Best roof for a cob circle house

So I may have access to 18 acres in Eastern South Carolina and was considering building a cob roundhouse with a diameter of 12' and a wall height of 12'.

The biggest problem I'm having so far is coming up with the best roof design.

I like the idea of a roof like this, but am unsure of what the center fixing is called, or even this kind of roof style.



Does anyone have any suggestions for a first time builder?

I was also thinking about a shed style roof, but worry about the elements on the higher side, plus it eliminates ceiling space.

Questions, comments, and ideas are much appreciated!

Why not try a Reciprocal roof? It can be made with most types of timber and for a round house would be ideal. Look it up on the net and I also think that there are a couple of videos on u-tube. Good luck.

The biggest thing I worry about with the reciprocal roofs is that it will just be me and maybe one other person working on it frequently, and I fear that may impede on my ability to make it.

I do think I came up with a good roof structure though.

If you know of any step by step tutorials on the reciprocals I'd love to see them and learn more about the process of making them.

Maybe somthing like a Yurt roof.? or maybe a design with a center pole.?

Cactusdan wrote:
So I may have access to 18 acres in Eastern South Carolina and was considering building a cob roundhouse with a diameter of 12' and a wall height of 12'.

The biggest problem I'm having so far is coming up with the best roof design.

I like the idea of a roof like this, but am unsure of what the center fixing is called, or even this kind of roof style.

Does anyone have any suggestions for a first time builder?

I was also thinking about a shed style roof, but worry about the elements on the higher side, plus it eliminates ceiling space.

Questions, comments, and ideas are much appreciated!

Anyone know if recycled concrete rubble could be used effectively for the stone foundation?

I agree with dugs. Reciprocal roof is the way to go!

What will you use for the actual roofing material over the reciprocal frame?

H Ludi Tyler wrote:
What will you use for the actual roofing material over the reciprocal frame?

I think you hit the nail on the head there. As cool as they are, finishing them is the problem most people will face.
I'm hoping to using one for the cabin. My plan is to do a light weight earth roof.

For a first time builder a Yurt style roof like Cyric30 suggested would probably be easiest.

You would place a bond beam at the top of the cob wall, ideally concrete and rebar, or alternatively airplane cable connecting all the members of the roof in one big circle. They way the roof is designed is to act in compression, while exerting force on the bond beam which will act in tension. There is not really any need for a center post taking up space in your cob round house.

check out this link below. its a similar idea, but for a earthbag round house.

http://www.instructables.com/id/How-to-Build-an-Earthbag-Roundhouse/step10/Build-the-Roof/

Hate to say it, but I'm going to disagree with just about all the advice previously given.
I suppose I've got to give my creds first, just so you know I'm NOT some kind of upstart loudmouth. I'm a Natural Builder, been at it for a bit over a decade now and have been teaching with Cob Cottage for 7+ years. Cob's NOT the only material I use but, seeing as how it's the "duct tape" of Natural Building, it's old hat to me now.
Ok, on with the rest..

Urbanite (recycled broken concrete) makes for GREAT foundations. You can stack it like good building rock and it's FREE which is good. It takes a little getting used to as it has it's own character but it ain't all that hard in the long run.
Concrete is passable as a foundation and it's fine to stick cob directly on (as in, mud on TOP OF concrete) but NEVER, NEVER put it on top of cob! DO NOT build a concrete bond beam on the walls, DO NOT use cementitious (stucco and friends) plasters! Portland cement is hydrophilic, it WILL wick and sweat moisture to the cob. Concrete plasters ESPECIALLY will tend to hold moisture against the wall(s) and hide the damage from view until disaster strikes. A tour of California's missions will prove the point, stucco and concrete patches on the old adobe walls has become the NUMBER 1 failure point of those buildings. What looked like a good idea in the 50's and 60's is now the death of the buildings, or will be if it's not stripped off again soon.

Reciprocal roofs look like a good idea at first, but the reality is that they can be fiendishly complicated to roof over. Unless you plan to use a VERY flexible material over it, like thatch or a pond liner, you'll find yourself deep in compounding difficulty VERY quickly. As a first timer, I'd stay away from reciprocals (thatch too), heck I don't do 'em just cause they're a headache and I'm not new to building.
Yurt style roofs are doable, though a bit difficult for a first timer. The key(s) are two-fold: first you need a good hub connection that won't allow rotation of the rafters and second you need a strong cable around the rafter tails to tension them in. A temporary center post, or better yet, center scaffold (so you can get up and down to work there) is necessary, otherwise it'll be really hard to get the thing together.  Good "dead-men" are important to hold the roof on (down) and can be a bit tricky to place properly and attach.

