Q: I have an existing external deck with the floor boards consisting of what I believe is referred to as "flex-boards". My question is whether or not lightweight concrete can be applied on top of the flooring (providing an edge were to be installed to keep from running off) to serve as a water-proof ceiling & flooring? My intent would then be to lay ceramic tile on top of the cement. I did consider regular concrete, but was informed that it would not be a good idea to pour cement over wood. Also, my builder assures me the foundation structure can support the weight.

A: (Scott MacHardy) I wouldn't think it a good idea to use any kind of concrete over wood deck materially like that, just because of the rot factor on down the road.

Q: I want to make my own lightweight concrete as a slab for radiant floor heating because of the much lighter weight than regular concrete.
Using Vermiculite, what would you suggest the mixture of of mortar / sand / Vermiculite should be? The floor will be finished with ceramic
tiles, and the water temperature in the heating pipes is about 140F maximum.

A (Kelly): The use of any lightweight concrete for a radiant floor system would not be advisable, because the heat from the pipes needs to be able to permeate the entire mass and be radiated into the tiles and the room. Lightweight concrete tends to be insulating material which resists the transfer of heat, so such a system would not be very efficient. You would be much better off to use ordinary concrete for this purpose.

Q: I live in Florida with high humidity and the dangers of high winds from tornados and hurricanes. I am interested in building a home using a poured concrete method. Can the lightweight concrete stand up to high winds ?

A: (Kelly) I would think that many lightweight concrete formulas would stand up fine to the weather, especially if you are imagining domed shapes, which are particularly sturdy in the wind.

Q: I was wondering if the lightweight concrete can ever be made light enough to be used as furniture ? How heavy is it compared to solid hardwood furniture?

A: (Kelly) I haven't actually heard of anybody doing this, but that doesn't mean it couldn't be done. It would take some experimentation. I have heard of people making outdoor tables out of concrete that stand up to the weather very well.

Q: I live in New Hampshire and am looking for a product to use for building a full walkout basement. Is pumice concrete mix a good alternative to a standard poured foundation? Also, need details like cost, r value, etc. I look forward to hearing from you. Going to build in two months so need to make a decision soon. thanks!

A: (Kelly) In general I would say that pumicecrete would be a good choice for a poured basement/foundation because it is relatively insulating (I don't have figures for R-value). The building officials in your area might require regular cement, however, because it has known compression qualities that relate to code requirements.

Q: We are a custom cabinet shop experimenting with concrete counters and are searching for a better/lighter product than standard sand mix. Any advise on mixtures? Will a lightweight concrete be too porous? What is the thinnest application, would 1.5" be workable?

A: (Kelly) The trouble with many lightweight aggregates for concrete is that they tend to be rather porous and/or soft, so that durability of the surface could be an issue. I would suggest some experimentation with various materials, thickness, and possible finish coatings that might make for a harder surface. It does seem possible to me that some combination of things would work well.

Q: I am considering two different designs for a geodesic dome (about 40' diameter) that involve lightweight concrete. The first is to use a rebar/mesh form, which seems pretty straight-forward and do-able. The second (and more interesting) possibility is to make forms for the triangular panels, and mortar the poured panels together to form the dome. Would you have any feedback on the feasibility or drawbacks of these methods?

A: (Kelly) As you say, the first method is fairly straight-forward, and has been done many times in many different ways. Monolithic domes are basically done this way. I have a neighbor who single-handedly made his home this way, mixing all the concrete by hand (and he is in his 60's). The advantage of the approach is that you end up with a more monolithic structure that is less likely to have leakage problems, and the process of building is probably easier.

Your other suggestion I see fraught with difficulties: Geodesic structures are not simple (there are several different triangular shapes to any geodesic), and require very precise measurements to fit together . Even light-weight concrete is heavy, so you need to figure out a way to erect the thing once the triangles are cast. The biggest problem with most geodesic domes is leakage between the segments, and your cement mortared joints would likely be no exception. All-in-all, I would think that this method would take you about three times as long to accomplish, with less satisfactory results.

Q: I need a lightweight version of a limestone fireplace surround mantle. They are now producing cast stone ones from concrete. Would any lightweight product work? It would have zero clearance to the firebox? Would it be okay with the heat?

