Structural Concerns?

Question: I am working on an old adobe (130+ years) and need to know the maximum height for a 12in. thick wall. Would a poured in place concrete beam help to strengthen the wall.

Answer: Maximum wall height is ten times the wall thickness. That is called the aspect ratio and has been known since biblical times. It is in some codes although many times a code just states that certain brick sizes are suitable for one- or two-story construction. A poured in place concrete bond beam would indeed tie the building together and spread roof loads on the wall. In NM wood bond beams are also allowed and might be easier to do. Three layers of 2" lumber or four layers of 1-1/2" lumber give the required six-inch high by 10-inch wide bond beam. Boards are staggered especially at corners to give interlocking lap joints.

Question: I am an architecture student. I will design a project outside of Phoenix, Arizona, made out of earth. I want to know what is the maximal height for a tower out of earth, and which will be the best structural material if it is not possible to realize one tower (possibly with a rectangular base), only with earth for a height of 20 floors for example.

Answer: The highest existing adobe buildings are in South Yemen. They are seven- to eleven- stories high. The Tower of Babel may have been higher. Adobe has an aspect ratio of 1/10. That is the wall height can be ten times the thickness of the base. To go one-hundred feet high, the wall at the base should be ten-feet thick and should be battered (tapered) to ten or twenty-inches thick at the top. If you built a round tower the total diameter of the tower might be considered as a basis for the aspect ratio and the walls could be thinner allowing for an internal stair or habitation area, but I would not know how to calculate that. Right now the New Mexico building code limits construction to two stories.

Question: I have a friend's home in Carmel CA which was partially built with adobe bricks. We are having a deterioration problem with the walls. One wall acts also as a retaining wall so is subject to constant moisture, the other walls are typical exterior walls. I would appreciate it if you could direct me to some source of maintenance information to deal with this type of problem.

Answer: Ageless Adobe: History and Preservation in Southwestern Architecture by Jerome Iowa, 1985. This history of architecture in the American Southwest presents conservation and preservation of adobe buildings and suggestions for solar options. Photographs and drawings make this a document of unusual value for all interested in preservation, rehabilitation or construction.

Question: I'm looking at building a hybrid adobe/Earth sheltered cabin that would be built half into a hill. If I were to choose this approach, what measures need to be taken to protect the adobe that is bermed, if any?

Answer: Shouldn't build adobe below the finished grade.

Question: What sort of insulation and structural design would you suggest for the ceiling/roof of the structure?

Answer: Heavy timbers/joists lots of straw, twelve inches of dirt.

Question: We are building a home on Bowen Island Canada. We have had to build over an area which is partly fill. It is pretty solid material, and we can remove all organic material, put some rubble in and so forth to stabilize it and prepare for the adobe flooring, but the process is costly. It looks like it may be cheaper to build a standard hanging wood floor with a crawl space, instead of building directly onto the sub-floor as we had planned. We would still like to use the adobe flooring, with radiant pipes in it. What is the experience base of building adobe onto a wood subfloor along with the standard gravel, vapor barrier, insulation etc. Does the very slight give in the wood make adobe untenable?

Answer: Adobe does just fine over wood subfloors. It will take longer to dry due to the required vapor barrier below it and moisture can only move upward. There is no need for gravel. The techniques for the floor would be the same as pouring a concrete floor over a suspended wood floor: it has to be very strong to handle the dead load of the adobe which at three inches thickness will be about 30 to 35 pounds per square foot added to whatever the dead load of the wood system would be.

Question: I purchased a ranch with an adobe roof on one of the buildings. The roof leaks. Without walking on the roof (makes more leaks) is there something I can spray on the roof to retain most, if not all, of the look and seal the leaks?

Answer: Oddly enough, more dirt might help. Sometimes it blows off or washes off in a rain. If walking on it makes things worse, then it could be that the boards underneath have gotten rotten and dirt falls through. In that case, add more dirt carefully. Sometimes we have just had to remove the dirt, replace the boards and then put the dirt back or in worse cases, use modern foam insulation covered by a hot mop roof. Lots of old dirt roofs in the Santa Fe area got a hot mop roof on top. I can't think of any easy fix that can be sprayed on. Asphalt driveways can be sealed with cold process asphalt, also known as emulsified asphalt. It is used also to stabilize adobe bricks. Home Building Suppliers usually have it. I have no experience with it but it might be worth the experiment. Farm ponds are sometimes sealed with a Bentonite clay but I think it has to be continuously wet for the Bentonite to work.

