How to add planter box drainage?

ScottH888

Hello,

I built a raised garden bed last year and the agi pipe drainage failed.

Water flowed out the side of the pipe rather than through it and out to pvc under the wall.

Ive removed all soil and plants and wish to get it right this time.

once plumbing is roughed in plan is to fill the inside base with concrete then box it out with blue board or HDPE liner and waterproof it.

this will leave water nowhere to go other than through the pipes.

question is what is best drainage design for water flow?

ag pipe or slotted pvc?

Being an L shape planter will water flow around corner ok provided slope is sufficient?

should it exit the end rather than through the side as it is currently to prevent a bend.

any help appreciated 

MitchellMc

Welcome to the Bunnings Workshop community @ScottH888. It's terrific to have you join us, and many thanks for your question about garden drainage.

From what you described, it seems the issue was that water was bypassing the ag pipe and flowing out through the side, which could have been due to improper installation or insufficient drainage around the pipe. If the pipe wasn’t surrounded by enough permeable material, like gravel, or if it was positioned incorrectly, water may have had no choice but to flow around it.

For your new setup, your plan to fill the base with concrete and waterproof it with blue board or HDPE liner sounds solid. This will create a contained environment, ensuring that water flows only through the drainage pipes. As for the best drainage design, I’d recommend using socked slotted ag pipe around the perimeter of the planter box. The socked version is wrapped in fabric, which prevents soil from clogging the pipe and ensures better drainage. While slotted PVC can also work, it’s less porous and may be more prone to clogs in garden settings. The key is to ensure proper water flow, so ag pipe with the fabric sock is generally the better choice for garden beds.

Regarding the L-shaped planter, water should still flow around the corner, provided the slope is correct. Aim for a slope of about 1:100 (1 cm drop for every 100 cm), which will ensure water flows toward the exit point. However, corners can sometimes cause water to pool, so make sure the slope is consistent, particularly around the corner. Avoid sharp bends in the pipe, as these can impede water flow. It might be best to move the exit point to the end of the planter instead of the side, as this will allow the water to flow straight out without making a turn. With the right slope and no sharp bends, water should flow freely to the exit.

Finally, ensure the pipe is surrounded by gravel or coarse sand to promote good drainage and prevent clogging, and leave space for the pipes to sit in these materials if you’re pouring concrete in certain areas. Once the pipes are in place, backfill the surrounding area with gravel or drainage stone to keep the soil from entering the pipe. 

Please let me know if you have any questions.

Mitchell