A: (Bruce Schundler) Not all lightweight concrete products are made from stone aggregates---and not all are immune to high temperatures. Nevertheless, if the aggregates are lightweight mineral, inert and fire-proof aggregates like perlite or vermiculite or other aggregates like pumice or expanded shale, then the concrete probably would be OK and could withstand a lot of heat. The key would be whether there is air entrainment, and whether the concrete is designed for any kind of refractory application. And if the mantle doesn't get particularly hot, then almost concrete made with mineral aggregates would be OK(e.g. again, not polystyrene or other organic aggregates).

Q: I'd like to make stair treads inlaid with natural stone. Local contractors seem lost. Specifically I'd like to make 2.5" x 11" x 60" treads, reinforced with 3 1/2" rebar. I'd like to press 1/2" thick ledgestone into the wet concrete. These 11 treads to be supported by 1/8" x 2" x 2" galvanized angle iron brackets bolted to three 4" x 10" stringers, no risers. This is in southern California, More details than you need. What lightweight concrete formula is strong enough to support normal residential traffic on such a stair tread? Can you suggest a local source for materials?

A: (Bruce Schundler)Usually lightweight concretes would not be used for this. They don't have the same structural or bonding strengths as regular concretes. But if weight really is an issue, a fairly strong 1 to four mix (one part Portland cement to four parts perlite or vermiculite) with fiber reinforcements might work.

Q: Hi, I am in a watershed area in North Carolina. I purchased a home a few years ago and would like to expand the driveway. My problem is that I have a limit to the amount of impervious area I may have on my property. I have researched it and am not able to work around this. I was at a standstill when someone told me to check into porous concrete. Can I use this to extend my driveway? If so, should it meet the porous requirements? And if so, will it look greatly different from the traditional concrete, already in place?

A: (Kelly) I'm not sure that lightweight concrete would meet the standards for allowing water to drain through it...I suspect that it would not. My recommendation would be to use pavers that are designed for this purpose.They come in a variety of patterns and are just assembled like patio pavers, but with more open space to let the water drain. They would not look just like concrete, but they are quite pretty.

Q: Your advice to not use any lightweight concrete to use as a thin-slab for a radiant heating system contradicts the information on many websites, including, of course, manufacturers of supplies (pex tubing and "liquid" lightweight concrete or gypsum). I am at a loss when I read experts contradict each other so flat out, but you have no vested interest, so...thanks for the website.

A: (Kelly) You are right that I have no vested interest in this question, nor would I necessary consider myself an expert. I am really just relating what seems like common sense to me. Radiant floors need to radiate their heat, and anything that interferes with that process (such as insulating the heat tubes from the room) would lessen their effectiveness. Also, the more thermal mass involved, the better the system will retain heat over time and moderate temperatures in general. I can certainly see using lightweight concrete beneath the heat tubes to insulate them from below, and then using a denser material above that to actually embed the tubes. Or, if weight is an issue, then lightweight concrete might be a choice of compromise. I know that radiant tubes can be installed with wood floors, which would be somewhat insulating, but again I would expect the performance to be less efficient and comfortable than using a better thermal mass material. If you encounter any tests that have been done to prove any of this, please let me know.

Q: I live in Southern California where it is very hot during the summer. My asphalt shingle roof is beginning to crack and we had a serious leak during heavy rains in December. I have been looking at many different types of material to replace the shingles. This would entail removing all existing shingles and taking them to the landfill. I have been looking at different versions of concrete and am wondering if putting a 1 inch layer of lightweight concrete on the roof using an artistic design is possible? There are elastomeric coatings with very high UV radiant capabilities that can be painted on to waterproof and insulate the coating. What is your opinion?

A: (Bruce Schundler) Perlite concrete has been used very successfully---both on flat roofs, pitched roofs, and in roofing tiles. And there is one company in California that has developed a roof deck that incorporates a thin perlite layer on top of the regular water proofing of a flat deck, and some of these decks have lasted 40-50 years.What can be done will depend a little on the slope of the roof, and on the structural integrity of the roof. There are three perlite manufacturers in California, and the one in North Hollywood (Redco II--- www.perlite.net ) has been involved with perlite concretes for years. They might have some suggestions and might be able to help.

Q: I want to make my own balusters for my deck. I live in the northeast corner of Wyoming, so we have a freeze thaw factor. I would like to know if lightweight concrete can be poured into molds and if it would hold up in my climate? If so, please advise a formula.

A: (Bruce Schundler) Probably lightweight concrete should not be used. Balusters normally are used to prevent children, dogs, and very short people from falling off decks and staircases. As such, their function is more like a fence, and they need to be able to withstand horizontal and lateral forces, and a fair amount of abuse. Regular dense concrete would not be good for this, and if used, would need reinforcing rods or rebar to make the baluster as strong and safe as it should be---and in this application, lightweight concrete would be even worse in terms of strength, durability, etc. Hope this helps (e.g. hope you don't try to use lightweight concrete), and good luck!