Oddly enough, even on a perfectly circular building, gable roofs are the EASIEST to build. They will slightly break up the feeling of the perfect circle (which I think is good) as the gable ends will necessarily be higher than where the rafter tails land. Some of the beauty of the gable is that rafter spans are short so you don't need hefty wood anywhere but the ridge beam, the rafters DON'T NEED to be on a level plane and can be "flown" about which can make for a very organic and interesting looking roof and they are the easiest to build which makes sense especially for a first timer. On a circle, at the gable ends, it can be a little difficult to get the overhang that you need. Rafters can be splayed, wider at the tails than at the ridge and something I've been calling a "shoulder beam" can be used. Shoulder beams sit on the wall, run parallel with the ridge beam and stick out from the building, their function is to get those last couple (or even one) rafters to hang out in space and provide easy attachment and support for eves.

Ok.. This post has gone on long enough, I've gotta go out and stack some urbanite today. If you got more questions I'd be glad to attempt an answer.. I typically check in once (sometimes twice) a day.. Anyhow, out.

I second the gable roof. I've been planning my cob cabin for years, and after examining many roofing options and building a practice hut, I've decided to put native logs on top of the cob as beams, then straight commercial lumber as rafters, then plywood, roofing felt, and steel. Also going with an urbanite foundation.

Awesome advice guys!

I actually am almost finished building the first layer of the urbanite foundation/stem wall, and I think I will be going with the gabled roof.

Would you recommend mortaring the urbanite together with cob to seal any holes?

Cactusdan wrote:
Awesome advice guys!

I actually am almost finished building the first layer of the urbanite foundation/stem wall, and I think I will be going with the gabled roof.

Would you recommend mortaring the urbanite together with cob to seal any holes?

Stuffing the holes with cob works well, though I'd recommend a sand/clay mix thats HEAVY on the sand (like here, I'm using 4 to 6 parts sand for each part clay soil) with no straw as a mortar. Ya gotta mess with it, do tests and whatnot as the soil in one spot ain't like the soil in another. The mix should have as much sand as you can reasonably get in and just hold together when you squish it, it's OK if the edges are crumbly when dry but it should mostly become a hard little ball.
It needs to be at least one if not two courses above ground level to keep it out of rising damp, etc. If you need mortar below grade, go for cement/sand or lime/sand.
Remember, mortar is for holding the stones APART, not for sticking them together.. Yes, modern mortars will stick them together too but DON'T BUILD IT WITH THAT IN MIND! Use as little of the stuff as possible.

Donkey wrote:
something I've been calling a "shoulder beam" can be used. Shoulder beams sit on the wall, run parallel with the ridge beam and stick out from the building, their function is to get those last couple (or even one) rafters to hang out in space and provide easy attachment and support for eves.

How much of the wall does the shoulder beam sit on - just the very outside of the circle, or....?

H Ludi Tyler wrote:
How much of the wall does the shoulder beam sit on - just the very outside of the circle, or....?

As much as is needed to hold up the rafters. Generally, the more on the wall it is, the better. If the curve is too tight to allow a good seat for the shoulder, I'll run it inside the room a little (across from end to end, parallel with the ridge).
It's good practice to dead-man the shoulder beams down well. Great way to attach rafters without the need to dead-man every one, leaves room to change rafter placement right up to the last minuet, while providing secure attachment anywhere along it's length.

Oh, been meaning to ask..
Why a circular building??
I tend to steer clear of true circles,  they're difficult to turn into good living spaces (especially if the plan is to partition off internal "rooms") and provide little in the way of comfy, secure, "being in" space. (if you know what I mean)
I've found (independently, though I've read others that agree) that circular rooms (buildings) are great for meeting or sacred spaces but not generally restful enough for home-space. No place to truly come to rest. What the human critter tends to like is little alcoves, sheltered corners and whatnot where one can find rest from active energies, etc. It's why I advocate for curvilinear, lobed buildings, lots of shelter out of the way where no one need try to rest in the aisle or passage. Circles tend to be ALL aisle or passage.
There ARE exceptions, VERY small circular rooms for one or an intimate two can work very well.

IMHO, Yurts done the western way make little sense. The Mongolian Yurt, with all it's traditions and ways intact, is the perfect appropriate technology. OTOH, the American copy, sans climate appropriateness, sans the ALL IMPORTANT yurt ventilation system, sans ancient tradition, made from plasticized canvas and bubble wrap insulation -- well, it's a disaster.. OK, disaster may be too strong.. How about less than appropriate and uncomfortable in most circumstances..??

Thanks Donkey.  I wish I could see a diagram or something of the shoulder beams and especially of the deadmen.  I'm familiar with deadmen holding thing UP, but not DOWN. 

In my case the round building is a to surround a round water tank and the fact that I really want to make a round building because I like the shape. 

Well my building will be quite small at about 12' in diameter, and designed to room just myself and maybe one other person.

I like the different shape, as well as the supposed stability of a circular house.

I'm planning on putting a loft ~7' up, which should maximize living space, and was considering having storage that would give that "room" the feel of being a niche.

I'm having trouble visualizing a gable roof on a circular house, cob or otherwise. Can anyone point me toward some pictures?

I've never built a round structure or worked with cob, but I think a reciprocal roof matches the look and functionality of a round cob house perfect. Here is a simplified walk thru that I like. How to build a reciprocal Roof I don't think the actual framing of it is any more difficult, even including the dead men, but the actual roofing material would be tricky since its a constant curve. But I would think it would be perfect for a living roof.

