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Old 10-14-2008, 10:19 AM
Join Date: Oct 2008
USDA Zone 10
Posts: 3
zebramidge is on a distinguished road
Flagstone cap on stone wall

DIY project. Just built 18"x18"x125' perimeter garden seating wall out of river rock. I've roughcut various flagstone varieties at appx 20" wide to cover the wall. The wall itself is close to level, but not perfect because of the materials. Before I start capping I have some questions:

What's the best way to level out the flag? My initial plan is to set a level line at the highest piece and then dry shim all the other flag pieces to this level, finally setting everything in mortar. I'm wondering whether I can just mix stiff mud and tap everything into place without the shims?

Should I use a masonry bonding agent on the bottom side of the flag, or should I add beads of adhesive on top of the mortar before setting?

Can I grind and rough up my cut edges after setting without worrying about dislodging the cap? If so, how long should I wait? Will a hand grinder with grinding blade work? What are the techniques to rough up the edges so they don't look so unnatural?

Great site. Thanks in advance for any help.
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Old 10-14-2008, 01:44 PM
i.e. Design's Avatar
Join Date: Aug 2008
Location: St. Paul, MN
USDA Zone 4
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i.e. Design is on a distinguished road
I've done several walls this way, boulders and other types of stone. The best way I've found is to find the high point, like you said. Considering the length of the wall, you'll probably want to find a couple of high points (a little variation in hight over that kind of distance shouldn't matter). Say you find a point every 30 or 40 ft. Make sure that point is not too far out of wack with the rest of the span (if it is, maybe replace a stone or two to bring it down). Use a medium to stiff mortar mix and set that piece of cap. Then move out from there setting each stone to that level. I wouldn't bother shimming, because you'll end up having to lift each stone to get the mud under there, making it a wasted step. You could shim and then tuck point mud in from the sides, But I think a bed of mud and a rubber mallet are faster and less aggravating.

As far as adhesion, I would go with a spec mix or any pre-mixed type M or S mortar mix. Considering the length of the wall, you might do better cost wise to mix it yourself (bags of type M and course wash sand at a 1 to 3.5 ratio). Depends if you're comfortable with your crew getting it consistently right.

A couple other things I've learned the hard way:
-Go back along the underside of the cap and shim in little stones to avoid huge spots where the mortar is really visible.
-The joints between the stones on the topside of the cap make the difference between a cap and a very well done cap. go back over them with a sponge after they dry a little and make sure you get all the mortar off the stone.
-Depending on the material, try to rock face the cut edges with a chisel before you set the caps. Most of the stuff I do is Bluestone, and this works well. A grinder wheel works on softer stone, and there's a way to thermalize it with a torch, but it's really hard to explain.

Sorry about the rambling message, but I hope a least some of this helps. Post a pic and let me know how it goes.

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Old 10-14-2008, 08:09 PM
agla's Avatar
Join Date: Feb 2003
Location: Cape Cod
USDA Zone 6
Posts: 2,208
agla is just really nice agla is just really nice agla is just really nice agla is just really nice
Is this a mortared wall, or are you trying to only mortar the cap?

Cape Cod Landscape Architect
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Old 10-14-2008, 11:17 PM
Join Date: Oct 2008
USDA Zone 10
Posts: 3
zebramidge is on a distinguished road
Wall is fully mortared on 8" reinforced footing. As Ian noted, it is basically level within an inch throughout its length, with some stones a little higher here and there, and of course the individual pieces of flagstone vary in thickness. I suggested the shims since I wasn't sure the mortar would support some of the heavier pieces of flagstone, but based on his experience it sounds like it will.
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Old 10-16-2008, 10:39 PM
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Posts: 180
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have done lot's of dry stacked walls - just use lime based mortar to level them up
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