ᐅ Foundation/Base for WPC Decking

Hello dear forum members,

The boss asked me to build a WPC deck. Currently, there is still grass growing there. The area measures 4 x 5 m (13 x 16 ft) and starts from the patio door. I am only concerned with the foundation.

This is what I have planned:
1. Dig to a depth of about 30 cm (12 inches).
2. Lay a geotextile membrane.
3. Spread a 20 cm (8 inch) layer of crushed stone, compacted with a plate compactor, with a 4% slope.
4. Add a 10 cm (4 inch) bedding layer of fine crushed stone, compacted again, with a 2% slope.

On this, I want to place individual rows of concrete edging stones, on which I can install the substructure.
I also plan to edge the whole area to prevent lateral slipping.

So far, so good. This also complies with the manufacturer’s specifications. Then a representative from a landscaping company came to make me an offer and said that he would never do it without concrete and reinforcing mesh and that he could not guarantee the work if done the way I am planning.

As I said, I wanted to do it myself anyway, but now I am a bit unsure.

Please share your opinions and knowledge on this subject.

Thank you in advance and have a nice weekend!

Skip all the unnecessary hassle.

1. Excavate only at the points where the substructure will rest
2. Fill and compact gravel at these spots
3. Fill freezer bags with lean concrete, place them on the prepared spots, and position the substructure on top

Cover the remaining lawn with a geotextile fabric.

That’s all there is to it. I’ve built four terraces this way myself and have also heard of similar methods from wooden deck builders.

This is what it looks like in reality, based on my description. In our case, there was still compacted gravel, and I had to adjust to the correct heights using leftover paving stones. You have it a bit easier there.

That does indeed sound simpler. How deep did you excavate at the specific points and fill with gravel?
In your picture, there are also "small concrete mounds" and paving stones visible on the geotextile fabric. Is that the height adjustment you mentioned?
Thanks in advance! That would save a lot of costs and work.

I didn’t excavate anything. The base underneath the geotextile fabric is compacted gravel (provided by the civil engineer). The fabric was laid directly on top of that.
I spread a bit of concrete onto the fabric so the paving stones (which are used for height adjustment) could be positioned properly. On top of that, I placed the small bags of concrete that support my wooden substructure.

Since the wood (Bongossi) should not be in direct contact with water, it rests on small pads to keep it dry. You don’t really have this issue with WPC.

Addendum/Alternative: I’m also planning to build a terrace around our pool. However, this will be slightly different since there is currently a lawn area there and I don’t want to do any digging.
1. Rent a post hole digger.
2. Drill holes about 35–40 cm (14–16 inches) deep, reaching the fill soil in my case, then insert 100 mm (4 inch) rigid sewer pipes (KG pipes) into the holes.
3. Level the height of the pipes.
4. Fill the pipes with gravel or, alternatively, concrete.
This way, you create stable points on which the substructure can rest.

I’m also interested in this topic, but I’m rather lazy and was thinking about buying those height-adjustable decking pedestals to easily set the height and slope. Is that really unnecessary, or how do you make sure there’s still a reasonably level surface after compacting? Because a solution with concrete slabs is obviously much cheaper price-wise.

Basically, I need to build up about 30cm (12 inches) including frost protection.

I hope it’s okay that I just join in here.