What Causes Slab Leaks And How to Repair Them
Slab leaks are a fact of life in areas were soils are unstable. Homes are built on concrete slabs or using pier and beam construction because basement walls can crack and break from the pressure of shifting soils.
Slab construction became popular during the post-WWII construction boom because it was faster and cheaper. A rebar-reinforced 4”-6” thick concrete slab is poured directly on a prepared surface. Plumbing lines are run beneath the slab, where they are difficult to reach.
Sometimes the plumbing joints leak or the pipes crack beneath a slab, and then you have one of the most serious problems you could imagine, a slab leak.
As water runs under the house, it erodes the soil. This removes support for the slab and puts more strain on it and may cause it to develop cracks from hairline width to an inch or more. Then all that water has a clear path into your house.
Depending on the extent of the leak, you may experience serious water damage to flooring, walls, furniture, appliances and more. And, as you can imagine, fixing a leak beneath tons of concrete is no walk in the park, and it’s definitely not a DIY plumbing job.
Plumbing Materials Can Cause Slab Leaks
Slab leaks result from some type of failure in a home’s plumbing system. The failure could be caused by faulty installation, weakened water lines, chemical reactions of metals in the earth to metals in the plumbing system or, drum roll, soil shifting beneath the slab.
We’ll come back to that last one in a minute. First, let’s consider the materials the pipes are made of, which often is a function of the age of the plumbing system. The materials themselves sometimes are the problem. Unless you were able to see the original plumbing work during construction, you will have little idea of where the lines run, what they’re made of and how good the installation work was.
Cast iron pipe was the staple for most homes built prior to the 1960s, although plastics have become more popular since. Cast iron plumbing lines above ground in a house do offer some advantages. Cast iron is quieter than plastic because it vibrates less, so you don’t hear water running through the lines as readily. It is very durable and will not burn, which can be important in case of fire.
But unprotected cast iron installed as underground service lines can be corroded by water and minerals in the soil. To address that, some cast iron pipes made today are sheathed in a protective material. These are far superior to the old pipes commonly used into the 1960s.
Galvanized steel pipes also were common in homes through the 1950s. Like cast iron, galvanized steel is strong but susceptible to corrosion. Galvanized pipe is also more prone to clogging. The coating inside the pipes eventually wears off, leaving the pipes vulnerable to rust and mineral buildup that can clog the lines solid with minerals and waste matter. The useful life galvanized steel plumbing pipe is about 50 years.
Rigid copper pipe remains a popular choice today, especially for hot and cold water supply lines. Flexible copper pipe is used for water supply lines to appliances such as refrigerator icemakers and dishwashers. Copper is durable and corrosion-resistant. Copper pipe comes in different wall thicknesses. Underground lines should be run using copper pipe with the thickest walls. Even then, careful installation is called for. A dent in a water line can eventually wear thin from the constant flow of water and open a small hole in the pipe.
Plastic pipes are corrosion-resistant and can be used above or below the ground. Plastic resists corrosion and is relatively easy to work with. Most common types include:
- ABS (acrylonitrile butadiene styrene) – black in color, ABS was the first plastic pipe used in residential plumbing. There have been problems keeping ABS joints together, which has led some municipalities to disallow it in new construction. A failed joint beneath a slab will result in a costly slab leak. Also, ABS can deform when exposed to direct sunlight, so it’s not suitable for all above-ground installations.
- PVC (polyvinyl chloride pipe) – This is the more commonly used plastic plumbing pipe now. Colors indicate intended usage, including white for drain lines and irrigation, blue for potable water, green for sewer water, purple for reclaimed water and gray for electrical conduit.
- CPVC (chlorinated polyvinyl chloride pipe) – This is similar to PVC, but the chlorination process gives CPVC different physical properties. CPVC’s ability to withstand higher temperatures makes it more suitable than PVC for hot water lines.
- PEX (cross-linked polyethylene) – the newest plastic plumbing pipe. PEX is used as an alternative to PVC, CPVC and copper. It is flexible, cuts easily and uses compression fittings, which makes PEX relatively simple to install. It also has some drawbacks. Making secure connections requires some skill, and it’s flexibility can cause vibration and movement problems. If not carefully secured PEX can be abraded by movement over structures such as floor joists and rafters.
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