Question: We recently moved to the grasslands of Sonoita, Arizona. We are considering an adobe patio wall, parts of it will be a retaining wall. Is this a problem with adobe, especially if it has some concrete in the block and if I seal it on the earth side of the wall?

Answer: The New Mexico Adobe Code does not allow using adobe below grade. Adobes need to be on a concrete foundation that extends 6- to 8- inches above the finished grade level. Adobes have been used as retaining walls historically and sometimes they last a surprisingly long time. Nevertheless, neither the Code nor I recommend it. Adobe walls have little resistance to lateral forces so if the run of the wall is long and the height of the soil is significant (12-inches or more) the wall would be expected to move before long. There is not much on the market that can truly seal off a wall from moisture.

Question: The person who is helping design the yard suggested the adobe and thought we would have to add rebar for the strength needed. I wondered if drilling the adobe would ultimately weaken it, even if we used some concrete in the adobe mixture. Would that possibly work? Also, we have about 19 inches annual rainfall, so that might present another problem with the longevity of the wall. If adobe is not a realistic option, what would you suggest for a natural looking wall that will blend with the environment?

Answer: (Kelly) You are right that drilling holes through the adobe blocks would either weaken, or more likely actually crack the adobe. I agree with Quentin that this is not a good material for this purpose. My suggestion would be to build the wall with natural stone, either dry stacked or mortared. Dry stacking has the advantage that it will allow the wall to weep water if necessary, so pressure will not build up behind it. It takes some experience to do a good job of stacking stones for a retaining wall, so either read up on it, or find someone with experience to help.

Question: I have exposed viga posts and I have found that these posts are rotting as well. These are vertical. Apparently, the footing was poured too low and now the bottoms of he posts are decayed. The worst one (as far as I can tell - since I can't see into the interior of the posts) has lost about 1 foot of length at the bottom. This has to be replaced, I know. I think I should replace them all after pouring the concrete footer up a little higher - above the brick porch floor so no water ever contacts the posts. I understand there is a metal viga base that can be used to cover the concrete which would otherwise show.

Answer: It would be relatively easy to support the porch on each side of each post with 2x4's or 4x4's while the rotted part of the viga posts are cut off. A sharp carpenter could make a form for a concrete pilar that would come up to the bottom of the intact viga. With a little care the pillar can be designed with a slope on it's top surface all the way around so that any rainwater that hits it will drain off and not puddle up under the viga. The concrete pillar could have beveled edges to make it look neater or it could have flagstone stacked around it to conceal the concrete. In fact the pillar can be built with stacked, mortared flagstone. A rebar from top to bottom of the pillar would be in order.

Question: Do you know of anyone who would be willing/able to do this and do it correctly, in this area?

Answer: I have had the experience of dealing with recently arrived Mexicans who do not present themselves very well but turn out to be spectacular workers and problems solvers. Or you can do pickup truck profiling:
2004 Dodge Diesel Dually: Too expensive, you have to help him make his payment this month.
1976 Ford with the tailpipe dragging and two fenders bent: A bit careless and fly-by-night.
1998 Toyota T-100 with a lumber rack and no more than 30% of the dashboard covered with paperwork: Perfect.

Question and Answer: My husband and I are making an offer to by a 1939 Tudor cottage style home in Phoenix Arizona. The home is made of adobe and has an adobe roof, (Probably clay tile. Adobe roofs are flat and very rare in USA) or so says the agent and the listing information. We like the house and we LOVE the idea of living in an adobe house but how does someone who isn't an expert find out the condition of the adobe both in the structure and the roof? (There is a lot of adobe construction old and new around Tucson. There should be some knowledgeable home inspectors in the area. Sorry to say, I am out of touch with the Tucson scene.) The structure is painted and looks solid but the roof has tiles that look like shingles and are curved upward. They are a whitish color that has black residue perhaps from tree residue or who knows what that is? (Leaf detritus is often the culprit on roofs.) I did a search for adobe roofs...hmmm. There were very few pitched roof adobe houses except in the Tudor revival style but no mention of adobe in the roof. What is you experience with this? (As above) What questions would you ask the seller or even better the inspector to make sure that we are purchasing a sound home with a roof that will last. (I am sure that the roofing material is something that most Tucson area roofers would be familiar with and would be able to evaluate. If you have a good roof and a good foundation, the wall material in between will last forever whether it is adobe, frame, metal or masonry. So check the foundation also.)

