Q: Do you recommend any particular mortar mix for use with flagstone paths? Also for a 3 foot retaining wall using Tennessee fieldstone, do you recommend a dry stack or mortar in the rear section of the wall?

A: I don't often use mortar on a flagstone path. I like to use a thick stone, an inch and a half thick if you can find it, set on a crushed rock base. Large thick stones will stay in place without the need for mortar. When I need mortar between stones, or to edge the path, I use the same crushed rock, or breeze which is 3/8 minus with fines, in a 3 to 1 mix with portland cement. I sweep in the mix dry and wet thru with a fine spray of water. Don't expect this mortar to last forever but is very easy to repair.

I would sure recommend that you use a dry stacked retaining wall if you can as water accumulating behind the wall adds a lot of pressure that can be avoided if the wall can weep. If conditions demand a mortared wall consider a gravel drain behind the wall with weep holes. A gravel drain also gives some protection against frost heaving. Good luck.

Q: I'm planning on building a dry stacked stone retaining wall. The land is sloped, so the wall will be four feet high on one end, and six inches on the other, with a arch shape. Should I build a concrete retaining wall, behind the stone for support.

A: No need to build a concrete wall behind the dry stack if you build the rock wall properly. A dry stack rock wall relies on gravity to hold up the bank so the rock has to weigh enough to counteract the force of the dirt pushing on it. The four foot end of the wall will need stones of considerable weight. A tight fitting wall with angular stone has the added advantage of friction working for it. Make sure that you start the wall below grade at least 6 inches to toe it into the slope. You can also reduce the pressure on the wall by sloping the face of the wall back into the bank a few degrees. Good compaction of the soil behind the wall is essential. Building a dry stack stone wall can be challenging the first time but well worth the effort. Don't be afraid to disassemble and relay to get a better fit as you get better at it.

Q: I want to build natural rock steps into a hillside on our lawn, but we have frost in the ground in winter here in New Brunswick, Canada. Do I need to put steel mesh underneath to keep the rocks from shifting or is there a better way to stabilize the steps?

A: (Kelly) We have pretty cold winters here in the mountains of Colorado and the rock steps that I built into the hillside leading to our front door have not shifted. They are pretty hefty rocks (bigger that I can lift) and are just bedded in the sandy soil. It might be that the fact that the soil drains so wells eliminates the possibility of frost upheaval. If such upheaval is a problem (it might not be) then the use of sand to bed the rocks in might be a solution.

Q: I have collected lots of river rock, enough to make a low rock wall around a garden. There is a concrete footing [from a defunct greenhouse] that I want to cover with the rock. It's about 2' tall and I want to cover the outside and top. Do I simply [ha!] use concrete and stack and "glue" to the rocks to the footing? I live in northern New Mexico, dry and dusty here.

A: (Kelly) I would say that you have a couple of choices here: you can either do as you suggest and mortar the rock wall up against the old foundation, or you could probably simply dry stack (without mortar) the rocks. Since it will not be a bearing wall and is not all that high, I might suggest the later. You might gain some insight into creating this type of wall by reading one of the several books listed on the rock page.

Q: We have an existing flagstone patio, but the pebbles and sand that they used in between each stone is always everywhere. Can we pour cement in all the cracks right over the pebbles and sand? What other suggestions do you have that doesn't require us removing everything and starting over?

A: The easy fix to your problem is to dig out the pebbles and sand to a couple of inches deep. Mix what you have excavated 3 to 1 with portland cement dry and sweep back into the cracks. You can use new sand instead or crushed rock, 3/8 minus is best. I try to pick a color that contrasts the flagstone. After you have swept the dry mix into the cracks, use a fine spray of water to soak the mortar. You will get some cement that wants to settle on the flags and stain them so start at one end with your spray and sweep the stones as you soak the mortar joints. This will give you a hard joint that will keep your flags clean but don't count on it lasting forever. Mix more dry mortar than you need and keep a bucket in the garage for easy repairs.

