Aug 1 2017

Basement Questions: Grading – Soil Types #flooded #basement


The right soil grade can prevent basement leaks before they start by directing the water flow away from or around the house.

The first rule to grading is: the soil should always slope away from your home.

It sounds like common sense but foundations are often set too deep in the ground during construction causing marshy ground, wet basements or flooded slabs.

Many building code officials respond to this problem by requiring that the top of the foundations or slabs sit at least six inches above the highest point of the soil at any location around the house. Also the ground must fall away from the foundation at least six inches within the first 10 feet around the perimeter of the house. Remember, this is just the minimum requirement. When it comes to residential grading – the more slope the better.

Land around your home needs to be graded so all water flows away from the foundation and does not accumulate near the foundation.

If you have grade problems for an existing house, you first have to look at the lot to know how to fix it. Look at the overall lot grading and the layout. Understand surface water may enter from adjacent properties.

Sloped Lots

If you are lucky enough to have a sloped lot, your task of establishing grade can be accomplished. All it will take is a small piece of earth moving equipment like a Bobcat or skid-steer loader.

If you have a situation where ground is slopping towards your house (houses built on hillsides), the trick is to slope the ground gently by creating a swale. This swale, or ditch, allows you to do two things. It gets water away from the house and at the same time collects the water which runs downhill towards your house. You direct this swale around a corner of the house and continue until the natural slope of the ground is falling away from your structure.

Flat Lots

If you have a flat lot you face a more serious challenge. Sometimes the ground is so flat you can’t create a swale or sloping condition. In this case, you will need to pipe roof water as far away from the house as possible. Downspouts that dump water onto the ground near the house can cause serious problems with hydrostatic pressure.

You can also consider surrounding your house with a moat, something like the old castles used to have. This moat is simply a ditch that is dug around the problem areas of your house. A two foot wide by two foot deep trench can be very effective. Once this trench is excavated, fill it with large 1-inch washed gravel up to within one inch of the top. This trench acts as a collection area for surface water. As long as your soil can absorb water (even at a slow rate) you will have improved drainage conditions around your house.

Soil Types

The type of soil around your home and on which the foundation is built will determine the degree of slope and drainage properties.

The soil types in your area can determine the effectiveness of your drainage system, or dry well. Soil scientists refer to soil types by texture or by how much sand, silt and clay is present. Many times the topsoil is porous (as would be used for planting) and absorbs the surface water. The sub-layer of clay or similar non-porous soil prevents the water from continuing in a downward movement and directs the water laterally. If non-porous soil next to the foundation slopes toward the house, water will begin to accumulate.

It is also important to understand your soil type because expansive soil can cause serious structural problems to your foundation. Your house can move if it is on unstable soil or if the moisture level in the soil changes. Soil movement can cause damage to the foundation and framing, evidenced by cracks in the slab or foundation, cracks in the exterior or interior wall covering, uneven floors and/or misaligned doors and windows. This type of movement is usually associated with slab on grade construction; however, this may also occur in structures with basements and crawlspaces. Expansive soils on slopped lots can also cause a house to creep down hill or even cause a landslide.

Although expansive soil is found throughout the United States, it is most commonly seen in Texas, California, Virginia and Colorado.

Possible Solutions for Unstable Soil

If you already have a home on expansive soils, you can take preventative measures by maintaining a uniform and constant level of moisture in the soil to prevent shrinking and using proper drainage systems and grading techniques to prevent swelling.

If your home is already damaged by unstable soils, you can learn our options with our foundation repair articles.

Proper grading (in conjunction with a gutter and downspout system) is one of the easiest ways to manage surface water, reduce the possibility of water penetration and structural damage from hydrostatic pressure, and control the water content in expansive soils.

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