How to select the correct drainage for side paths?

marish

Hi all,
I was planning to install French drains along both side passages of my house (Melbourne) as part of a paving job, mainly to manage stormwater runoff from my place and the neighbour’s. I’ve had a contractor look at it and their feedback was:

• The areas aren’t currently ready for French drains — they’d need to be at least 300-400mm deep

• In their view, it’d be a waste of money — they think it’s very unlikely I’d actually have water issues that would justify the expense

Before I decide whether to drop the French drains from the scope altogether, I wanted to get some other opinions:

1. Is 300-400mm depth pretty standard for a French drain, or does it depend on the situation?

2. How would I know if I actually need drainage here vs. it being unnecessary? (e.g. signs of pooling, damp walls, runoff from next door, etc.)
3. Has anyone had a similar “you probably don’t need this” call from a tradie that turned out to be right (or wrong)?
4. If I skip the French drain, is there a simpler/cheaper way to manage runoff along a narrow side passage?

JacobZ

Hi @marish,

Welcome to the Bunnings Workshop community, it's fantastic to see you make your first post.

If you have a couple of photos you can upload, that would certainly help our members to better understand what you are working with. If you need a hand uploading images, please let me know.

Did your contractor happen to give you any indication as to why they think it isn't worthwhile in this situation? I can't imagine anyone reputable tradesperson would give you that advice without some form of reasoning attached to it.

I think the first thing to establish is where the water you are trying to manage actually is.

Before proceeding, I'd look for evidence of an actual drainage problem. After some decent rain, check whether water pools in the side passages, whether the ground remains saturated for long periods, whether water is visibly flowing in from the neighbouring property, or whether there is dampness developing against the house. If none of those things are occurring, your contractor may well be right that a french drain would add considerable cost without achieving much.

A french drain is primarily useful for intercepting water that is moving through or sitting within the ground. If the issue is rainwater running across the surface from paving, the house or the neighbouring property, then a french drain may not be the most effective solution. In that situation, correctly grading the paving and directing surface water towards a channel drain running to an appropriate drainage point, ideally a stormwater pipe, would generally make more sense. If the issue is more about the ground remaining saturated for long periods after rain, then a french drain is more likely to be worthwhile.

There isn't really a universal depth that makes a french drain suitable, however 300-400mm is pretty common.

If the concern is mainly surface runoff, I'd concentrate on the finished levels of the paving. Ideally, water should be encouraged to move away from the house and towards an appropriate collection point. A grated channel drain can also be useful in a narrow side passage where there is a predictable path of surface water.

Allow me to tag some experienced members to see what they think - @mich1972, @Dave-1, @Nailbag, @R4addZ.

Let me know what you think and if you have further questions, please don't hesitate to ask.

Jacob