Q: I'm building a dry stack stone retaining wall 70 feet long 2 to 2 and half feet high. After I have cut back the lawn to provide a 2 foot crushed stone base of 6 to 8 inches deep I plan to bury my first row of stones. Do I need to apply landscape cloth on the incline behind the retaining wall? Also do you advise backfilling with dirt or crushed stone? Would either or both of these assist in the longevity of the retaining wall?

A: I don't think you need to use a landscape cloth behind the wall unless you have a very sandy soil that will seep through the stone wall. A layer of crushed rock behind the wall will provide protection against frost and water pressure that can damage a dry stack wall. You can get away without either option if the conditions are right, but without knowing more than I do I would say skip the filter fabric and put a drain of crushed rock behind the wall 3 to 4 inches thick.

Q: We recently had a large (1500 sq ft) flagstone patio put in that was set in mortar over a concrete base. We are seeing a significant amount of "flaking" of the stone. In some cases entire sheets 1/8" thick are coming off. Would a sealer stop this flaking? Do you have any other suggestions?

A: No, a sealer will not stop the flaking. You obviously have a poor quality stone for a patio. I would hope that only a few of the stones will flake and soon the patio will stabilize. Picking the right stone is step number one of any patio job. If you had to pour concrete under the flags you probably are using a thin stone to begin with. Here's hoping that some of the stone was inferior and the rest hold up.

Q: We have some property on a lake in the Texas hill country. The property is very steep and treacherous to navigate. We would like to cut/carve stone steps right out of the hillside rather than install stone steps. Is this possible and how do you go about finding someone who can do this?

A: It may be possible to cut steps out of the rock on your site. It's impossible to say without a closer examination than you can give me via email. As far as someone to do the work, I sure don't know anyone in that part of the country.

Q: How deep and wide do I dig a hole for 6-8 ft retaining wall?

A: Give me some information. An 8 foot tall dry stack garden wall requires no hole per se, just start the stone a foot or so below grade. A mortared house wall that retains 8 ft. of dirt will have to start out about 3 feet wide and need a foundation that gets below frost line which varies from place to place. And that's just the start. ...What kind of dirt will you be holding back? What kind of stone will you use? How big are the stones? Do you have water issues?

Q: The rear corner of our house is within 2-3 feet of a drainage ditch which separates our property from our neighbors. The bottom of the ditch is 5-6 feet below the level of our house. We have noticed erosion of soil along the edge of the property and realize that we need to build a retaining wall. Due to the closeness of the house to the ditch only limited excavation of soil behind the wall is possible. The soil is clayey and we have annual rainfall in excess of 3 feet. Is dry wall stacking feasible for this situation?

A: Sure a dry stack wall would work. Dry stacking is probably the best way to build a wall with so much water. It is really hard to comment on your situation without more information or a good look but it sounds like you might not need to excavate a wall at all. In similar situations I have laid small stones on the existing grade to solve an erosion problem. I like to line the ditch like a dry creek bed with river rock about 3 to 6 inches deep. Using a filter fabric underneath would probably be advisable with so much rain. Next line the slope to the house with stones like cobbles, this is called rip rap. You don't need to be too precise with the fit because you want things to grow in between them. If you can collect the stones yourself you can assure that you get a variety of sizes for the rip rap and dig some of the larger stones into the bank to anchor.

Q: I am planning on building a rock wall around my stand which is 1/4 acre in Zimbabwe. What would be a standard size wall that would compliment my house.

A: You have asked me a question that only you can answer. The rock that you will use will probably be the determining factor in the thickness of the wall and whether you will mortar the stone or dry stack. The height of the wall may be more affected by your skill as a mason and influenced by the function of the wall. Are you just delineating the property or trying to keep wildlife out?

Q: We are updating the backyard of our new home and want to cover the existing treated lumber retaining walls. We are hoping we can just cover them with tar paper (or something similar/better?), chicken wire, then cover with a cement or a stone veneer. The walls are 3-4' high. We are not sure how old the walls are and are afraid that (1) they need to be replaced by a certain age, (2) they will continue to age/degrade to the point where we would need to tear down the veneer and walls to replace, or (3) that we can't cover them at all. We have been unable to find any mention of covering wood retaining walls thus far and any advice you can provide will be greatly appreciated.

