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Mistakes to avoid when building a home on rocky soil

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You can clearly see the bedrock in the hillside where this home is being built. (Tim Carter)

The lot where I’m building my new home is quite rocky. The soil cover is somewhat thin, perhaps only three feet deep at most. Fortunately, no blasting is required to dig into the rock. What are some of the challenges I’ll face dealing with this rock? What are a few of the best practices to ensure I’ll have a dry basement for years to come? What’s the biggest mistake my builder could make, and how do I prevent it?Tim J., Spring Hill, Tenn.

Congratulations on your new home project! You’re asking all the right questions so far, and I hope all goes well throughout the entire process. My college degree is in geology, and I had quite an interest in both hydrogeology and engineering geology while in school.

Hydrogeology is the study of groundwater, and engineering geology focuses on how man-made structures of all types interact with soil and rock. If you’ve ever had the pleasure to fill a water jug or bottle from a natural spring, then you know that water loves to follow tilted and flat pathways within bedrock.

It’s not too hard to build a home when you’re dealing with rock unless you need to blast. Blasting can be quite expensive. Two lots above my own home have never been built on because several previous owners have spent tens of thousands of dollars blasting holes in solid granite only to exhaust their budgets and go bankrupt. You’re lucky that excavators can dig through your rock.

I feel the biggest thing you need to understand is the relationship between water and rock. Rainwater penetrates topsoil and starts to head into the ground. Depending on the subsoil makeup and depth to bedrock, the water tends to begin to travel sideways or downslope along the contact zone between the soil and bedrock. Your builder’s job is to install a fantastic foundation drainage system that collects this water as it gets close to your foundation and channels it away from your home to a low spot on your land.

Not too long ago I was sent a photograph of a structural concrete basement slab that an uninformed builder poured directly up against the bedrock at a building site. He then placed the foundation on top of this slab. Immediately, groundwater collected against the foundation and seeped into the basement. Be sure your builder doesn’t make this rookie mistake.

It’s best to place the footing of the foundation at least one foot away from the side of the excavated hole. This spacing gives you plenty of room to place a perforated drain pipe alongside the poured or cast-concrete footing. The drain pipe gets covered with clean, washed gravel that’s similar in size to a large grape.

I prefer to use the solid plastic pipe that has half-inch holes drilled in it in two rows. The holes should face down and the pipe should be placed on a two-inch-thick layer of the clean gravel.

This pipe should then be backfilled with the clean washed gravel that has no sand in it. The thicker this layer of gravel, the better. Before it gets covered with soil, put a thick layer of straw on it or cover it with a layer of roofing felt paper. This prevents the gravel from getting clogged with silt from the soil.

The foundation walls need to be waterproofed, not just damproofed. Damproofing is just a spray coating of hot asphalt. It does a great job of stopping water vapor passing through the foundation walls, but if the concrete foundation cracks, the thin asphalt coating will not bridge the crack.

Waterproofing foundation systems are designed to deal with foundation cracks. I used a magnificent system on my last house that was a blend of rubber and asphalt. It was applied about 1/4-inch thick on the foundation walls and then covered with a rigid dense fiberglass panel. This fiberglass protected the rubberized asphalt from backfill damage and it channeled water directly down to the foundation drain tile.

You need to be very careful when backfilling any trenches that contain utilities. Buried water, gas, electric and cable lines need to have a six-inch bedding layer of sand that they lie on in the trench. Never put a utility line in a trench in direct contact with rocks.

Then, once the utility lines are laid, an 18-inch protective layer of sand should cover them. Sharp rocks coming into contact with these lines can injure them and cause lots of pain and suffering.

Each week I’m reminded of the biggest mistake most builders make when they build. Many builders, no matter what the soil type, dig the foundation hole too deep and set the house too far into the ground. All too often the top of the foundation is just a few inches above the grade line around the house.

If you look at homes built around the early 1900s in many cities you’ll discover the top of the foundation was often three feet out of the ground. This was done for any number of reasons, and it might be considered extreme today.

The top of the foundation should, in my opinion, be no less than 18 inches above the highest point of land within 10 feet of the foundation. This allows you to have plenty of foundation above the grade line and create a good slope so water drains away from the foundation.

If you could read my incoming email each week, you’d agree this is a primary pain point felt by many new homeowners. I see photographs each week of foundations that are only three or four inches above grade and the soil around the house is flat or slopes toward the house. No wonder these homeowners have water pouring into their basements and crawlspaces! Don’t let it happen to you.

Need an answer? All of Tim’s past columns are archived free at www.AsktheBuilder.com. You can also watch hundreds of videos, download Quick Start Guides and more.

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