What ground cover for elevated granny flat?

Question

Hi all looking for some feedback around ground cover for an elevated granny flat build. The building is on sloped ground varying between 450mm and 1m above natural ground draining away from the building. I’m proposing to place some heavy weed matting (permeable) on the ground footprint and between the 16x piers topped with 20mm of 5mm granite stone. To make it more permanent I’ll probably apply it as no fines concrete to retain the permeability as pavelock and similar is non permanent. And recommendations or alternatives? No images as yet as I’m a month or so away from building approvals.

Dan.

Dave-1 · Answer

Good Morning @Dan1966

Its a bit hard to picture what you are describing tho think I have it. I would daydream about it and even sketch up some ideas but would wait before buying any materials to put down. The distance between the ground of 450mm to 1m is the reaon I say wait. You may decide on a skirt around the houses edge depending on where you live and bushfire regulations.  Maybe a solid skirt half way under your house so once the ground to floor gets to be around the 600mm mark thats a solid skirt on the high side and your gravel idea on the low side?

I find nothing really grows within 600mm of teh edge of a covered building. you might get some hardy type weeds but they want sun so generally dont colinise under the house at that height.

Dave

Dan1966 · Answer

Thanks Eric. For inspection I’ll be making the concrete piers cap above any finished ground level providing a clear inspection area for the steel pier hardware joined to them. When I did a similar base under the trampoline I edged the area with pavers but still saw some stone depart the area into the grass but perhaps it would be simpler to restrict the use of any binder to the edge area only. No doubt the chickens had something to do with the trampoline experience :-)

EricL · Answer

Hello @Dan1966

For an elevated granny flat with 450mm to 1m clearance, the main goals are drainage, weed suppression, and long-term stability without trapping moisture under the structure. A permeable ground treatment is definitely the right approach, but I’d be cautious about going straight to no-fines concrete unless you really need a hardened surface. Once it’s in, it’s permanent, harder to service plumbing later, and can still collect debris over time which slowly reduces permeability.

Heavy-duty geotextile with compacted drainage aggregate is usually the simplest and most reliable solution. A commercial-grade weed mat laid over trimmed soil, pinned well, then covered with 40–50 mm of 5–7 mm granite or 10 mm drainage aggregate tends to stay stable and drain extremely well. The extra depth is important; 20 mm tends to migrate, expose the fabric, and allow weeds to re-establish. With good fall away from the building, this setup stays dry and maintenance is minimal.

Also consider airflow and moisture control. Keeping the ground surface well drained and not sealed helps prevent damp air building under the floor. Avoid plastic-type weed mat that traps moisture; a proper non-woven geotextile is better because it breathes while stopping fine sand and soil from pumping up into the stone. Maintaining a small inspection strip around each pier is also helpful for long-term monitoring.

Overall, I’d lean toward heavy-duty geotextile plus a thicker layer of drainage aggregate. It’s simpler, fully permeable, easier to maintain, and still looks clean and permanent without committing to concrete.

Let me call on our experienced members @Nailbag, @Dave-1 and @Noyade for their recommendations.

If you need further assistance, please let us know.

Eric