. This can, naturally, cause further problems inside the building.

One way of reducing the possible effects of this is to install a bell bead in the render, approximately 150mm (2 bricks) above the ground or, if there is one, just above DPC. This allows some drying to take place at the bottom of the wall and stops the render from drawing moisture out of the ground. If you have a situation where the render is already in place, it could be worth knocking the bottom off and installing bell beads in this manner. NB: Smooth render is very difficult to 'patch in' and so re-rendering of the wall may be necessary is some cases, but consult your plasterer for advice on this. This will not stop rising dampness inside the building, but it may reduce it and should generally lower the moisture content of the wall, which is generally beneficial. A 'bell' bead is preferable to a 'stop' bead because its shape deflects rainwater and prevents drips from creeping back towards the building due to the effects of surface tension.

4) Applying plaster down past a new, injected chemical damp proof course - When a modern chemical DPC has been used, it should be injected (or installed, with regards to Dryzone Dryrods) into the lowest mortar course visible in the wall above the floor. When the waterproof render, or one of the modern, Dryzone breathable, damp resistant plasters, are spread on the wall later to complete the system, care should be given to avoid plastering down to the floor; as this can bridge the new DPC. Once bridged, the DPC can be rendered ineffective and rising damp can once again reappear.

5) Butting up new masonry against old masonry - Obviously this is often unavoidable when building extensions or undertaking renovation work on old buildings. However, there are things that can be done to control the lateral spread of dampness from one wall to another.

The most common connection, where a new wall is butted to an old wall, is probably the trusty, block-work starter kit. When installing one of these, it takes almost no time and costs very little, to install a vertical PVC DPC behind the kit. The bolts that hold the metal to the wall will press the DPC in place nicely. Simply make sure that you choose a DPC that is wider than your block-work! If you wanted to be really 'belt and bracers' you could even fill the drill holes with silicon too, before inserting your Rawl Plugs!

Using DPC this way can make a huge difference but it may not be the whole story. If you are plastering directly onto the walls or using plasterboards in the 'dot and dab' method, it may also be worth installing a membrane waterproofing system like Oldroyd Xp Plaster Membrane or Drybase Flex Membrane. This will ensure that your new plaster won't draw water out of an old, untreated wall. It is advisable that the membrane extend 1.2m laterally, away from the old wall as a minimum and that your new plaster work does not touch the old wall directly, in any way.

Laterally penetrating dampness can, typically, travel as far as rising damp, as it also relies on capillary action to promote the movement of moisture through the fabric. NB: We have seen instances of rising and penetrating dampness go much further than this, although it is rare - every project should be assessed individually.

Damp Bridging, Rising Damp and Penetrating Damp are all linked by Capillary Action and can occur all at once in the same area.

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