ᐅ Building a Terrace: Open Questions About Construction and Drainage

Hello everyone,

I want to build a terrace and have some uncertainties about a few things.
It is a concrete slab attached to the house, with exposed aggregate concrete slabs on top.
The concrete slab now has a steep slope (about 7%), which I want to level out at the same time.
A lawn is adjacent to the terrace.

The plan is to install WPC decking with the appropriate substructure over it.
Due to the slope, I will need to raise the lawn area accordingly, as I don’t want a step.

1. Do I need something like a drainage mat on the existing slabs for drainage? I want to protect the lower floor as well as possible from water, and there is often a lot of water standing on the lawn in autumn/winter.
2. Is a drainage channel necessary along the lawn edge? Currently, there are two small ones (one of them along the side path in the photo at the top, so definitely not running the full length of the terrace), which I think don’t really help much anyway, so theoretically they could be connected.
3. If I need that, how do I build it exactly? I assume it would have to be installed at the level of the concrete slab since the water drains there? How do I handle the soil then, which I plan to raise for the lawn surface?

I hope this makes some sense, and please excuse my beginner questions.

Thanks in advance and best regards.

Here is a sketch, sorry it was made on my phone.

Hey,

with a 7 percent slope, the cleanest way to install WPC (wood-plastic composite) decking is on height-adjustable supports. The substructure should have its own slope of about 2 percent away from the building. For drainage, you don’t need a drainage mat under the WPC if the decking ventilation is unobstructed and water can drain onto the concrete slab. The edge area is critical. Plan a linear drain channel with a slope at the outer edge and connect it to a soakaway or an approved discharge system. Alternatively, use a gravel strip with a permeable subbase and a DN 100 (4 inch) drainage pipe wrapped in geotextile fabric. Leave at least 2 to 3 centimeters (about 1 inch) of air gap between the decking and the building with no soil contact. Also, pay attention to the threshold height at doors.

Good luck!

Nauer schrieb:

Hey,

With a 7 percent slope, the cleanest way to install WPC decking is on height-adjustable pedestals. The substructure should have its own slope of about 2 percent away from the house. For drainage under WPC, you don’t need a drainage mat if the decking is properly ventilated and water can run off onto the concrete slab. The critical part is the edge. Plan a linear drain at the outer edge with a slope, leading into a soakaway or an approved outlet. Alternatively, a gravel strip with a permeable base and a DN 100 drainage pipe wrapped in geotextile fabric. Leave at least 2 to 3 centimeters (1 inch) of air gap from the house and avoid soil contact. Also, pay attention to the door threshold height.

Good luck!

Hey, thanks for the reply.
Yes, height-adjustable pedestals were my first thought too. I just need a very large number of them for my area.
The decking boards will be laid lengthwise (my wife wants it that way 😀). Would that be less ideal for drainage?

So you suggest installing the drain level with the WPC boards? Then I could connect it to the existing drains in the concrete slab.

What do you think about the edge detail? I’d like to keep it flush with the ground, meaning soil contact, and raise the lawn slightly. Then I assume I would need to install new edge stones there?

Thanks and best regards