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Roofs for Strawbale Buildings

Dr. Owen Geiger, Ph.D.( in Social and Economic Development,) is the former Director of Builders Without Borders and current member of the BWB Steering Committee. Dr. Geiger is Founder and Director of the Geiger Research Institute of Sustainable Building (www.grisb.org). He is an author, engineer and licensed contractor specializing in strawbale construction and other types of sustainable building. He co-authored the Builders Without Borders Straw-Bale Construction Guides and contributed to Building Without Borders: Sustainable Construction for the Global Village. Dr. Geiger has consulted on numerous international housing projects, worked closely with Habitat for Humanity for seven years and mentored housing officials with the United Nations Institute of Training and Research. He is also a correspondent for The Last Straw Journal. Dr. Geiger's Global Straw-Bale Construction Certification Program provides high quality strawbale training via a unique program that combines hands-on experiences with research and assignments; this is a distance learning program for those within reach of the internet and with an adequate knowledge of English. See www.grisb.org for more information.

Questions and Answers

Q: I hope to build my house "Santa Fe" style in the following manner: 1. bales flat, not on the side (I'm hoping this will provide more structural strength) 2. flat roof, i.e.,
a. walls to ~9'.
b. plywood bond beam.
c. 4x6's on 24" spacing, spanning no more than 16'.
d. plywood sheeting over 4x6's.
e. bales laid flat over plywood, over the whole roof.
f. 1-2" earthen plaster over bales with slight incline for run-off.

Climate - Southeast Arizona, annual precipitation ~10". Note - I'm not hoping to build a house to last a hundred years, 40-50 years will suffice. Q. Do you have any advice for or against this type of construction using straw bales?

A: It's an interesting concept for low-cost houses where life span is not important.  However, the bales will decompose in a few years if left exposed.  Then what will you do? I'd spend a little more money on parallel chord trusses (and metal roofing).  You could make them or buy them.  Place the bales between the trusses.  This provides excellent roof insulation but also keeps the bales protected.  You'll also have a stronger truss.  Remember, bales are fairly heavy.

Q: I am especially interested in traditional, natural, renewable, British Reed Thatch - which has been in use for at least 3000 years, and, to some extent, Cedar Shakes, the North American Thatch. The only mention you make on the site is safarithatch.com which apparently doesn't even offer Thatching Materials!!! Just plastic 'look alike' copies & thatch 'shingles' that must be used over a waterproof membrane, since they are only 99% waterproof!!! Completely loosing one of the best qualities of Thatch - it's breathability! And, to me, even 'only' a '1% leak' is 100% unacceptable!!! Especially since I want to use it over Straw Bale!

I've been considering all the options I can find. I want a system that is 100% waterproof and breathes... so far only Thatch and Cedar Shake seems to fit the bill, without building an energy inefficient 'vented/cold roof system', which also requires more timber, sheathing, fasteners and other materials. Can You offer any other suggestions? I believe that both Reed Thatch and Cedar can be fastened directly to purlins. By the way, I'm thinking about a Straw Bale Vault design, where the roof and walls are 'one' - so that the purlins serve double purpose - for lateral structural ties and roofing attachment points. Of course, with straw bale, breathability is paramount !!!

A: (Kelly) You make some excellent points about the importance of roofs and roofing materials and concepts. This is one area that needs to be developed further at www.greenhomebuilding.com. Do you know of any sources for genuine thatch and related products? A breathable strawbale vault is a difficult proposition. If you were building in a fairly arid environment, you might experiment with a papercrete plaster, as I have done with my earthbag home. The papercrete is completely unsealed, and thus breathable. It also absorbs a tremendous amount of water, which it holds until the sun comes out and evaporates it. The papercrete is much more hydroscopic than straw or other earthen materials, so it would actually draw moisture out of the bales, if it ever got that far. I have never had a drop of water get through my earthbag domes, and the walls are completely breathable. This is a completely new concept for a roof, where you actually absorb the moisture, rather than repel it!

