Backfilling Guide: Choosing Between Sand, Topsoil, and Fill Dirt
When tackling a backfilling project, whether it is around a foundation, behind a retaining wall, or in a low spot in your yard, picking the right material is key. The wrong choice can cause settling, drainage issues, or even structural problems later.
Quick Decision Guide
- Goal: Improve drainage Best Material: Sand Why: Water passes through quickly.
- Goal: Grow plants Best Material: Topsoil Why: Nutrient rich and loose texture.
- Goal: Fill large holes Best Material: Fill dirt Why: Affordable, stable, and easy to compact.
- Goal: Level base for pavers Best Material: Sand Why: Creates an even, stable surface.
- Goal: Raise yard grade Best Material: Fill dirt plus topsoil Why: Fill dirt provides volume, while topsoil allows for planting or grass.
When to Use Sand
Best for: Drainage and stability
- Around foundations: Sand compacts well and allows water to drain away quickly, reducing the risk of water pressure against basement walls.
- Under pavers or slabs: Creates a level, stable base that does not shift as easily as soil.
- Play areas: Clean, screened sand works well under swing sets or in sandboxes.
Why sand? It is free draining, so water will not pool. It is also easy to spread and grade. The downside is that it does not hold nutrients for plants, so it is not ideal for growing grass or gardens directly on top.
When to Use Topsoil
Best for: Planting and landscaping finishes
- Top layer after fill dirt: Once you have raised an area with fill dirt, a few inches of topsoil provide the nutrient rich layer plants need.
- Garden beds and lawns: Ideal for reseeding grass or building up garden areas.
- Leveling uneven spots before planting: Gives you a fertile surface to work with.
Why topsoil? It contains organic matter and nutrients that plants need to grow. However, it is not suitable for structural backfill because it can hold water and shift over time.
When to Use Fill Dirt
Best for: Raising ground levels and structural support
- Filling large holes: Fill dirt is inexpensive and easy to compact.
- Backfilling behind retaining walls: Provides bulk without the cost of premium materials when drainage is not the primary concern.
- Building up low areas: Fills volume efficiently before adding a top layer of soil or gravel.
Why fill dirt? It is made of subsoil with little to no organic matter, so it will not decompose or settle as much as topsoil. It is stable, but it does not drain as well as sand, so drainage design remains important.
Pro Tips for Choosing and Using Backfill
- Think about water movement: If you need drainage, lean toward sand or a sand and gravel mix.
- Layer smartly: Use fill dirt for bulk, then topsoil for planting areas.
- Compact in layers: Every 6 to 12 inches, compact the material to reduce settling later.
- Avoid deep topsoil: Do not put topsoil deep underground as it will hold water, attract roots, and rot.
- Match material to purpose: Use fill dirt or sand for structural support and topsoil for growing plants.
Common Backfilling Mistakes to Avoid
- Using topsoil deep underground: This leads to settling and water retention issues.
- Skipping compaction: This causes uneven settling over time.
- No drainage planning: This can lead to water pooling or damage to walls.
- Mixing random materials: Different textures settle at different rates, which can create dangerous voids.
Visual Reference Guide
- Sand: Light tan to beige with fine grains. Ideal for drainage and leveling bases.
- Topsoil: Dark brown to black with a loose texture and visible organic matter. Best for planting and finishing layers.
- Fill Dirt: Light brown, clay heavy soil with occasional small rocks. Great for volume filling and structural support.