Rain Penetration

By rain penetration is meant that rain water penetrates into a wall either through the surface wall, or due to leakage at windows or similar installations. It is not necessary that water penetrates so far that it may be discernible on the inside of the wall.
Rain penetration in modern cavity wall tends to show as a well defined roughly circular area on internal finishes. Sometimes surface salts will define the outer limits of such wetting. If the wetting persists, most of the wall may become visibly damp.
In older, solid wall buildings, wetting may not be visible because successive coats of emulsion paint or vinyl wall paper have masked effects. The extent of the dampness, or if dry the salts which define it, can be traced with a moisture meter.

Moisture can be deposited on external surfaces in several ways:

• Gentle rain or drizzle falls vertically and will accumulate on flat surfaces. Some splashing may wet adjacent surfaces.
• Driving rain, which is heavy rain blown by a strong wind on to horizontal and vertical surfaces. Water can also be blown uphill on sloping surfaces.
• Snowfall and wind blown snow drifts have little effect at the time but when the snow melts, it can cause severe wetting, particularly very fine snow blown into pitched roofs.
• Fog wets external surfaces but in small quantities and has little effect.
• Condensation can occur on the out side surfaces in tropical climates, particularly with air conditioned buildings. Storms in these climates are more likely to be a test of weather tightness.
  

Rain penetration in walls

Water moves into or through a wall under the action of several forces including capillary action, diffusion, wind, gravity and sorption. Wind can give rise to the effects of turbulence which increase speed and change the direction of its influence. Pressure difference across a building can result in negative pressures in a cavity or void which will suck water into that area.
A wide variety of materials have been used for walling and with varying levels of resistance to driving rain. The concept is introduced of raincoat construction where the surfacing material is intended to shed water and the overcoat effect where water absorbed into the masonry evaporates before penetrating the full thickness of the wall. For both these effects the resistance to rain penetration of a single leaf wall depends on its thickness.

Brick or stone walls comprise masonry units bound together with mortar; both vary in porosity and suction. If the mortar is strong, it shrinks away from one side of the vertical joint and minor cracks develop. When rainwater hits a wall, it is first sucked into the wall material. If rain falls at a rate greater than the effect of the suction, a water film forms on the wall and run off occurs. Some of this can enter cracks being also sucked into the bricks or stone. When the storm ceases, drying occurs by evaporation from the external surface. If rain falls for a long time, the wall is likely to become saturated with the pore structure full of water. This is the overcoat effect.

Areas covered Dorset – Bournemouth – Poole – Weymouth - Christchurch – Ringwood – Wimborne – Ferndown – Corfe Mullen – Parkstone – Dorchester – Shaftesbury – Blandford - Westbourne – Wareham – Oakdale – Maiden Newton – Bridport – Yeovil – Warminster – Gillingham – Mere – Sturminster Newton – Salisbury – Southampton – New Forest – Fordingbridge – Wiltshire - Hampshire