Negative Grade

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This week’s blog is on a topic that has been fresh on my mind as of late, that being negative grade around building foundations.

Many people don’t acknowledge that this plays a crucial role in the longevity of your buildings foundation and the way water moves around your property.  Grade can be defined as slope, and you’ll want to ensure that the soils around your home are sloped in a positive fashion so as to direct water away.  All too often when we get to a property experiencing moisture related problems we find that the soils are pitched back towards the buildings foundation, allowing water to flow back in the same direction.

You may be asking yourself what happened to the soils around my home?  Why did they settle?  Soils around a building will settle over time.  When your home was new, the soils were probably placed in a positive fashion by the builder.  However, most builders will not take the time to compact the soils used to backfill your home because that takes time and resources, and let’s face it, time is money.  You may also want to consider the type of soil your builder may have used to backfill your home.  Sure, you may have some black top soil underneath your landscaping but what’s underneath that?  We typically see a lot of natural clay used for backfill purposes during our investigations.  This is likely because it’s essentially free to use for the builder and it’s already located onsite.  The time of year the soil was placed back in against your foundation will also play a crucial role in things.  Had the soil experienced a month of solid rain, or maybe it was frozen in chunks during the winter months?

Like any building product, soils contain certain properties in the way they move and how well they accept moisture.  Clay for example is a very cohesive soil, meaning it can get very hard when dry but will expand and become something you can throw on a potter’s wheel in art class when saturated.  Throw the seasonal freeze and thaw cycles experienced in a year’s time in Minnesota into the mix and you now understand why this becomes so crucial to the longevity of your homes foundation, surrounding driveways/sidewalks, patios, deck footings, etc.  When correcting the grade around your home we often recommend a soil correction before achieving the final, positive grade away from the foundation.  This entails removing the saturated, cohesive, unsuitable soils used by the builder and replacing them with a compactable, more permeable soil such as recycled asphalt and concrete (con-bit), red rock gravel, or equivalent.  Compaction of this soil in layers is a must to ensure your newly achieved positive grade does not settle leaving you in the same boat you may currently be in.

I briefly touched on moisture in the soils in the last paragraph.  One other item to consider as you look at the settled landscaping around your property is whether or not you have rain gutters along the eaves of your roof and where the rain gutter downspouts drain to.  Rain gutters are a must to direct your roof water where you want it to go and this includes the downspouts.  CBS recommends placing extensions on your downspouts that take the rain water away from all landscaping along your foundation, sidewalks/driveways, deck footings, etc.  Did you know a 1” rainfall will produce .6 gallons of water per square foot of roof area??  That’s a lot of rain water allowed to access your building foundation should you not have rain gutters and downspouts directing this away!

I’m going to leave you with a couple of photos of the damages to building foundations we’ve witnessed due to prolonged exposure to water and negative grade.  The first images are of deck joists that are pulling away from the ledger board due to vertical movement in the outer deck post footings.  Not only may this cause costly damage but may also present a safety factor to its users.

deck joists that are pulling away from the ledger board due to vertical movement in the outer deck post footings

deck joists that are pulling away from the ledger board due to vertical movement in the outer deck post footings

deck joists that are pulling away from the ledger board due to vertical movement in the outer deck post footings

deck joists that are pulling away from the ledger board due to vertical movement in the outer deck post footings

The last image is of a deteriorated concrete block foundation that has been exposed to water in the soils for the last 25+ years.  We actually see this quite commonly, believe it or not.  This moisture in the soils, when combined with the movement in the freeze and thaw cycles experienced in Minnesota, has broken down the concrete blocks to mere sand if you will and can be broken just by kicking them with your shoe.  Concrete products are not designed to take on such moisture and movement in the surrounding soils.

deteriorated concrete block foundation that has been exposed to water in the soils for the last 25+ years

deteriorated concrete block foundation that has been exposed to water in the soils for the last 25+ years

Hopefully after viewing this blog you have taken away something about the importance of having a proper grade around your building that you may want to apply to your own projects this coming spring.  Until next time, stay warm and keep an eye on your buildings!