By Jon Eakes
How soap can
rot your exterior walls
Soap is often used to clean house siding, usually with a pressure washer. Soap is also often added to stucco to make it more workable. Soap is good stuff - isn't it? If you believe that, read on.
Let's start with taking a look at just what soap does that makes it work so well both for cleaning surfaces and for making stucco flow better. Pure water has a tendency to hug itself. That is why water drips off the end of a faucet in individual droplets rather than just flowing as a tiny stream. The surface tension of the water wants to keep it together in the smallest form possible, so it tries to form a ball or sphere. The tear drop shape of a drip is simply the elongation of the sphere before it lets go of the faucet. In fact if you use a stop action camera to see that same drip in the air, it is an almost perfect round sphere.
We see that with water on a newly waxed surface. Drips of water will actually form balls sitting on top of the surface. When you add more water it can't stand up high as a large ball so it flattens out, but the outer edge of a puddle of water on a waxed surface will be curved, even rolling back under like that ball. You can see that in the top picture. When you add the smallest quantity of soap to that water, it instantly looses that surface tension and flows out, as you see in the bottom picture.
Soapy water works for removing dirt because the soapy water has little surface tension and can flow under - even through - materials that would not absorb water without soap. This same "slippery" function is what allows soap to make stucco work in a more fluid way.
So, to this point, soap is doing what we want it to do. The problem with soap and houses is that the two photos show water on a house wrap. House wraps are sheets of material designed to stop liquid water from penetrating but they are porous enough to allow water vapour to pass through. They do this by having a sort of waxy surface and are made of a woven material, or small pin holes, that have openings too small to allow water that is being held together by its own surface tension from getting through the holes. However, water vapour, which has no surface tension, can get easily through. If you add soap to that water, its surface tension is gone and now the water can flow through the material as easily as it will flow through paper. When I left those two sheets of house wrap sit for a while, the first one simply dried up. The second one dripped most of the water through to the other side before it dried up.
The point is, soap on a building paper or house wrap removes its waterproofing qualities. This is not a good thing. The first signs of this problem came from houses which had stucco finishes and soap had been added during application. It has also shown up with soap being deposited by pressure washers shooting up into ventilation passages of siding. You might think that the soap would eventually rinse off and the problem would go away. That would probably be true if there was enough water flow to actually rinse off the soap but, more likely, siding leaks will only provide enough water to wet the house wraps, not rinse them.
Hence, allowing soap from any source to come into contact with building papers and house wraps could eliminate one of the most important water control mechanisms we put on our walls. Use proper stucco additives, not soap, to make stucco more workable. Spray downhill or horizontally, not uphill, with power washers when cleaning walls - and explain this to your customers so they won't accidentally sabotage the protective systems you have built into their homes.
Problems in walls are not caused by the presence of some moisture, but by having just a little too much for the given conditions. Non-waterproof building paper could let in that little bit of moisture that becomes too much for a particular wall. HB
broadcaster, author, home renovation and tool expert Jon Eakes provides a
tool feature in each edition of Home BUILDER.