A: I have a lot more questions for you than you had for me. What's your climate like? Is the ground moist? Are the timbers treated? etc...You could probably put a veneer over a tight stable timber wall if you weren't trapping too much water. A timber wall is designed to weep and relieve hydraulic pressure. A concrete and stone veneer would trap the water. A lot of timbers though treated might rot out faster with a stone veneer. Oak railroad ties on the other hand might take the covering just fine in a dry environment. If you think your wall can take it skip the tar paper and use expanded metal lath instead of chicken wire. I would be tempted to consider a drystack wall in front of the timbers as an alternative. The wall could still breath and weep and the drystack would give you the enduring natural look. Still more questions. What kind of materials are available to you? What are your skills? Timbers of any kind or treatment will eventually rot and of course the stone veneer will fall off but some timbers might outlast you and justify the easier veneer fix. The rock will outlast us all.

Q: Based on your advice, my husband and I decided to remove the timbers and replace them with cement blocks, then veneer over that. Helping that decision: We noticed some rotting already in full swing on a smaller wall near the wood deck.

Though I absolutely love the look of a dry stacked wall we feel that since the wall is in between the grassy area and pool, which will get a lot of traffic (especially with 4 active dogs, we want the wall to be as stable as possible. As far as what materials are available, we have many home improvement stores and specialty stores nearby, to include nurseries.

Our skills are somewhat limited. My husband does have experience remodeling homes (tearing down/building up walls, etc) but limited masonry experience. That being said, can you advise us on how best to prep the ground? For example, how big of a footer do we need? Are there special considerations we are unaware of being novices? Everything I've read up to this point tells me one of our most important tools will be a level to ensure the wall is built straight up and level across, but that's about it.

If you feel this job is too big for a couple of novices (our wall is "L" shaped an approximately 7' long (shorter area) and 13' long (long part) and roughly 3' - 4' high) could you provide a ballpark estimate of how much it would cost us to have a professional replace the wall with concrete blocks? I'm confident we can do the veneer ourselves (unless you suggest otherwise!) We really need to contain costs as much as possible but don't want to waste money doing a poor job ourselves, or worse, create a safety hazard.

A: A well built drystack wall will almost always be stronger and less expensive than anything you can do with cinder block. It does take some skill but skill that the rocks themselves can teach you. The only drawback is that the drystack is a gravity wall, the weight of the wall counteracts the pressure that the dirt exerts on it. A 4 foot high wall will necessitate some heavy stones in the base, maybe up to a couple of hundred pounds. Cinder block by itself has very little structural integrity. Filling the cores with cement and rebar helps but more is needed to keep the wall from tipping. Most walls of this kind have a footer that extends a couple of feet under the bank to be retained connected with rebar to the wall itself. Water behind the wall adds to the pressure on the wall and should be relieved with a gravel drain and weep holes. You add all that up and a drystack wall looks a lot more attractive. I would generally bid a job like this at around $1500 for the rock work itself given that you can get a good stone for around $40 per ton. It sounds like you will need about 10 ton. Site prep, backfill, and site considerations, like can you get the stone to the work site with a loader or do you have to wheelbarrow through the living room, add to the overall cost. Of course some stone can cost up to $300 a ton. A flagstone drystack is beautiful but pricey.

Q: Building a retaining dry wall with field stones, in Ontario Canada. I have built many retaining walls with prefab materials, but never with fields stones. Should I make the base the same as I would with my other installs?(compacted base) Also, would it help to mortar the stones on the inside of the wall (I don't want any visible mortar). Lastly, the wall is on an incline. When I step up, can I overlap the the next layer on the first layer? The wall is only about 2 ft high, with 3 step ups.

A: You should definitely compact the base of your drystack wall with a gravel type material; after that the process changes from the prefab materials. Your field stone will not have the uniformity and interlocking characteristics of concrete block. A good drystack wall can be even stronger though. I would try and stay away from rows of rock and take the time to interlock the stone on an individual basis compacting the fill behind the wall as you go. Mortar behind the wall can be problematic by trapping water and adding pressure on the wall so use it sparingly if at all. If you can keep from laying the rock in rows you can step up as the grade demands trying to keep a random look to the face of the wall.

