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A classic building defect in the outdoor area is a terrace without a slope. Serious water damage is the result. Do-it-yourselfers can read here practice-oriented tips on the correct inclination of a perfect terrace surface.

In a nutshell

  • A terrace needs a slope so that water can drain off quickly
  • the correct slope averages 2 percent
  • The percentage slope is to be calculated precisely using the formula height difference (cm)/terrace length (cm) x 100
  • The slope is determined and marked with strings stretched between wooden pegs
  • the marking cord is to be moved on one peg for each running meter, while the opposite peg indicates the final height

Why do patios need a slope?

Terraces are exposed to the elements without any protection. Driving rain, pounding hail, autumnal dew and winter snowfall massively affect the area. If water cannot flow away quickly, standing puddles and waterlogging attack the construction and cause serious damage. Furthermore, fungi, moss and algae appreciate a permanently moist terrace without a slope, because ideal growth conditions prevail here. Every step on the green terrace surface becomes a daring slide.
A gradient provides a remedy. A slight incline of the terrace allows rainwater to run off quickly, so that the surface dries in no time.

Which inclination is recommended?

The material of the terrace surface largely determines the slope. The following overview sums up the angle of inclination that has proven to be necessary and practicable in practice:

  • Natural stone slabs and fine stoneware: 2.0 to 2.5%
  • Concrete and natural stone paving: 2.5 to 3.0%
  • Wood and WPC boards: 1.0 to 2.0%

Act according to the rule of thumb: the rougher the surface covering, the greater the angle of inclination.

Notice: The orientation of a patio slope is just as important as the degree of incline. The correct slope should be sloping away from the house. This means that rainwater cannot run towards the façade and seep into the building.

How to calculate the terrace slope?

The slope defines the degree of incline. In order to calculate this value correctly, you need precise information about the route and the difference in altitude. The resulting formula is:

  • Height difference in cm divided by the length of the route in cm = gradient
  • Gradient multiplied by 100 = gradient in percent (as car drivers know from traffic signs)

The following example illustrates the arithmetic operation: Your terrace is 600 cm long and has a height difference of 12 cm between point A and point B. 12 cm divided by 600 cm gives 0.02. This value multiplied by 100 gives a slope of 2%.

How can you determine the slope?

Before the start of the construction work, the obligatory gradient can be determined in just a few simple steps. Ideally, you should devote yourself to the marking work in pairs. The materials and tools required are: wooden pegs, string, hammer, tape measure, spirit level and a thick pencil. How to determine the terrace slope correctly:

  • Drive in the first stake at a corner of the terrace area
  • drive the second stake into the ground at the opposite corner
  • Mark the final height of the terrace construction with a pencil on the first stake
  • attach the cord to the marking, stretch it to the opposite stake and knot it
  • Use the spirit level to determine whether the cord is at exactly the same height on both woods

The stretched cords now mark a terrace without a slope. To make the necessary inclination visible, proceed as follows:

  • For every meter of stretch, move the cord up or down 2 cm on a peg, depending on the orientation
  • on the second wooden peg leave the cord at the end height of the terrace construction
  • repeat the process on the opposite side of the terrace with two more wooden pegs

At the end of the marking work, the stretched cords visualize the correct degree of inclination of the future terrace area.

Tip: In relation to a sample distance of 300 cm, there is a total difference in height of 6 cm with a 2% gradient. A 6 cm higher mark on the first peg results in a slope towards the second peg. A 6 cm lower mark on the first peg indicates a slope from the direction of the second peg.

frequently asked Questions

Can a terrace be built in the gravel bed without a slope?

Private builders are often under the misconception that a gravel bed under terrace areas is sufficient for the complete infiltration of rainwater. In fact, the water from a light rain shower can easily drain away via the path of a gravel pad. In heavy rain, on the other hand, the foundation reaches its limits because it cannot absorb large amounts of water quickly enough. For this reason, a slope is also necessary when terraces rest in a bed of gravel.

What if the subsoil prevents an even slope for the wooden terrace?

A common problem for the perfect substructure of a wooden terrace is an uneven, bumpy subsoil. The differences in height prevent the terrace surface from sloping evenly, so that rainwater can run off freely. Smaller bumps between 3 mm and 20 mm can be leveled out with pads or rollers made of rubber granulate. Infinitely variable adjustable feet are available from specialist retailers to compensate for larger height differences in the subsoil.

How to lay terrace tiles on a concrete slab without a gradient?

So that patio tiles do not sink into waterlogging after every rain, the surface should have a gradient of 2 percent. Create the degree of inclination with screed on a level concrete slab. The tiles themselves are laid on the sloping screed either in a water-permeable bed of gravel, directly in a bed of mortar or on a slab support.

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