How to add drainage at rear of house?

Tomahawk1

A concrete area behind my house falls towards the rear facade, which is not ideal. Before I dug it up, the concrete was up to the weatherboards at one end, with most of the subfloor brick wall buried. The weatherboards had water damage/rot at one end.

So, I dug a trench along the base of the wall thinking I would install a grated trench, which would then connect to a SW pit, which would then connect to the council SW system.

I then changed tactics and went for an ag pipe (as shown), mainly because it seemed cheaper and easier.

After reading some more online, I am now having second thoughts. It seems that managing surface water at surface level is better than inviting the water to pool around my foundations. I don’t really know if groundwater is an issue or not, the soil is heavy clay.

I used a permeable landscape fabric to line the trench, which in retrospect might not have been a good idea, as it won’t keep water away from the house.

I am thinking I probably need to go back to the grated trench idea. I could keep gravel in the trench and concrete over the top, if it’s better to have gravel against the brick rather than clay…?

I’d appreciate any advice people can offer before I get myself into further trouble here. Thanks!

JacobZ

Hi @Tomahawk1,

Welcome to the Bunnings Workshop community, it is fantastic to have you with us.

As I'm sure you are already aware, there are two main types of drainage, surface level in the form of channel drains, swales, kerbs, graded slabs, drainage pits, etcetera, and then subsurface drainage, typically using slotted ag pipe surrounded by drainage gravel. They deal with different sources of water, so it really depends on the situation which one is better, or if a combination of both might be the best solution.

Knowing which one is better for the situation really comes down to how the water acts in that area. Having a slab graded towards your foundation is never a good idea, so removing it was definitely a good start, and it is about the best thing you could have done.

Clay soil typically holds a lot of water, so having agi pipe run through it is not necessarily a bad thing, but if your main issue is surface water flowing over the concrete towards the foundation, adding a channel drain could also be worthwhile.

The main issue with having water run along your foundation is that it can erode the soil and undercut the foundation. Having your geotextile fabric in place will do a lot to stop that erosion, so I wouldn't say it is a bad choice. As long as the bottom of your drainage channel and your agi pipe sit below the dampcourse line in the brick, I don't think you'll really have any issues.

I think in this situation, you have already done the best thing you can, which is remove the slab that was graded towards the foundation. From here, I wouldn't make any drastic decisions just yet. I'd encourage you to examine how the water is moving the next time you have some significant rain and evaluate whether surface water is a larger issue. If you are seeing a lot of water streaming off the remaining concrete into the gravel channel, then it is likely worth adding a channel drain along the concrete's edge alongside what you have already done.

Keep in mind that any connections made to a council stormwater drain must be made by a licenced plumber, so if that is your intention, it would also be worth getting their professional opinion as they would be dealing with this sort of issue regularly. Their ability to examine the site and provide a professional opinion would definitely be valuable.

Allow me to tag @Nailbag, @Dave-1 and @Noyade to see what they think.

Jacob