Q: We are having an in-ground pool put in this week. They will be pouring concrete around the pool. We do not want to look at the stark white concrete so we have checked into other options such as stamped, acid etc., and have found those options to be very costly. We can lay real flagstone cheaper. Can we lay the flagstone into the concrete when they pour it? Will this hold up well?

A (Kelly): I'll give you my short answer, since Jose may not respond in time. Yes, you can lay the flags in wet cement, but it can be a bit tricky. I would suggest that you have the pattern of stones already laid out the way you want them, so you don't have to make lots of decisions as you set them. Wet the bottom of the stones before you place them. Also just press them down into the cement to the level that you want, because if you get cement on the top it is hard to remove. And, of course, don't walk on them until the cement has set up.

Q: What is the "best" foundation for a flagstone patio and what is the "minimum" foundation for a flagstone patio. I live in Wisconsin and temperatures and get 20-30 degrees below 0 in the winter. In the summer, like today -- it "feels like 105" with the humidity and heat. What type of stone/sand is the best for the foundation? Does it need to be framed in?

A: No sense in worrying about the frost the whole patio will float with the freeze and thaw. I like to use a breeze under the flags. This is a crushed rock with fines, 3/8ths minus, that holds its shape better than sand. Sand will do just fine if it's all you can get. Like I mentioned, the patio will float on the soil you have so the sand or breeze is mainly to ease the installation process and only needs to be a couple of inches deep. Try and get a stone that is an inch and a half thick to two inches. Your back will be sore from all the lifting but you'll have peace of mind for years to come. I use a dry diamond blade on a skill saw to cut a few of the stone to get tighter joints and fill the cracks with breeze or topsoil and lemon thyme or some other ground cover.

Q: We had some ledger stone installed on the front side of a large flower box/retaining wall. The stones are falling off, my guess is from the moisture of the flower box. The retaining wall is made of cinder block. Would you be so kind to suggest what we can use to adhere the ledger stone to the blocks with out having to redo the whole thing?

A (Kelly):You might try attaching the stones with "Thin-set" mortar used for attaching tiles to walls and floors. It is a bit expensive, but it has better adhesion than ordinary cement mortar. Both the stones and the cinder blocks should be as clean as possible before doing this.

Q: I'm building a wet stone stone retaining wall approximately 4' high from grade. How often do weep holes go in, what height do they go in at and what type/size pipe should I put in? Also, the drainage pipe running the whole length of the base of the wall, should it be perforated or not?

A: I've never known weep holes to do much good in a mortared wall. I would put a curtain drain behind the entire wall from ground level to the perforated pipe at the base. You will have to daylight the pipe of course. This technique relieves the pressure that water puts on the wall. The 6" of gravel behind the wall will also absorb any soil expansion due to moisture and freezing. The best way to build a wall in a wet situation is to dry stack the stone. You will need a good size stone to hold back a 4 ft. bank. I imagine the base stones would have to be of a hundred pounds or more with smaller stones as you go up. This technique requires a good eye or as I like to say a good ear as the stone will tell you what to do if you listen. No need for weep holes or a curtain drain in a dry stack wall, just good compaction behind the wall as you go up.

Q: Last year we laid a large flagstone patio on a 3" bed of crushed rock (gold fines) We are continuing the project this year, but I'm disturbed at a lot of discoloration in the stones we laid last year. The discoloration appears that maybe part of the stone is wet, but it isn't. It could also give the impression of oil on the stone, but it isn't. I've tried everything I know to try to "clean" it, but nothing seems to make a difference. The discoloration is random, and I can't figure out why some would be, and some would not be. Can you offer any possibilities for this discoloration, and how to prevent it on the newer end of the project?

A: I don't have anywhere enough information to give you a good answer. There are way too many types of flagstone, all with differing textures, coloring and densities for me to comment on yours. My guess is that it's an environmental stain and if it doesn't even out over time I'd look to the cat as the culprit.

Q: I am building a free-standing dry-laid or dry-laid look stone wall about 30" high 2'wide of a blue stone type material. The wall will be largely atop ledge rock which is primarily a shale type composition. I am wondering how best to construct the wall and base to restrict movement. I will not be able to excavate for a base over much of the area.