Q: I'm building affordable housing from sip type panels in Latin America, tropical. I'm looking for a concrete base material recipe that is lightweight for a floor of 12' x 20' room. I plan to pour the floor after the house is erected and can use the inner walls as part of my form. The floor will have very light loads. I would like to keep thin and low cost for the market. In some cases, the house will be on pylons off the ground and will be suspended so I would use reusable forms under the floor while pouring. I have available a sisal type natural fiber I was thinking of using as a filler. I would appreciate any thoughts or suggestions you may have.

A: (Bruce) Perlite and vermiculite concretes are used for floor fills and under some floors, but these floors usually are covered with something after curing. The problem is that perlite and vermiculite concretes tend to be fairly "soft" and cannot withstand scuffs, hard scratches, etc.As a result, they are covered with carpeting, tiles, wood flooring, etc. The end result is a very insulating floor, that also resists noise transmission through the floor. They also aren't good for a suspended floor that would require some structural function. Essentially perlite and vermiculite concrete could not be used and shouldn't be used for a floor that is above grade unless it is poured on a surface that can withhold all the loadings normally required of floor designs. Having said all that, they could be used if placed on or over the ground, if the aggregates were readily available from local sources, and if an extremely hard surface is not required.

Q: I recently produced a concrete replica of a human posterior a a joke for a friend. We left the center hollow and provided a hole to allow the posterior to be use as a cigarette butt collector. Numerous people have displayed interest in having one of these "Butt" cans for their patios. Originally I used regular 5 bag with 3/4 minus aggregate. As you can imagine this was really heavy. I was wondering if you could recommend a reasonable priced LWC that would exhibit really good heat resistance wile lowering the unit weight to around 35 to 70 pounds/ft3. The LWC would also have to be moldable and castable.

A: (Bruce) Perlite could be used and the possible mix designs are all on our website (schundler.com) in the "construction" section. Interesting use.....

Q: Can your product be used to pour a new driveway over a old one ? Half the old driveway is cracking and falling apart.

A: I can see no advantage to using pumicecrete over an old driveway, since it might not hold up to the wear as well as standard concrete.

Q: We have an aircraft hangar that has a 6 foot wide and 58 foot long deck. It’s a metal structure covered with ¾ inch plywood. The plywood is covered with a heavy neoprene roofing material. We used indoor/outdoor carpet (glued down) on top of the neoprene roofing; unfortunately it deteriorated after a few years and it was a lot of work getting it off the deck. We are now looking for other options. We want to protect the deck from leaks because it is over our shop area. The deck covers about 3 feet of shop area and extends out another 3 feet; the deck is 12 feet off the ground. During our annual air show the deck receives a lot of traffic. We have discussed using carpet, but would prefer a longer lasting solution. Could light weight concrete tiles be used on the deck? How heavy our the tiles and what shape/size do they come in?

A: (Schundler) Light-weight concrete is used regularly in building as well as restoring flat roof decks. As part of a complete system, perlite concrete can be feathered out to re-angle or fill low spots, but must be protected from the elements, i.e. water, ice and ultraviolet rays. If heavy traffic occurs you can refer to mix designs to ‘beef-up' the psi. We only manufacturer to aggregate. As for light-weight concrete tiles, check with the manufacturer for their psi.

Q: I am interested in light weight concrete which can be use as paving block because I think I can use it in my project.

A: (Kelly) Light-weight concrete can obviously be formed into blocks for use in paving projects, but this would not be my first choice of materials for this purpose, because such blocks would not be as durable over time as denser masonry products.

Q: I am building a 6 ft x 6 ft shower with a mortar (concrete) base. The bathroom is on the 2nd floor. What do I need to use (and in what proportion) to lighten the weight of the concrete? The lighter the better but I also need strength.

A: (Kelly)You can replace the stone aggregate in concrete with crushed volcanic rock, or perlite to make it lighter for your shower pan. I would suggest using  wire mesh to reinforce it as you would for ordinary concrete. You might experiment with different mixes to see what works best. Also, you may want to use a tough paint to keep it from absorbing moisture.