Q: I live in Oklahoma. Lots of very hot humid weather in the summer and cold and humid in the winter. Thinking of building a straw bale home. Been reading some of your Q&A on-line. I have a place to build with good drainage and have decided to build up off the ground about 2 to 3 feet. I like the adobe style house with no overhangs shed style porches in front and back. The main house and both porches will have a shed style sloped roof, with metal sheeting. Also stucco walls inside and out. Would like info on the best type of stucco and coloring, also the best ways to do a metal roof over a bale house.

A: Sorry to break this to you, but a Santa Fe style house with no roof overhangs is doomed for failure in a rainy climate. Even in dry climates like New Mexico (where this style is popular), these type of homes experience significant moisture damage. It's not a matter of if there will be moisture damage, only when. And by building with straw instead of adobe, the risk is greatly compounded. This is true no matter what type of stucco is used. With proper roof overhangs, humidity is not a problem with strawbale (unless you live in a tropical rainforest). Then it would make sense to use materials that will not rot.

One option: Investigate the farmhouse style that's popular in northern New Mexico. It uses gable roofs with roof overhangs that would protect the bale walls. This style started about 100-150 years ago when pioneers realized the drawbacks of the Santa Fe style.

Q: If I put over hangs on the house and made a few roof changes, what
about the metal roof and can I build a load bearing wall about 12 ft. tall?
Also out of all these books I see on straw bale houses, what would be the
best one for me to get?

A: Almost any design/style with adequate roof overhangs will work for strawbale. Provide an adequate pitch on the roof to help shed the water. I would use at least a 4:12 pitch. And add a moisture barrier on the top of the walls in case there are any roof leaks (but not on the sides of the walls).

Best resources:
1. The Last Straw Journal
2. More Straw Bale Building - code issues, insurance, etc.
3. Build it With Bales (only available through Dirt Cheap books/Charmaine Taylor) - best how-to information.

12' load-bearing walls: Yes this is doable, but it is tricky. I would add external pins on both sides of the wall as it is built, lay the bales flat vs. on edge, keep the length of the wall to a minimum, tie-in interior partition walls if possible, and use temporary bracing. Depending on the size and the design of the wall, you may even need some extra bracing. Above all, make sure your bales are very dense so they don't compress too much. Get "builder-quality" bales that are recommended by experienced strawbale builders. Use tie-downs to
pre-compress the walls. Allow time for settling. Don't make the door and window openings too big, etc. Use caution!

Q: My husband and I live in a strawbale/solar home in southwestern Colorado. We would like to add on using a combination of log and frame. The addition would be 2 stories, with three walls of log on the first story and one wall of frame (the one that abuts the existing strawbale), and four walls of frame on the second story (one of which will also abut the existing). What special considerations would we need to make to ensure the integrity and breathability of our existing strawbale?

A: In general, you shouldn't have any problems.  But, as always, use common sense and good design details.
- Vent bathrooms, washer/dryer, etc. (anything that can generate excessive humidity) to the exterior.
- Pay extra attention to merging roof lines.  You don't want one roof shedding lots of water onto another roof.  And you don't want water from the roof splashing up against a wall.  (These problems are more common on remodels because less thought often goes into the design.)

Q: I live in a typical 1960's built wood frame bungalow, in Alberta, Canada. We could use better insulation in the roof. Can we use straw bales instead of the usual bags of pink insulation? Its dry up there, but we have no idea if this is even an option, ie weight, bugs, etc.) We live in farming country and access to unlimited bales! Hope this isn't a silly question!

A: Yes, you can use straw bales as roof insulation.  First, you need to check the strength of your roof system.  Make sure it's in good condition (no rot, termite damage, cracked boards, etc.  Are there any very long spans?  Do you see any sagging ceiling joists or trusses?  Do you have partition walls that help shore up the ceiling?  Check for roof leaks.  You don't want those bales getting wet!  Also, this may be a good time to reroute wires and cables.  The bales would make it difficult to get to them later.  And you may need to lay some boards across the ceiling joists for the bales to sit on.  And, of course, you'll need a good sized scuttle hole to move the bales through.