A: Any wall should have a toe hold in the ground to prevent movement. A freestanding drystack wall should have little, if any, impetus to "walk" though. You should be able to lay atop the shale as if it were the first stone in the wall, as long as the shale is fairly level and matches the stone you will lay on it. If not hopefully you can chisel a bed for each stone in the base of your wall. An alternative to that would be to pour a concrete bed for your base stones to rest on.

Q: We live in the desert in southern CA and have an abundant supply of granite boulders of all sizes. I want to build a rock wall about 30" tall. I want it to have mass and taper from the base. I will insert pipe at 6' - 8' intervals to later slip in fence posts and then take fence up to 6'. It never freezes here. I am thinking 24" at base and 18" at top. Do those dimensions seem sensible?

A: (Kelly) Yes, this seems good to me.

1. Do I need a concrete footer, or can I just dig out 6' or so?

Giving the wall a 6" toe-hold should suffice.

2. I would like to make a slip form. These rocks are very irregular in shape, but mostly round. I would like to have the appearance of almost no mortar sticking out of the outside rocks. Any suggestions?

A slip form will not give you the appearance you seek. If you don't want mortar to show, you will need to individually set each rock.

3. This wall will bear no real weight, but should I integrate rebar?

A well-built stone wall, with overlapping stone, requires no rebar.

4. Can you suggest a book or WEB site to help me with mixing details, and building slip forms?

There are various books listed at http://greenhomebuilding.com/rock.htm#books , including one about building with slip forms.

Q: My husband and I want to put a small (8" deep 10" high) stacked stone wall around our pool. It will NOT be backfilled. We have purchased some mohave stone, but have no idea how to make the wall uniform on both sides. Do we use small, rectangle pieces on both sides then fill the middle with mortar? We were trying to use random pieces and fit them all together and stack, but have found it very difficult to keep the mortar from getting on the outsides and it looks too gappy. Help :)

A: (Kelly) Stone masonry is an art that takes time to learn, and patience to accomplish well. The type of stones have to be chosen with the final appearance in mind. If you want a flat appearance on both sides of a wall, you will need to start with stones that generally have flat surfaces to orient that way. Otherwise more rounded stones can be used to create a wall that has more relief from the "pointing" of the stones. It takes care to keep the mortar from spilling onto faces of the stones, but if some does, it is best to clean it off with a wet rag before it sets up.

Q: I want to build a raised bed garden (3 by 8 ft) using flat stone for an arugala garden. Do I need to create a footer or can I build it on flat ground. I'm in Park City Utah (elevation 6500 and footers require 42 inches). I would like to use mortar. Should I dry stack it?

A: (Kelly) A small garden bed should not require the massive foundation of a house. I would suggest that you dig a small trench foundation, about a foot deep, fill it half way with loose gravel, and then start laying your stones on this rubble trench. You can either dry stack the stones or mortar them, according to your preference; the mortared stones will hold more moisture than the stacked ones.

Q: I have 15 tons of 4 man rock/stone that I cannot use as originally planned on my building site. The only space I have to put them is in the front of the property as a fence. Is it possible to make a fence with such large stone? I was thinking of using the large stone as a base and use small stone on top but don't know if that would look right. Do you have any suggestions?

A: (Kelly) I think that what you suggest is excellent, and should work just fine. It is often recommended to put the larger stones at the base of a wall.

Q: We have been building granite rock walls in our garden . I have noticed that occasionally a rock will disintegrate (within a couple of months sometimes). I was told by an old friend once that stone masons had told him that a rock will disintegrate if it is laid upside down to how it was originally formed . Do you agree with this theory?

A: No I don't agree with this theory of decomposing granite. I have worked with more granite than any other stone and can assure you that I have laid half of the rocks in any given wall upside down without them falling apart. An experienced eye can tell you which rock will crumble over time while still in the pile waiting to be placed. Place any questionable stone near the top of the wall so it can be replaced easily. Decomposed granite makes great pathway gravel.