Q: I make large expanded polystyrene spheres with Hexagons and Pentagons, up to 6 ft. in diameter. I would like to make a large dome about 15 ft. in diameter at the base using the same technique as the spheres with the Pentagons & Hexagons. I envisage coating the outside of this shape with some cement mix; somebody suggested adding Vermiculite to the sand & cement mix? I guess I would also encapsulate the cement coating in a steel or mesh frame around the polystyrene shape. Can you suggest a suitable mix that is light but strong please.

A: (Kelly) You might look at the formula suggestions at Formulas. In general, I am sure you will have to do some experimenting to see what works well for your application. Besides using wire mesh reinforcement, you might experiment with the commercial fiber reinforcing material that it used in the concrete industry these days....it would probably be a lot easier to work with.

Q: We want to produce large outdoor sculptures that will be displayed in a public place. People will be sitting, standing, or walking on them, so I am wondering if lightweight concrete is the material to build the sculptures with.

A (Kelly): Light-weight concrete can be a very sculptural medium, and for that reason it might work well for you. On the other hand, it is not as scuff resistant and durable as ordinary concrete, so I would recommend that you stay with this.

Q: My application is for fireproofing gable-end fascia boards on a high-roofed chalet I am building in the midst of the foothill Sierra Nevada mountains in California. I have a unique shape to my fascia boards, which are now formed with 1/2" thick OSB clad to 2x10 rough framing fascia boards. My overhang is 2 feet. I will be boxing in under the overhang and eaves and stuccoing the house. On the fascia boards themselves, I want to clad about 3/4 inch to 1 1/2 inch of lightweight concrete that will be exposed directly to sunlight and weather. I can (will) use chicken wire to cling the concrete to the fascia boards. I will be applying the job myself with a small mixer. What lightweight concrete do you recommend for this job, and how to I find the mix ratio formula?s This house is a completely do-it-yourself, earth-friendly home.

A (Kelly): I actually think you would be better off using a standard stucco mix for the application that you describe. It will likely be easier to mix and apply, and more durable than something of less weight.

Q: I have been interested in lightweight concrete as a building material for quite some time. I am wondering about a more "conventional" use, i.e.: sound deadening in sub floor construction. I need to patch an existing floor where we have had to make a few openings. Also, I would like to apply a layer in my own condo before adding a wood floor. I believe it is possible to hand mix small quantities of lightweight concrete. This would enable a reasonable patch job for the existing floor. My main question is what might be the thinnest practical dimension of the membrane for a sub floor using any "newly available" materials? I need to keep it thin, thin, thin. I have seen a few lightweight sub floors. These, including my patch job seem to be around 1,1/2" but I can't go this thick for my own floor.

A (Kelly): It seems that lightweight concrete is fairly commonly used as a subfloor, and I would imagine that 1 1/2" is about as thin as it would normally be. You might experiment with adding fiberglass fibers that are conventionally used to reinforce concrete to see if that might make a thinner application practical; try this in a test sample.

Q: We live in Washington State and were wondering if we could use concrete (preferable lightweight) as a countertop for our outdoor BBQ center? If so how thick would it have to be and could you lead us to a source for buying the supplies we need and maybe a web link or book to learn how to apply it and stain/texture it? My husband is a builder and is experienced with regular concrete but would need detailed instructions for this new light weight concrete.

A (Kelly): I suggest that you stick with a cement-rich ordinary concrete for this application; lightweight concrete will not be as strong and would tend to be more porous than ideal.

Q: What material would you recommend for making a decorative cover for a range hood. It doesn't need to hold weight, but it must be able to withstand cooking heat from a high heat stove and oven. I can make an armature out of plywood or wire. It is over an island range and will be held up from above, so weight is an issue. I am making this for my own home, but would eventually like to do this as a business.

A: I suppose that it would be possible to fashion a range hood from lightweight cement, if you were very careful about handling it. Masonry materials of this sort are inherently fragile and can crack easily. If you attempt this, I suggest that you make a wire mesh armature and try using perlite as the aggregate. This will take some experimentation. If it were me, I would fabricate the hood out of thin metal, which is commonly done and is not fought with the problems of masonry.

Q: I'm concerned that the metal and concrete will expand and contract at different rates when they are exposed to heat. Does that happen?

A: I doubt that this would be a problem, since steel-reinforced concrete is used in many applications where they get pretty hot, like out in the blistering sun, without breaking apart.

Q: Can lightweight concrete be used over radiant pipes in a home? We have wood floors with new construction.

A: (Kelly) I suppose that lightweight concrete could be used over radiant pipes, but I wouldn't recommend it because this concrete is more insulating than standard concrete, and this would inhibit the movement of the heat.