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jbk 10-29-2010 04:19 PM

Flagstone Path - How to Fix Unstable Stones
We just put in a small flagstone path ourselves and the stones are pretty unstable. We did it with little research and no experience, and on a very tight budget so we cut a lot of corners. So we probably did several things wrong.

I think the main problem is that we didn't actually use flagstones. Someone was giving away granite tiles (12x12x3/8") that we liked, so we broke those up and used them as the main stones. I think they're probably just too light. But I'd like to understand all the things we did wrong before attempting a fix.

The path is on our front yard, about 3 ft wide and 20 ft long, gently curving, and gently downsloped -- there is about a 1 ft decline over the 20 ft distance.

First, we put in benderboards on the sides of the path. We did this without scraping the underlying dirt to be perfectly flat, so the tops of the benderboards aren't even from one side to the other.

Second, we poured in several bags of moist, all-purpose sand. Because the dirt wasn't flat and tops of the benderboards are uneven, the sand depth varies from 3/4 inch to 1.5 inch. We tamped it by pushing down with a piece of wood board -- we did not use a heavy, metal tamper.

Third, we laid the granite tile pieces. Their sizes varied, but on average they are about the size of a business-size envelope. Then we filled the spaces between them (which are about 1" width) with 1/4" black beach pebbles.

Right away the stones did not feel solid. They sank and shifted when walked on. After just one day of use, several steps are already out of place. And to make things worse, the pebbles that were between the stones are are all over the place.

So, I think the main problem is that our stones pieces are just too small and thin. But I wonder if not tamping the sand was also a problem? In a normal flagstone path, what keeps each flagstone from shifting under the weight of a person? It seems to me that even if the sand had been packed very tight, it would eventually dry out and become fluid, and would not keep flagstones from shifting under people's weight. How does that really work?

I'd appreciate any input. Thanks.

Daniel Holzman 10-29-2010 04:48 PM

We did a brick path about 18 years ago, and a flagstone patio about 12 years ago, so I have some personal experience. By the way, this topic has been discussed numerous times on this forum, do a search on "walkway" or "patio" or "flagstone" and you can get a lot of information.

For starters, if you are in a frost prone region, you need to replace poor soil such as silt or organic soil with either coarse sand or crushed stone. This may require excavation and replacement of a foot or more of poor soil. In our case, we replaced about 18 inches of poor soil, creating a bed of crushed stone about 12 inches deep, then placed 6 inches of coarse sand above that. We live in New England, cold winters, and we wanted to avoid frost heave. So it sounds to me like you did not use enough bedding material for starters.

Second, it is essential to compact the sand. Not so much the crushed stone, it pretty much self compacts, but the sand must be compacted vigorously, in small lifts. We used a heavy tamper and three inch lifts to create a very firm surface for placing the stones.

Third, your flagstones are probably too small. Our paver bricks were standard 8 inch brick, they worked well, and our flagstones were Vermont slate, variable size, but mostly about 1 inch thick, and a minimum of about 12 inches long by 8 inches wide. We filled the gap between the stones and brick with masons sand. Some people use stone dust, some use a special kind of sand called polymeric sand that is stickier than normal sand. We have had no real problems using ordinary sand, it got between the cracks of the bricks and stones, and hasn't moved in years.

So in summary, I think your issues include inadequate base, inadequate compaction of the base, and too small flagstones.

jbk 10-29-2010 04:59 PM


Originally Posted by Daniel Holzman (Post 525046)
... So in summary, I think your issues include inadequate base, inadequate compaction of the base, and too small flagstones.

Thanks! That's helpful to hear.

We're in San Diego, so frost is not an issue. Based on what you're saying, it sounds like the stability of the stones come from well-compacted sand, the sheer weight of the stones, and the mason sand between the gaps.

It seems like, over time, the compacted sand used as the base would dry and loosen up. I guess they do, but over a long period of time.

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