Backfilling Your New Home Foundation

How to Build a Home, Step 29

Believe it or not, backilling your new home foundation requires careful attention to detail.  Carelessly pushing dirt up against the foundation wall can create structural and water problems with your new home.  Here we will cover the essential elements for a proper backfill while building a house.

Backfilling:  Timing is Important

If you’ve chosen a poured concrete foundation wall for your new home, it’s imperative to give the concrete plenty of time to strengthen prior to backfilling.  Dirt and moisture put a tremendous amount a pressure on a foundation wall.  A brand new, green concrete wall can fail if it’s not allowed to strengthen prior to placing the load onto it.  With normal outside temperatures, a poured concrete basement wall should be allowed to cure for a minimum of one week prior to backfilling.  When it’s cold outside, be sure to allow more time and cover the wall with blankets to speed up the hydration process.

One other note about timing your backfilling activity…make absolutely certain you have been approved by the local building inspector to cover up the wall.  You, or your builder consultant, will need to schedule the inspection with the local building department and make sure you have approval before proceeding.  The inspector will check to make sure the wall was waterproofed/dampproofed properly and will check the drainage system.  Ideally, you will get the approval in writing, either as an approved permit sticker or a piece of paper signed by the inspector.

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Material

For most new homes built today, backfilling is accomplished with one of two materials.  If there is enough compactable dirt on site, your new home will most likely have dirt placed up next to the foundation.  This dirt needs to be free of organic materials that will decompose and cause excess settlement.  Examples of organic material that will decompose include top soil, tree roots and stumps, scraps of wood,…etc.  By using fill dirt already on the lot, you can save a great deal of money.

If there is not enough quality fill dirt on site, stone (or gravel) will most likely be trucked in and used when backfilling the foundation.  This is expensive because you must pay for the material, loading and trucking to the job site.  Although it does cost more, stone or gravel is the best alternative because it compacts easily and allows water to quickly move through it. By getting water away from the foundation wall quickly, you can prevent leaks through foundation cracks.

Methods for Backfilling

Typically, the dirt or stone is placed into the trench next to the foundation (a.k.a. over dig area) with a bull dozer or back hoe.  Notice we used the word “placed” versus…dumped or dropped.  There could be sharp rocks inside the dirt that can cause damage to the foundation wall, waterproofing or drainage system if backfilling isn’t done very carefully.

Ideally, the material placed in the hole next to the foundation would be mechanically compacted with a vibrator every six inches of depth.  Unfortunately, most homes do not get this kind of treatment due to the extra costs. So it’s important to realize the backfilled area around the home will settle over time.  Therefore, you don’t want to place any structural elements into this area.  Patios will need to be placed outside of the foundation backfill over dig unless you make special arrangements for proper compaction in these areas.  If you are building a deck, make sure the posts are not placed within the backfilled area.

Quality Considerations

There are a few very important things to keep in mind when backfilling a new home.  Make sure your contractors follow these tips…

• Brace Basement Foundation Walls – Basement walls have a tremendous amount of pressure placed on them from the soil and water (also called hydrostatic pressure) outside.  Once the house is built, the first floor framing provides lateral pressure at the top of the foundation wall to counteract the force from the outside.  The basement floor provides additional support for the foundation wall at the bottom.  Unfortunately, the basement floor and first floor framing aren’t typically installed yet when backfilling occurs.  So, it’s important to temporarily brace the foundation wall.  If you’d like to see a foundation properly braced, check out this article:  Best Practices: Foundation.
• Remove Organic Material – It’s important to remove any material from the area to be backfilled that will decompose and cause excess settlement.  The photo below shows straw covering the footings that needs to be removed prior to backfilling.

• Compaction in Slab Locations – The concrete slabs in your new home will need proper support to prevent cracks and settlement.  This can be accomplished by placing a properly compacted fill material beneath the slabs.  Stone or gravel is the easiest material to use to ensure proper support.  Don’t forget, in a home with a basement, you will need eight to nine feet of fill up next to the house foundation in an attached garage.  It will be difficult to make sure this is compacted properly without using stone or gravel.
• Heavy Equipment Operation – Most heavy equipment operators know the equipment should never be driven parallel to the foundation in the backfilled area.  A bull dozer can easily weigh 50k pounds.  By placing the tracks parallel to the foundation at the over dig, a tremendous amount of pressure is placed on the newly formed foundation wall.  Therefore, all heavy equipment should be operated at a 45 degree angle to the home when backfilling.  Be sure to include this note in your Scopes of Work and Specifications.
• Plot Plan – Review the plot plan with the machine operator to make sure the proper rough grade is set at the time of backfilling.  You don’t want to create drainage problems during construction.  This can lead to a muddy job site or worse yet, a wet basement.
• Check for Damage – Because backfilling requires heavy equipment that can easily damage improvements on site, you will want to carefully check all curbs, water stop boxes, sewer risers, sewer manholes…etc. for damage.
• Driveway Stone – This is also a good time to core out the top soil in the driveway location and place stone down.  By removing the topsoil and prepping the driveway, the delivery trucks will have a nice spot to place material while at the same time, helping to compact the stone for the future driveway.

Although backfilling a new home foundation sounds simple, major problems can be created without the proper care.  Follow these simple professional builder tips and you will help protect your brand new foundation and drainage system.  Are you planning on building a new home as an owner builder?  If so, be sure to stop by our Build Your Own House resources page for more great articles, videos, and digital resources.  Either way, stay tuned as we will continue the Armchair Builder blog series…How to Build a Home.

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