Setting up your new above ground pool is exciting, but figuring out what to put under above ground pool on grass can be tricky. Getting the foundation right is the single most important step for a safe, stable, and long-lasting pool.
A wobbly or uneven base leads to liner wrinkles, structural stress, and a shortened pool life. Grass alone is never a suitable base. It decomposes, shifts, and invites pests. This guide walks you through the best materials and methods to create a perfect, stable foundation on your lawn.
What To Put Under Above Ground Pool On Grass
Your foundation is a multi-layer system designed for stability, protection, and drainage. Each layer has a specific job. Skipping a layer might save a little money upfront, but it risks costly damage later.
Why You Can’t Just Place the Pool on Grass
Grass and soil are living, moving surfaces. When you place thousands of gallons of water on top, problems start. The grass will die and decompose, creating soft, uneven spots. Earthworms and insects will vacate, causing tiny sinkholes. Moisture gets trapped, promoting rust on metal components and creating a muddy mess.
A proper base prevents all this. It distributes the pool’s weight evenly, protects the liner from punctures, and allows for proper water drainage away from the pool wall.
Essential Materials for Your Pool Base
Here’s your shopping list for a professional-grade foundation. You’ll combine these based on your specific yard:
- Mason Sand or Stone Dust: A fine, compactable material for the top leveling layer. It creates a smooth, soft surface for the liner.
- Crushed Stone or Gravel: A coarse drainage layer. Use ¾” crushed stone without sharp edges. This is crucial for wet areas.
- Ground Cloth or Geotextile Fabric: A weed barrier that separates soil from your base materials, preventing mixing.
- Pool Pad or Commercial Underlayment: A thick, felt-like or foam layer that goes directly under the liner for extra cushioning and protection.
- Pressure-Treated Lumber or Pavers: For creating a perfectly level retaining box for your base materials on sloped ground.
Step-by-Step Guide to Building Your Foundation
Follow these steps carefully for a result that will last for years.
Step 1: Mark and Clear the Area
Mark a circle at least 12 inches wider than your pool’s diameter. Remove all grass, roots, and rocks from this area. You need bare, undisturbed soil. A sod cutter makes this job much easier.
Step 2: Level the Ground
This is the most critical part. Use a long, straight 2×4 with a level on top to check for high and low spots. Remove soil from high areas; never add loose soil to low spots, as it will settle. The entire area must be level within 1 inch. Take your time here.
Step 3: Install the Ground Cloth
Lay down the geotextile fabric over the entire cleared area. This stops weeds and keeps your base materials from sinking into the soil. Overlap seams by a few inches.
Step 4: Add the Drainage Layer (If Needed)
If your yard has poor drainage or holds water, add a 2-inch layer of crushed stone. Rake it level and use a plate compactor to tamp it down firmly. This layer isn’t always required on very sandy, well-draining soil, but it’s often a good precaution.
Step 5: Spread and Compact Your Sand
Spread a 2-3 inch layer of mason sand over the area. Get it as level as possible with a rake. Then, thoroughly soak it with a garden hose and use the plate compactor. This creates a hard, stable, and perfectly smooth surface. Let it dry completely.
Step 6: Lay the Final Protective Layer
Unroll your pool pad or underlayment over the compacted sand. Trim it to fit your circle. This layer absorbs tiny imperfections, protects the liner from abrasion, and provides extra insulation. Tape any seams according to the manufacturer’s instructions.
Common Materials to Avoid
Some materials seem like a good idea but can cause major issues.
- Tarps or Plastic Sheeting: These trap moisture, causing mold, mildew, and rust. They also create slippery wrinkles under the liner.
- Play Sand: Too fine and loose; it will wash away and not compact properly.
- Cardboard or Newspaper: They biodegrade quickly, leaving voids and attracting insects.
- Just Dirt: Soil compacts unevenly and turns to mud, guaranteeing an unstable pool.
Special Considerations for Sloped Yards
If your yard has a slope, you must build a level retaining box. Use pressure-treated lumber or concrete blocks to create a frame within your marked circle. Fill and level the inside of the box with your base materials (gravel then sand). This prevents your sand from washing out downhill. It’s more work but is essential for safety on uneven ground.
Maintaining Your Pool’s Foundation
Once your pool is up, a little maintenance keeps the base in good shape. Avoid dragging heavy objects across the floor. Keep the water chemistry balanced, as acidic water can weaken the base materials over time. When you winterize, keep the cover tight to prevent debris and standing water from sitting on the base.
FAQ: Your Foundation Questions Answered
Can I use just a pool pad on grass?
No. A pool pad is a protective top layer, not a structural base. It cannot level ground or prevent grass decomposition. You must always prepare the ground first.
What’s the cheapest safe option for a pool base?
The most budget-friendly safe method is a well-compacted 2-3 inch layer of mason sand over leveled soil, with a ground cloth underneath. Never skip the ground cloth to save money.
How do I level ground for a pool without digging?
You almost always have to dig. The proper method is to remove high spots to match the lowest point. Adding fill to low spots leads to settling and an unstable pool. A rented laser level can make this process much more accurate.
Is it okay to put an above ground pool on a concrete patio?
Yes, if the patio is perfectly level and structurally sound to hold the immense weight. You must still use a protective pool pad or foam tiles under the liner to prevent abrasion and provide cushioning for the pool’s bottom structure.
Can I build a base over existing grass?
It is not recommended. Even with a ground cloth, the grass will die and decompose, creating voids and an uneven surface. The small amount of work to remove the grass upfront prevents major problems later. Its worth the extra effort.
Choosing the right materials and taking the time to level properly makes all the difference. A stable foundation means a safer pool, a smoother liner, and many more seasons of enjoyment without worry. By following these steps, you’ll create a base that supports your pool perfectly from it’s first fill to the end of the season.