Question: I own a thirty year old adobe house with a flat roof and would like to consider putting on a pitched metal roof. The house is a single story on a concrete slab. The walls are adobe and are one brick thick. I know that this is difficult to answer without seeing the home but but do you have any thoughts or advice? Is the weight a problem? Do the parapets need to be removed?

Answer: Should you be in the state of New Mexico, the adobe part of the building code has been recently interpreted to mean that fourteen inch walls are required below if the pitched roof system includes any second floor living space. I got one retrofit accepted on the condition that the pitched roof start right above the vigas on ten-inch adobe walls below. Interpretation and enforcement of the code rules are not consistent. You may be in an area where a permit is not needed. The short answer, then is that you can feel pretty secure in adding a pitched roof to your home. It is a routine addition to homes here in the high country of NM. I would indeed remove the parapet and begin the pitched roof on a wood plate shimmed to level and well nailed or screwed or lagged to the vigas. If you do want attic space we should continue the conversation.

Question: We are replacing the covered porch supports of our Boulder, CO home. We are installing upright vigas and corbels with a cross beam that supports the existing 2x4 rafters. The vigas will sit on the concrete pad of the porch. (We have been planning to hammer drill holes in the concrete for large pegs to go up inside the viga and down into the concrete.) We are trying to figure out attachments - How to attach the corbel to the viga, how to attach the corbel to the cross beam and how to attach the bottom of the viga to the concrete pad. We have looked at pictures and can't see any hardware or obvious means of attachment. How is it done?

Answer: Use a long lag screw that gets six- to eight-inches into the viga. You may want to counter-bore the corbel so that the hex head and washer under the lag screw are not above the corbel. Screw up from the corbel into the cross beam using 6-inch deck screws or timber screws or lag screws. Pick the place where it will show the least and again, counter-boring may be useful. You can counter-bore enough to fill the hole with wood filler or even a wood plug. If you can work from above it will give a neater look to lag from the beam into the corbel. Try to find something to go under the viga post to keep it above the concrete and standing water: flagstone; a metal base such as those made by Simpson Company which specializes in metal connectors and is found at most any builders' supply; or a round poured cement shoulder. Drill into the concrete and epoxy in a piece of rebar that will go eight-inches or more into the viga post. For a 1/2-inch rebar, pre-drill the viga at that diameter. The knurling on the rebar makes it larger and the viga will have to be pounded down onto the rebar. A Simpson 6 x6 post base can be modified to do the entire job but you might have to think up some trim at the base of the viga to hide it. If you are pouring new concrete an anchor bolt and Simpson post base would be easiest. A clever welder could make you a similar unit that was round - for a price. If you cannot drop the elements down from the top and have to shoehorn them into position, it can all be toe-screwed together with those 6" deck screws. Predrilling with a slightly undersized extra long drill bit will allow you to get the screws in just where you want them before the Phillips head screw strips. Counter-boring can make it all look neat. Timber screws are also great, but they have a larger head that is harder to hide. They are hex heads and never break or strip. I use TimberLok brand from OMG FastenMaster. 800-633-3800. It is a pleasure to answer an actual construction question.

Question: I just finished an Adobe floor in Tucson AZ. It is beautiful, but I could use some advice. We laid down an octagonal form made of 2x2's. we leveled the floor and pounded in the adobe. The problem is that the adobe , although packed against the form but not above, had seemed to pull away from the form and the form is "popping up" in a few places. Do you have any suggestions on how to solve the problem. I have thought of re packing the adobe and pre drilling the form and possibly finding some long screws to act as an anchor. Please help.

