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Problems With Dirt Under the Deck

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Building a new deck as an addition to your home adds value and creates space for enjoying the outdoors. Most decks are built over the dirt naturally found around the foundation of the home, not on a separate concrete slab. Installing a structure like a deck over dirt can lead to a variety of challenges and problems.


When groundhogs decide to burrow in the dirt below your deck, they may cause it to become seriously damaged or even collapse. Groundhogs can dig extensive burrows, even under full concrete deck slabs, according to AA Animal Control. These burrows collapse during rainstorms or when the load on the dirt increases. As the dirt above the burrows fills them in, cracks form in the concrete footers the deck is built on. This weakens them, and if enough dirt moves, the footers could slide with it and break the wooden support beams. Groundhogs are difficult to persuade to leave once they inhabit a certain area, but trapping and removing them will prevent them from creating a large burrow system below your deck.


Dirt without plant roots or other structures to keep it in place will slide when it gets wet, creating instability in the ground and damaging the concrete footers. Removing the sod, grass or other ground-cover plants in the area where you plan to build your deck will destroy important root systems, says Virginia Decking & Remodeling. If the site is bare from grading for construction of the home, planting grass or sod and allowing it to become firmly rooted will help prevent soil erosion. The plants may die from lack of sunlight, but the mesh of roots will remain. Putting down a layer of black polyethylene will prevent weeds from growing up through the deck seams without removing root systems.

Wet Soil

Soil that regularly becomes wet, but does not allow standing water and does not slide in heavy rains, is suitable for building a deck on. If the soil is too wet, it may not remain stable. Digging properly designed trenches will help drain water from the area and prevent swampy soil, says author Stephen Cory, in his book, "Decks: Step-by-Step Projects." Another alternative, French drains, constructed of corrugated pipes with drainage holes, can be buried under the soil to carry excess water away. Installing either of these drainage systems is easier before the deck is built, especially if a solid concrete slab will be poured.

Loose Soil

Soil that is loose from excavation or water rising from the water table will not support heavy concrete footers, Cory writes. Digging into the soil at least 16 inches should provide a stable and compacted base of soil for supporting the deck. Large chunks of rocks or tree roots may be present at this level in the soil, but the extra work will ensure that your deck's supports will survive all weather conditions.

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