When to Backfill Foundation Walls

Allow Concrete to Fully Cure before Backfilling Foundation Walls

By Mark J. Donovan

When building a new home or home addition the question frequently comes up on how soon can I backfill foundation walls. The backfilling of foundation walls should ideally be done only after the concrete has had time to fully cure.

Concrete foundation walls are defined as being 75% cured after 5-7 days, and fully cured after 28 days, under ideal temperature conditions. The definition of ideal temperature conditions is above 50 degrees Fahrenheit.

Consequently backfilling foundation walls should be done only after the 28 days have passed, assuming ideal temperatures.

Though many contractors would argue that it is not necessary to wait the 28 days, it is at this point that the concrete reaches its full hardness. If you are building foundation walls with a 3,000 psi rated concrete mix, and decide to backfill the foundation walls after just one week of pouring the concrete, the concrete strength is only at a level of 2,250 psi.

By backfilling foundation walls too quickly you will run the risk of cracking the foundation walls.

Ideally it is best to backfill foundation walls after the concrete has had 28 days to cure, the basement slab has been poured, and the first level flooring framing has been installed. By pouring the basement slab and capping the top of the foundation walls with framing, you provide lateral support for both the base and top of the foundation walls. 

Backfilling Foundations

Combined with waiting the full 28 days for the concrete to cure, you significantly reduce the risk of cracking the foundation when you backfill the foundation walls.

So push back on your contractor when he is pressuring you to backfill foundation walls. The pressure that is associated with the soils pushing up against the foundation walls is enormous and by backfilling to soon, particularly if the basement slab is not poured and the foundation not capped, puts the foundation walls at significant risk of being cracked or even worse, pushed off their concrete footing pads.

For more help on building a home addition, see HomeAdditionPlus.com’s Home Addition Bid Sheets. Our Home Addition Bid Sheets provide you with the knowledge and information on how to plan a home building project, and what to look for when hiring contractors. They also include detailed cost breakdown tables and spreadsheets for estimating your own new home construction building costs.

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