Property Slopes and Fill

Slpe House Solid State Home Inspections

Building a home on a flat property is ideal for builders. Flat sites make it simple to dig equal depth foundations on all sides, have even heights of walls, and pour flat basement and garage floors. Unfortunately, many building sites are not flat and builders need to address the slope of the property to build safe homes.

Hillside Foundations

One way to address a sloped property is to build into the hillside. Typically, a high concrete wall is built on the uphill side of the home with 'step' foundations moving down the sides of the building where only short foundation walls are needed on the slope side of the building. These homes commonly enjoy a full sized walk-out basement and full sized windows on the slope side of the building. 

For builders, this basement foundation method allows them to build at a high drainage point on the property with largely undisturbed native soil which should result in less building settlement. The disadvantage for builders is that foundation wall heights constantly change in the basement making construction and finishing more time consuming than consistent foundation walls would. The other disadvantage is that sin post-earthquake investigations, step foundations have been found to perform poorly due to the inconsistent seismic loading on different steps of the foundation.

Site Terracing

Another option for builders with sloped properties is to 'terrace' the slope so the entire building site (and often the yard) becomes flat. The builder can now more easily build even height walls and flat basement and garage floors on the building site. Owners of terraced sites enjoy a flat yard for easier maintenance and recreation and commonly less reliance on stairs than some hillside step foundation configurations. 

A disadvantage of terraced sites is a reliance on levelling 'fill' at the building site. Different fill depths along the property can settle at different rates and this could result in cracks and movement in the homes foundation. In some cases, properties which have been terraced 50 years earlier have been known to shift as logs and other organic material used as original fill has broken down over time and the fill levels have sunk.

Fill Materials

'Fill' is a term used to describe earth materials added to a construction site, typically to adjust grade levels. Ideally, when a builder lays home foundations, the foundations will rest on undisturbed, compacted, native soil which resists building settlement. However, when native soils are unsuitable or when the slope of the property is adjusted, compacted fill may be used under the home's foundation walls.

All homes will settle slightly as the concentrated weight of the foundations rests on the soils below. Builders are targeting for as little settlement as possible and for any settlement that does occur to be even. When houses settle unevenly, as can happen when different soils are under the foundations or when foundation fill is not compacted, cracks will form in the foundation. Foundation cracks are not uncommon but all cracks can allow water into the structure in time and in the worst cases, cracks may indicate a structural failure.

Another common use of fill on building sites is for 'back-fill'. Back-fill is used to replace soils removed during excavation for the foundation and under basement and garage floors to bring the earth back to grade. Back fill is looser than the compacted soil under the foundation walls and will settle differently around the foundation. Generally this can be improved by adding additional top soil where settlement has occurred but in worst case situations, the settlement can damage utility lines entering the home or cause cracks in unsupported concrete floor slabs.

Landslides and Soil Stability

Another impact of building slope to be considered is the stability of the building sites near major slopes like river banks, ravines, and escarpments. The weight of buildings and erosion effects of wind, gravity, and water can cause nearby slopes to give way and damage nearby buildings.

A good rule of thumb for stable building sites is no more than a 2-to-1 slope. This means for every 20' of property depth, the vertical change should be no more than 10'. The realm of soil movement is a science to itself. If you are purchasing a home on or near a high slope, you need to hire a specialist in site stability in addition to your home inspector.

What Does a Home Inspector Look at With Property Slopes?

Home Inspectors are concerned about the slope of the property and how it has or could impact the building in the future. Inspectors are looking for signs of uneven settlement of the building or site and how that change could be impacting the building. Cracks in foundations are common finds during home inspections as is uneven settlement of backfill soils around the property. If your home inspector raises concerns in an inspection about the building site, this could be a warning of a very serious and expensive problem. Further evaluation by experts should always be undertaken if the inspector raises concerns or the slope of the property is greater than 2-to-1.

Final Thoughts

Great homes are very complex systems. As occupants, we are commonly looking for a warm, dry place to entertain family and friends but as home owners, we need to be sophisticated about buying homes that will  be safe and solid for our families will into the future. When it comes to protecting your investment, hiring a great home inspector is one of your best investments.

By James Bell - Owner/Operator of Solid State Inspections Inc.