Answer: The form remains in place? If so there are 6-inch deck screws that might help hold it down depending on what is below it. Several companies also make timber frame connectors up to at least 10-inches if not longer. Where the adobe has pulled away from the form you can pour in a thin slip of adobe mud to fill the space. We just did an adobe floor today and poured it about 5-inches thick. The weather is still cool and we have no idea how long it will take to dry out.

Question: Restoring 100+ yr. old adobe in San Jorge, Nicaragua - the oldest house in the pueblo. The walls were repeatedly patched with concrete, and one side was plastered with concrete. This is a humble farmers house, so the walls are rustic and uneven. Can we replaster with adobe over the concrete patches or must they all be removed (ARRGH!) and replaced with mud. Suggestions to improve plaster adherence?

Answer: Adobe plaster usually sticks very well to cement/lime based plaster or concrete. If not a little wheat paste glue as used by wall paper hanger or made by someone's grandmother on the back of the stove will help. Elmer's white glue or its equivalent can also be used. Sometimes, there is great satisfaction in pulling off concrete and replacing it with adobe mud.

Question: We are purchasing a home in Penasco, New Mexico. It is an adobe home with a pitched roof, very old but in good condition. My husband and I are fixing it up to what we want, so some walls need to come down (old adobe houses are like mazes). But we aren't sure how to take an adobe wall down with out damaging some of the adobe for future use, any suggestions?

Answer: Have to make sure the walls are not holding anything up, like ceilings. It is not terribly difficult to enlarge doorways or windows with bigger lintels or even arches. Some folks remove bearing walls by placing a large beam under the vigas or joists and supporting the beam with posts or masonry columns at the ends and sometimes in the middle. Wear a hardhat.

Question: I'm a construction consultant in Santa Barbara, Calif. and I have the challenge of mitigating water entrapment that appears to be migrating through the walls of a beautiful adobe building here. I found your site through a search engine that pointed me to the article by Quentin Wilson that you have posted. In that article Quentin mentioned a product called Okon W-1 and W2. I thought it might be prudent to talk to him or to yourself and ask if there are any other products that you might recommend for application to the exterior of adobe walls to restrict or inhibit the migration of rainwater.

Answer: This situation can get complicated. If the moisture is seen in the lower portions of the wall, it may be rising from the foundation. Many buildings have had various types of impermeable materials applied to walls only to find that they hold in water that would otherwise be evaporating from the wall surfaces. If it can be determined with certainty that the water source is rain then there are several exterior treatments that can be made. I am not sure if Okon W-1 and -2 are appropriate for exterior surfaces but certainly one of their products should work. One advantage of the W's is that they are essentially invisible. The best product is one that is waterproof to stop incoming rain but vapor permeable so that any moisture that gets into the wall has the opportunity to get back out as water vapor. My recollection is that a permeablility of 5 or 6 perms is desirable. It is not easy to predict the permeability of a field applied material. Usually perm ratings are determined in a laboratory using manufactured building materials. El Rey Stucco in El Paso has a product called Adobe Sealer which they claim has appropriate permeablility. It may now called Crown Seal. The specifications seem the same.

Question: I am purchasing an old adobe place in San Fidel, N.M. that has 3600 sq. ft. of floor with 2000 sq.ft. of it with no roof. All Adobe walls and well worth renovating. One 800 sq. ft. room has for rafters, logs, that have been incorporated into the adobe walls themselves, and the logs have rotted out and need to be replaced. And since money is a problem I was wondering would it be cheaper if I were to remove the logs entirely and go to a more conventional type of rafter or dig the logs out of the wall and just replace them with newer logs? And any suggestions on a sealant for where the logs make contact with the adobe?

Answer: Conventional rafters might well be cheaper as long as they are big enough to hold up the roof load. Usually the logs (vigas) are more likely to be intact in the wall while they rot in the open air of the ceiling system. Lots of the old pueblo and Spanish buildings still have the intact stubs of the vigas in the walls while the rest of the roof system has disappeared. If the original vigas protruded outside the exterior surface of the wall, then that can be a source of moisture and rot. As for treating the new beams or vigas, there is little stuff available that will kill varmints, rot, or bugs that I would want to have inside the house with me. I would consider soaking wood in a solution of Boraxo, 20 Mule Team. It is effective but if it gets wet the Boraxo is washed out. Therefore, protecting the wood from moisture is very important.