A good French drain, a level driveway or a dry yard all need gravel for drainage; the right kind of gravel can make or break a drainage solution. Use the wrong type of material, and you'll have standing water, blocked pipes and expensive remediation. Use the right material, and water will recede from your home for years to come. Our guide explains how to determine the best gravel for drainage and how to buy it.
Why Gravel is Important for Drainage?
Gravel for drainage creates spaces between the gravel - called 'voids' - allowing water to move through the gravel instead of collecting on the surface and soaking into the earth near the foundations. The larger and more uniform these voids are, the quicker the water can pass through.
Not all gravel packs the same. Fine content or grading (mixed sizes of gravel) can form a tightly packed structure with minimal voids, reducing the rate of flow of water through it. That is why the characteristics, size and cleanliness of your gravel are so important.
The Consequences of Insufficient Drainage
Bad drainage isn't just about standing water in your yard - it's about the water you can't see that quietly intrudes on your property, inflicting damage on soil, structures and surfaces over time and causing costly repairs. The following are the most common issues:
- Cracking in foundations: Soil gets heavy with water and pushes down and causes shifting resulting in cracking of slabs, walls and footings.
- Failure of retaining wall: Between the wall and gravel it results in bulging, leaning, or toppling over.
- Pooling water causes potholes, washouts and ruts in the driveway.
- Mould and damp: Dampness penetrates walls and subfloors, destroying property and affecting health.
- Soil erosion: Water washes away surface soil, revealing plant roots and destroying landscape features.
- Pests: Standing water is a breeding ground for mosquitoes, termites, and other pests.
Better drainage, installed with good gravel at the earliest stage possible, costs less than repair work due to existing damage. Prevention is worth a pound of cure.
Key Factors When Choosing Drainage Gravel
1. Stone Size
Size, please. For the majority of drainage projects, anything between 14mm to 40mm is the ideal size. Stones of this size create plenty of room for water to flow between, while being heavy enough not to shift.
- 10mm - 14mm: ideal for a lighter garden drainage, use around small pipes and as a bedding layer.
- 20mm: The popular all-rounder and preferred for French drains, trenches and as backfill behind retaining walls.
- 40mm and above: Designed for more robust drainage such as large soakwells and the greatest volumes of water.
2. Shape of Stone
Angular, angular stone (crushed gravel) is angular and locks into place, making it the best option for driveways and high-traffic areas Rounded river stone is smooth and drains efficiently, but it does not always stay in place. The flattened shape of the river stones make them more susceptible to shifting underfoot. For drivable or walkable surfaces, either shape will work.
3. Cleanliness (Washed vs. Unwashed)
Use washed or screened gravel for your drain. The dust, clay and tiny particles in unwashed material settle between the stones, plugging the voids and slowly transforming the whole drainage layer into a solid mass. It's only a few dollars more per ton of washed gravel but it makes it last ten times longer.
4. Durability and Material
Hard, dense stone like blue metal, granite or basalt that will not weather or break apart under pressure may be better. Softer stone can weaken and disintegrate over time, producing fines that lower the aggregate's drainage capability. And if you are doing a long-term project consider high-grade aggregate from a reliable source.
Best Types of Gravel for Drainage
Here are the standard selections often used in Australian residential and commercial jobs:
- Drainage gravel: Uniform size, clean stone specifically prepared for drainage. Ideal for drainage trenches, agricultural pipe runs and retaining wall backfill.
- Blue Metal: A crystalline angular crushed rock in a range of sizes. It is high strength with excellent drainage characteristics and generally suitable for a sub base under driveways and slabs.
- River pebbles: Decorrative-shaped, rounded, well-draining stones for use in garden beds, dry creek beds and swales.
- Screened (crushed rock): Cheaper than mini-screened but still screened for general drainage use where appearance doesn't matter.
What not to use? Sub base, crusher dust and sub base style mixes all have fine particles that pack down tight, ideal for a solid foundation, poor for drainage.
Matching Gravel to Your Project
The best gravel for your project:
- French drains: Place clean 20mm drainage gravel across the area where a slotted pipe is to be laid, wrapping the pipe in geotextile fabric.
- Walls: Filling behind the wall with 20mm clean gravel to reduce water pressure and ensure the wall does not bulge.
- Driveways: Compact base with angular 14mm or 20mm blue metal for a surface that drains and supports well.
- Levels under pavers and slabs: Select a graded base that sufficiently compacts yet offers water the opportunity to drain laterally.
- Garden beds and landscaping: River pebbles or 10mm to 14mm gravel make for a tidy, well-draining, free-draining cover.
How Much Gravel Do You Need?
Calculate what you need beforehand and this can be ordered on. It will save you money and will not delay a project. To calculate simply times the length x width x depth (all in metres) to give a result in cubic metres. For example a trench which measures 10m long, 0.4m wide and 0.5m deep will require 2 cubic metres of gravel. Gravel is normally purchased per tonne and it is average that one cubic metre weighs between 1.4 and 1.7 tonnes depending on the material used. It would be worth ordering approximately 10% more.
Remember Geotextile Fabric
Just about even the finest gravel will clog if dirt enters it. Using a geotextile fabric (a bit expensive for the depth of trench we're talking about, but is your drainage field worth it?) will keep dirt and silt from entering the drain.
Evaluate Your Soil and Site Factors
Soil conditions: heavy clay soils retain water and require a thicker cover of larger stone and drainage material, sandy soils naturally drain but could use a thinner cover. Consider: rainfall, slope and volume of water that your landscape will have to drain. When in doubt, size up and let the water run!
Pitfalls to Steer Clear Of
- Unwashed/washed or mixed size gravel that can plug readily.
- Selecting stones that are so fine you're limiting water flow.
- Tossing aside the fabric liner and allowing soil to infiltrate the drainage layer.
- Avoid the slope. The drain will require some form of fall, so water runs away from the buildings.
- Underestimating quantities and running short mid-project.
Get the Right Gravel Delivered
Essentially, it's about size, shape, cleanliness and durability. A clean, angular or washed aggregate from 14mm to 40mm in size, correctly installed in a trench and with its performance matched to the project will safeguard your property from water damage long term.
Looking for expert advice or a trusted freight company? The friendly team at Formula Transport will assist you in sourcing quality bulk materials and getting them to your site when you need them. Head to formulatransport.com.au or contact us for a quote.
Common Questions
1. What Causes Poor Drainage Around the House?
Soil that has been tightly packed, soil that is sloping toward the house, debris in the gutters and drains, no drain gravel, all would keep water from being able to flow away.
2. Can bad drainage damage my foundation?
Yes. Rain waterlogged soil expands, moves and applies force to your footings and slabs. In the long term, you end up with cracked floors, sinking and uneven slabs, stuck doors and much more expensive repair bills which good drainage resolves.
3. How do I know if I have a drainage issue?
Search for muddy or musty odours, cracks in the wall or slab, eroded soil, patches of wet lawn, stains of water or mould around the bottom of your wall or fence and standing water after rain.
4. What makes gravel prevent drainage damage?
Drainage gravel. This clean gravel creates spaces for water to run away from building foundations. Used behind retaining walls, in trenches and in french drains, it's an excellent way to add support and reduce pooling.
5. What is the cost of fixing drainage damage?
Prices can be very different: Minor repairs can be a few hundred dollars, while foundation repairs, rebuilding walls, or rebuilding landscape features can amount to hundreds or even thousands. Early drainage solutions will be much cheaper.
6. When should I put drainage gravel?
Before issues start as part of new construction, landscaping, or driveway installations. For existing issues with standing water or injury to your property, drainage can be installed immediately to prevent damage and excess repairs.
