Regrading and installing AGI Pipe - What type of backfill to use to regrade and divert water away?

[Floyd25]

Hello Everyone,

We are planning to add decorative stones to our empty lawn that is currently crushed rocks. No budget for concrete.

Unfortunately, the slope of the ground moves towards the house and on the left is flat. 

1. Is it better to regrade the slope first before installing drainage? Or do we regrade to where we are planning to add drainage?  I understand the first priority is to try to move water away from the house then add needed drainage.

2. What type of back fill/base is best to use to help with the water run off/ away from the house? It will be topped with decorative stones. 

3 As my available storm water discharge is not deep enough to accommodate 100mm agi required trench depth of 300mm. 

Will multiple 65mm agi do the job? 

4. What is the recommended distance of french/agi drain from the house foundation? 

Here are some photos for reference. 

Low area at the bottom right of the pic

4 PONDING AREA

Crushed rocks? 

GROUND CLOSE -UP

Plan

 

Thank you so much in advance! 

Floyd

[MitchellMc]

Welcome to the Bunnings Workshop community @Floyd25. It's wonderful to have you join us, and many thanks for your question about drainage.
 

You're right that regrading the area should be your first priority. The ideal solution is to shape the ground so that water naturally flows away from the house, which may reduce or even eliminate the need for AGI pipe along the wall. This can be achieved by using compactable material like road base to create the slope, followed by thorough compaction to maintain that grade. However, depending on the existing site levels, you may find it difficult to create a sufficient fall away from the house without building up too much material against the wall, which isn’t recommended. In cases like that, a combination of regrading and installing a drain is often the best approach.

As for the base material to use under the decorative stones, a well-compacted road base is best. It provides both a solid foundation and aids in directing water. You can then top this with a layer decorative stones. If drainage is still required after regrading, a 65mm AGI pipe can be sufficient for moderate runoff, especially if you can’t achieve the 300mm trench depth needed for a 100mm pipe. If water pooling near the house is already a significant problem, it might still be worth trying to make room for the 100mm pipe, but 65mm is a practical alternative. Ensure the trench for the AGI pipe has a slight fall — typically 1 in 100 (1cm drop per 1m run) — to help water move efficiently toward your discharge point.

In terms of placement, keep the AGI pipe at least 600mm away from the house foundation if possible. This helps intercept water before it reaches the walls while reducing the chance of undermining the footings. If space is tight, you can go a little closer, but avoid running the pipe directly against the wall. Proper grading and drainage working together should give you the best long-term result for managing water and protecting your home.

Please let me know if you have any questions.

Mitchell
 

[Floyd25]

Hey Mitchell,

Thank you for the detailed response mate. I am in the process cutting and filling the existing material of the area. Little by little chiselling out the high part to fall away from the house, also backfilling the low area with road base as suggested.

I was able to get a descent slope of 1.5m perimeter from the house. . This is where I am planning to install the AGi pipe where the negative and  positive slope meets. Will this work or is this too far from the wall? 

I have read on a similar post that you can

install builders plastic from the wall to the slope  then down the agi pipe . Before adding the finish decorative stones on top. This should be able to eliminate or minimize any water between the house and Agi pipe correct?

Thank you. 

Floyd

[EricL · Accepted solution]

Hello @Floyd25 

Technically it should eliminate or minimize the water between the house and Agi pipe as you mentioned. I don't see any issues with putting in the builder's plastic as it will deflect the incoming rainwater. Let me tag @MitchellMc just in case he has any extra recommendations he would like to add.

Eric

[Floyd25]

@ashleerich @MitchellMc  Thank you guys.

In addition, if say I use 20mm or 40mm salt and pepper stones on top of the plastic. Will it puncture the plastic if someone steps over it? I will be using 140um, hoping it will be strong enough.  

Thank you.

Floyd25

[EricL]

Hello @Floyd25 

If these stones have sharp pointy edges on them, the odds of them puncturing the plastic will be very likely especially if you step on them. I propose looking at the Tuscan Path 10kg 10-25mm Mixed Polished Pebbles. The round shape and polished surfaces will prevent it from damaging the plastic underneath. Using builder's plastic with a 140um thickness should be sufficient for your project.

If you need further assistance, please let us know. 

Eric

[Floyd25]

Hi @EricL ,

Makes perfect sense, thank you. 

I was able to grade most of the areas and  with the help of builders plastic direct the water to the low spot near the existing storm drain. 

I was thinking of also adding a storm pit to the low are to collect as much ground water as possible and still have the french drains of course. 

Will a shallow flow away pit work? 

Thank you.

Floyd

[EricL]

Hello @Floyd25 

The purpose of a pit is to slow down the velocity of the flowing water allowing it to reflow into the other pipework at a slower speed. If the flow is very strong, I would advise using a deep pit to prevent it from overflowing. 

If you need further assistance, please let us know.

Eric

[Floyd25]

Noted mate. Thanks @EricL 

Floyd

[Floyd25]

@EricL Is it possible to connect the pipe higher as my storm water discharge is not low enough?  Thank you.