The main risk is fire.  Stay clear of chimneys and stove pipes.  Besides all the normal things you should do in any home, I would smear a coat of mud on each bale just in case.  This will also deter insects and pests.

Q: We are considering building a tepee strawbale house. It would be 40x40, in order to keep the roof looking natural is there a light weight concrete or a stucco that will keep the bales dry? We live in Upstate NY.

A: It sounds like you want to build a tapered straw bale structure that relies on plaster to protect the bales.  This will not work.  Bales need to be protected by a roof structure.

Q: I'm a builder in VT where winter snowfall can get several feet deep and frost averages about 5ft. I own a house site on a steep down slope where I want to build a strawbale dome type structure. I will be excavating into a steep hillside that will bear the front of my dome. The straw bale walls and roof need to be load bearing so I plan to wrap both sides with rebar and wire mesh. I'm considering spraying on several inches of lightweight concrete to both sides of the straw bales instead of stucco/plaster. The only bale wall exposed to full sunlight needs to stand about 20ft. tall and has to support the weight of the 2nd floor and our domed bale roof. The baled roof when done will be covered with soil and wild grasses about a foot above present road grade. My idea is for an off-grid energy efficient dome/shell design made entirely from straw-bales sandwiched with steel rebar, wire mesh and concrete. I do not plan on using any studs or joists for the exterior wall and roof structure if possible. I am open to suggestions on how to best compensate for drainage while allowing bales to breathe. Can straw bales when sandwiched with reinforced concrete be used for this purpose? Will bales carry weight of a curved roof design plus loads from soil and snow? Do I need worry about water/moisture seeping through the exterior coating into the straw bales?

A: It's widely agreed that bales are not suitable for domes.  Sooner or later (probably sooner) moisture will find a way in.  Strawbale construction is great, just don't expect the material to do the impossible.  Bales need a good roof and adequate overhang to keep moisture off the walls.

I highly recommend using earthbags filled with an insulating material like scoria or perlite.  Any water that finds its way in won't hurt anything.  You can also save on the concrete and rebar.  A properly constructed earthbag dome is very strong and doesn't need it.  Regular stucco would suffice.

Maintaining a living roof is a lot of work.  Think this over carefully.  I say this because I have a dome with a living roof and I baby sit it almost every day.  It takes regular maintenance.  I love my dome but many would tire of the ongoing work.  Living roofs are best on gently sloped roofs with slow runoff.  Water flows off a dome very quickly.  This washes away soil and nutrients, and dries out the soil.  Consider using trays or flats of potting containers filled with drought resistant plants.  The containers will help hold soil in place.  You'll also need a sprinkler on top.  You'll also need to figure out how to secure the trays in place to prevent wind damage.

Q: I couldn't think of any viable options for roof insulation because I was handicapped by cost and preference.  I am adamantly opposed to "the pink stuff" and all its cousins, so that's not an option.  The natural alternatives like denim and blown cellulose are more than I plan to spend. But then I found your article. Whew!  You've saved the day.  Rice hulls and perlite are far more palatable than my alternatives...and the concept fits my razor thin budget. Do you have any roofing materials that you prefer over others?  The place where I'm building has many cedar trees so I'm planning on cedar shakes if I can get the hang of making them.

A: You can add borax to the rice hull insulation to make them less attractive to mice and other pests (possibly added in a slight clay slurry and tossed on like salad dressing). Alternative roofing materials: tire shingles, recycled aluminum printing plates, terra cotta tiles (possibly recycled).  We know of one gentleman in the desert southwest who used layers of plastic and old carpet covered with clay soil on his house.  This isn't ideal, of course, but it worked in his area where it is very dry.  I've also heard of people in Mexico coating their roofs with clay coated with lots of cactus juice, linseed oil, etc.  Again, this may only work in very dry climates.

 


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