Wet subfloor

Hi everyone!


I've been building a new home on a sloping corner block on the side of a hill on the Central Coast, NSW. The house is built on bearers and joists and has a big subfloor area enclosed by brickwork. Eventually I'd like to concrete as much as I can and turn a part of it into a living space while the other parts into storage space.

Water flowing down from the hill is hitting the long edge of the home and seeping into the subfloor resulting in alot of water pooling under there and I reckon I got to deal with it sooner rather than later. Right now, the roof is on for a month and I've still got a fair bit of water pooling at the base of the bearers.

This is what it looks like with some contours:


Here's the Western side of the home:


This is what the southern side looks like:




I've got some ideas on how to deal with it, but I honestly don't know what I'm doing so any help would be appreciated.

From what I've been able to research, I'll need to dig a trench along the edges of the home the water is impacting against and expose some of the concrete footings, paint it with liquid rubber and/or some sort of plastic sheeting and run ag drain (I think it's called French Drain in some places?) along it, empty it into a stormwater pit with a submerged bilge pump, then pump it back up the hill into the stormwater line on the street.

I imagine something like this:

If this is the only way I can deal with it, I've got some questions regarding this solution:

1. How deep do I need to dig down in front of the footings to install the ag drain?
2. How do I waterproof the footings before laying the pipe? Paint on membrane, or a combination of that and sheeting?
3. How do I get around the downpipes to install the AG drain? Some of those spots seem quite awkward.
4. How do I size the sump and pump combination?
5. How likely will this keep my subfloor dry? Do I need to do even more to make that space habitable and dry enough to store things?

Any help would be really appreciated!

Hey BenH Welcome to the forum
I think you'll be battling Hydrostatic pressure, water ingress and associated damp problems
The basement needs to be waterproof and self contained beneath the House
Your options are create a sump accept that water will pool beneath the FFL and then pump it out
Ideally Concrete basement tanking would have been a better option
Seek out a local design engineer for the details, though now the house is built it may be a bit late for concrete tanking
Cheers
Chris

Thanks for the welcome Chris!

Yea, it's looking like a bigger and bigger problem as I go down the rabbit hole.

You're right on the concrete basement tanking, it looks like way too big a job while the house has already been built and yea, it sounds like that solution will turn it into sort of like an American style basement.

I'm not too sure what you mean by a 'sump accept', I think that may be the solution I mentioned in the original post?

Later when we do lay the slabs for the subfloor area, it won't cover the entire subfloor area as some of it just has too low a ceiling and I won't be able to excavate much without compromising the foundations of the home. I was told that potentially to control the damp situation there, I could put in an inline fan ducted to an exhaust point to draw air through the subfloor space and keep it dry that way, then i can waterproof the 'exterior' wall of the subfloor area facing the dirt subfloor space to keep it all dry. Would that be sufficient?

Search Basement Sump Pits
best to contact a local design engineer, they should do a site visit, geotech assessment,
Sump design and detail, compliance/certification , etc, etc.
It's always advisable to incorporate redundancy (Engineering) measures in basements...just in case.