Sloping concrete is primarily achieved by carefully setting the forms before the concrete is poured, ensuring a planned drop from a high point to a low point. This built-in incline allows water to drain effectively, preventing pooling.
Why Slope Concrete?
Creating a slope in concrete slabs, such as patios, driveways, or walkways, is crucial for drainage. Proper sloping helps to:
- Prevent water from collecting on the surface.
- Protect the concrete from freeze-thaw damage in colder climates.
- Reduce the risk of slippery surfaces due to standing water.
- Direct water away from structures like building foundations.
Determining the Right Slope
The degree of slope needed depends on factors like the size of the slab, expected rainfall, and surrounding landscape. However, a standard recommendation exists to ensure adequate drainage.
Based on common guidelines for exterior concrete, a good rule is to use a minimum slope of 2%, which is equivalent to a 1/4-inch drop for every foot of length. For areas needing very fast drainage or in wet climates, a steeper slope might be considered, but excessive slopes can be difficult to walk on or use.
Understanding Slope Measurement
Slope can be expressed in different ways:
- Percentage (%): The drop in elevation over 100 units of horizontal distance. A 2% slope means a 2-foot drop over 100 feet, or a 2-inch drop over 100 inches.
- Ratio (e.g., inch per foot): The drop in inches over a horizontal distance of one foot.
As stated in the reference, 2% is equivalent to 1/4 inch per foot.
Here's a simple comparison:
| Slope Type | Measurement | Example over 10 ft |
|---|---|---|
| Percentage | 2% | 0.2 ft (2.4 inches) |
| Ratio | 1/4 inch per foot | 2.5 inches |
Note: The 2% and 1/4" per foot are very close and often used interchangeably for practical purposes in concrete work.
Steps to Sloping Concrete
Sloping concrete is primarily done during the formwork stage:
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Plan Your Drainage:
- Identify the desired high point (where water will flow from).
- Identify the desired low point or drainage area (where water will flow to). This might be a lawn, a drain, or away from a structure.
- Measure the distance between the high and low points to calculate the total required drop.
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Calculate the Drop:
- Determine the desired slope (e.g., minimum 2% or 1/4 inch per foot).
- Measure the length of the area to be sloped in feet.
- Multiply the length by the slope ratio.
- Example: For a 20-foot long patio using a 1/4-inch per foot slope: 20 feet * 0.25 inches/foot = 5 inches. You need a total drop of 5 inches over the 20 feet.
-
Set the Forms:
- Install the form boards (typically wood or metal) around the perimeter of the area to be poured.
- Use stakes to support the forms.
- Set the form boards on the low side lower than the form boards on the high side according to your calculated drop.
- Use a string line and line level, or a laser level, to ensure the forms are set at the correct, consistent slope from the high point to the low point. This is the most critical step in achieving the slope.
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Pour and Screed:
- Pour the concrete within the sloped formwork.
- Use a screed board (a straight edge) resting on top of the sloped forms to level the concrete. As you pull the screed board along the forms, it will automatically follow the slope you've created, leaving the concrete surface at the correct incline.
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Finish:
- Perform bull floating, troweling, and edging as needed. Be careful not to disrupt the planned slope during finishing.
Tools Used
- Measuring tape
- Calculating tools (or just basic math)
- Stakes
- Form boards (wood, metal, plastic)
- Level (string line and line level, builder's level, or laser level)
- Screed board
- Shovels, wheelbarrow (for placing concrete)
- Finishing tools (bull float, trowels)
Practical Tips
- Double-Check Forms: Before pouring, always verify the slope of your forms with a level to ensure it matches your plan.
- Consistent Slope: Strive for a consistent slope across the entire surface for uniform drainage.
- Edge Forms: Pay close attention to setting the elevation of your edge forms, as they dictate the finished surface slope.
By carefully planning and setting your formwork with the correct elevation difference from one side to the other, you effectively "build" the desired slope into your concrete slab before it even hardens.