A (Kelly) I have never heard about this, and don't think that is generally true...at least not in my experience. It might be that there are some particular types of stone where this could happen, but I doubt that granite is one of them. I have seen certain granite decompose though; I think this can occur with some granite formations where the stone is not particularly well consolidated when it was formed.

Q: I want to build a stone gazebo. The idea is to have 5 pillars in a circle with railroad ties at the top (open to the sky). I live in Maine where frost is a concern. My idea was to dig a hole about 5 feet deep, use a sauna tube for the middle with poured concrete and then build around the cement column with rock and mortar. How thick should the columns be if I want them to stand 8 feet above grade? Do they have to be tapered? Does it matter if they are square or circular? Do you think they will topple?

A: (Kelly) Basically your idea is sound. My suggestion would be to reinforce the concrete columns with either rebar or wire mesh columns before pouring the concrete; that way they will stand firmly for centuries probably. I doesn't really matter what the shape is...they do not need to be tapered. You could use the sonotubes all the way up as a form if you want. Just make sure that the metal reinforcement is totally embedded in the concrete.

Q: We have recently added a path with tumbled black slate flagstone set in decomposed granite. Is there anything we can use as a "sealant" for the DG? We want to maintain the look of the path and keep the DG from washing away. Mortaring the flagstones into place will take away the look.

A: (Kelly) This could be a bit tricky to accomplish. If there are enough fines in the DG it might be possible to use something like the product offered at http://www.enviroad.com/   or  http://www.specialtysalesllc.com/nutra-bond.htm   to bind the DG somewhat.

Q: I'm building an above ground pond with Japanese Koi living in it year round. I will insure that the pond will never freeze over. The poured steel reinforced concrete wall will be 8" thick and up to 4 feet high. I am extending a footer 3" out from the bottom of the wall to support a round river stone facade to be attached to the wall. The stones will be 3-4" in diameter. Will I still need to attach metal lathe to the concrete wall and scratch it with mortar or will the mortar buttered stone attach to the mortar prebuttered concrete wall be satisfactory? Also would you recommend a layer of metal flashing to be placed under the top flagstone layer which will extend out a few inches to keep rain from getting in between the stones? In the coldest weather even though I can keep the water from freezing the outer layer of concrete may be susceptible to freezing and potentially popping out the stones...? I live just north of Atlanta so we don't get dramatic cold weather but some freezing has lasted up to two days with temps getting into the teens occasionally.

A: (Kelly) I would not rely solely on the mortar to hold those small stones in  place, so at least some occasional metal lathe or other physical method of  attachment would give you some security that they will stay in place. I doubt that it would be necessary to include metal flashing under your flagstone cap, as this will hinder the adhesion of this to the wall itself; also I doubt that the entire wall will ever actually freeze, since that huge thermal mass of water will be in direct contact with it and tend to warm the wall in freezing weather.

Incidentally, I once had a pond with several dozen gold fish in the mountains of Colorado, where we experienced freezing weather much of the year and ice would form on the top of the pond for several months...the fish survived fine and multiplied for several years.

Q: I would like to know what the best product is out on the market these days to attach flagstone to block columns for gate.

A: (Kelly) I would consider one of the mastics that are used to adhere tiles to walls for this job. I think there are several varieties; you might ask for the one with the best adhesion.

Q: I have been trying to lay a slate patio (about 20 by 8) on a budget. I have already put down a sand base and realized I am short on slate. I was thinking of placing the slate further apart and putting mortar or concrete in between the slate. What advice would you have about using mortar. I live in NY and was wondering about freezing issues.

A: It is true that horizontal, exterior surfaces, like patios, stand to get a lot of abuse from the weather. If water can get between or below the slate and the mortar or grout and then freeze, this could present problems. If the slate is simply laid directly on the sand and then a mortar is place surrounding them, it will be difficult to prevent moisture from migrating down between the seams. A well-draining sand may prevent most damage, but it is still possible. Another perhaps more stable approach to laying the slate would be to pour a layer of concrete (say 2" thick) and then embed the slate in it so that the tops are level with the surface of the concrete. Doing this requires more skill to accomplish, since any section must be accomplished within the set-up time of the concrete and you need to know what your slate pattern will be in advance...but it can be done.