How do I secure my Flagstone around a Koi pond?


Old 05-02-08, 02:44 PM
Thread Starter
Join Date: May 2008
Posts: 3
How do I secure my Flagstone around a Koi pond?

Question is: How do I keep soft material from falling into my pond? How do I stack flagstone around a pond? How do I fill the seams between the flagstone around the pond if Iím using moss everywhere else except for the area directly surrounding the pond because the moss wont root into the liner and second course of flagstone?

I dug a pond with the edge a little over 3 feet from the wood fence. The area between the pond and the fence is a walkway from the front of the house to the back yard. I want to double stack flagstone around the pond and lay flagstone from the front gate to the bridge that is not yet build (~50 feet) - it's a walkway and this lay of Flagstone would be the second course covering the first course of one side of the pond. I would like to set the flagstone in soft base and use no cement because I plan to fill the seams with topsoil and grow moss between the stones.

Iím planning on setting the first course of Flagstone directly on the underlay and pond liner, which sits on the clay barrier Iím trying to build, then use waterfall foam to glue the liner to the top of the flagstone. The second course would hide the liner and underlay. I'm in Colorado and I'm working with clay that is rock hard when dry and super soft the wetter it gets. I've been trying to build the clay into a barrier that can hold the base for the flagstone from falling into the pond, but this has been nightmarish.

I've been to the quarries, pond supply stores and Home Depot/Loweís for advice and keep getting looks like no one has ever does this before. I have to imagine with all the experience here, Iíll find some brilliant helpful individual. I tried to describe the project as best I could, but picture are best. I took photos of the project and have them loaded at: - go to the last photos to see the latest state of the project. If you can help, Thank you! Thank you! Thank you!
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Old 05-02-08, 07:27 PM
fewalt's Avatar
Join Date: Feb 2001
Location: sw VA
Posts: 3,100
Dirt and moss WILL wash into the pond.
Set the flagstone in mortar, and put mortar between the stone closest to the pond. Some dirt amd moss can be put between the flagstone as you get away from the pond edge.


sidenote: my pond was dug with a backhoe. Two sides of my pond (the softer sides) I set concrete block at the top edge. I drove rebar down thru the block holes and filled the block with concrete and capped it off. I placed the liner over the cap and then felt safe laying my flat edging stones in concrete/mortar on top. Alot of work but, I did not want a cave in!!!
That was four years ago. Still looking great. Six golden orfes, five large comets, two fall fish, and a few sunfish caught by my granddaughter.
Old 05-03-08, 04:21 AM
Join Date: Aug 2004
Location: Vancouver, Canada
Posts: 1,210
That sounds good Fewalt. We're always underestimating the persistent effect of gravity - overkill from the start proves itself a decade later.

Oliver_aranda, concrete/mortar needn't be an all or nothing proposition. You could have pretty deep joints to hold moss, and bed the stones and have some mortar deeper down between them. Keep it shaded and the moss will do fine - moss does not require soil!
Old 05-03-08, 10:14 AM
Thread Starter
Join Date: May 2008
Posts: 3

Would you happen to have a link to the technique you are describing. I've never laid mortar (or stone for that matter), but hiding the mortar by using deep joints is what I would like to do.

By the way, any usage of mortar and concrete is suggested for use only on the pond edge, right? The project/moss should look good if I use soft material in the areas just off the edge, right?
Old 05-03-08, 01:37 PM
Join Date: Aug 2004
Location: Vancouver, Canada
Posts: 1,210
No link, sorry. But if the joint between two stones is plopped over mortar, some mortar must squish up into the joint. Then if you shimmy the stones closer together, that mortar must rise in the joint. If you can get a finger into the joint, and smooth the mortar against the stone edges (but not to the top), it'll stick better. Better too if the stones are damp.

Here's a trick to getting moss started:

Gather moss, careful not to include sand or pebbles. Drop it into the blender. Pour some yogurt over that. Make a thick "paint". Apply to any surface you wish, with brush. The yogurt binds moss spores to nearly anything, and feeds their growth.
Old 05-04-08, 05:08 AM
Join Date: Nov 2005
Location: North Carolina
Posts: 948
Great Stuff makes a canned foam for building ponds.
Old 06-09-08, 08:49 PM
Quietman's Avatar
Join Date: Dec 2007
Location: Southeast New Hampshire, Zone 5A
Posts: 64
This might help...

Check this link, lots of good information there.

While I'm an engineer, I'm still learning a lot about pond building. I too have clay that is soft when wet, and dries rock hard (till it gets wet again). I'm installing coping (a stone necklace) that should hide and secure the liner, while re-enforcing the edge of my pond. If you are interested, I've got a "running" thread in this forum with my progress.

Looking at your photos, I believe (my opinion only) that you could be in serious trouble if you ever do complete the pond and populate with fish (especially expensive koi). Imagine having to move them to a temporary "pen" and working under the gun to repair/replace the old pond (before they die)?

Please understand, I mean no disrepsect! Your pictures show that you've been working very hard, beleive me, I sympathize!

Good luck!


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