I can sort of picture how a square gabled roof would fit on a round cob house, but I have a hard time not thinking it might look a bit odd. It would certainly be easier for roofing with metal or other conventional means. I would think it would look better, and function better, if the shoulder beam was on the outside of the circle but extended a ways out and there were posts outside the structure at the ends, allowing a generous overhang. I think that would look more deliberate and also serve to protect better, and allow for greater surface for catchment. Just my thoughts.

I have put a lot of thought (but not much actual practice) into this. I have discussed this with my teens alot, as I am trying to get them to build their own houses and not dig a debt hole to dig out of when they get to be my age. I would make a center column from a metal culvert pipe, fit stainless woodstove pipe up the middle, then fill it with gravel for mass. Probably wrap a water line around it for hot water, too. I was thinking of a 16 ft diameter, though, which makes a free-spanning loft a real stretch.

Why not build 6' riser walls in a 12' circle, then top with a geodesic dome? That way you still get your 12' height but combine walls and roof to achieve it. A 12' hemisphere is a cinch to build alone. Heck I put up a 24' one once by myself in one day. (Just the frame, not the sheathing.) You can finish with a pool liner to waterproof or cover it in 3 layers of chicken wire and make a ferro-cement roof. Guaranteed not to blow over in a tornado!

This is a good handy dome calculator if you are interested in seeing what it would take to build. You can use anything from conduit to saplings or regular standard lumber. A conduit dome that small would likely cost you less than $40 to frame. http://www.desertdomes.com/domecalc.html

There is a step by step tutorial on building a conduit dome on that site too.

Deb, I really like the idea of a riser wall with a geodesic dome on top. I've been mulling over various ways to roof a roundhouse, myself.

And the ease with which one can build a geodesic dome for a topper is a bonus for the owner/builder.

Max Hubbard wrote:Deb, I really like the idea of a riser wall with a geodesic dome on top. I've been mulling over various ways to roof a roundhouse, myself.

And the ease with which one can build a geodesic dome for a topper is a bonus for the owner/builder.

Yeah, domes are so simple and cheap to build (not to mention incredibly strong and storm-proof) that I really don't know why more people don't build them. Small ones are great super-strong and safe shelters for goats or other animals; and they make great storm and root cellars too. If you dig the circle down a few feet into the soil and build the dome inside that, then cover with dirt and plant, it will blend seamlessly into the landscape, like a small hill. It also will not be going anywhere and it will stay whatever temp the ground is year round. Of course, you will want to figure some options for drainage just as you would for a basement.

Putting a dome on a riser wall helps overcome some of the problems associated with the roof curve in smaller domes (like hitting your head before you get within a couple of feet of the edge), and the outside appearance is really neat. Think observatory or short silo. It is easy enough to add a wrap-around porch to one for shade and keeping rain from running down the walls. Just construct the porch as a free-standing unit lightly fastened to the house -- like a hat brim -- so that in a really big storm, it will break away and leave the dome undamaged. If you build it into the structure itself, you risk having wind catch up under the ovehang and lift the dome off. We live in an area with a lot of tornadoes and straight line winds so I think about these things a lot.

Domes aren't very common because in the long run, they're not very practical.
They are difficult to divide into useful spaces. Covering them properly say with plywood leaves behind a LOT of waste. Sealing up ALL of the seams is devilishly difficult and the things tend to leak EVERYWHERE. The MOST expensive parts of a building are the foundation and the roof. Domes are ALL roof!
In the book Shelter, Lloyd Kahn (one of the big proponents of domes who worked on the Domebooks) wrote a refutation of his earlier works called "Smart but Not Wise". I think this is an EXCELLENT title and pretty much speaks for itself. Check it out.

On the shoulder beams, I forgot to come 'round for more explanation.. So Here's a picture that might clarify a bit, it's not a round building but you can see how the shoulder works pretty clearly.


You can see that the roof isn't "square".. Yes, a square roof looks really odd on a round (curvilinear) building, but if you don't build 'em square they look kinda cool.



This roof uses a pond liner on top to make waterproof, so we could get away with a really low slope (in case yer wondering).
First, several layers of cardboard, which insulates and protects the pond liner from puncture, then the liner, then loose straw and dirt.. It's a VERY thin "living roof", so thin that it's mostly NOT living as it gets dry here in the summer and I won't be watering it.

Kirk Mobert wrote:Domes aren't very common because in the long run, they're not very practical.
They are difficult to divide into useful spaces. Covering them properly say with plywood leaves behind a LOT of waste. Sealing up ALL of the seams is devilishly difficult and the things tend to leak EVERYWHERE. The MOST expensive parts of a building are the foundation and the roof. Domes are ALL roof!

Apologies to all for digressing somewhat from the main topic here, but I feel I need to respond to this...

Kirk, With all due respect, I disagree with everything you have said here. Those are the usual anti-dome arguments that, despite evidence to the contrary, continue to be bandied about -- usually by people who have either never built a dome or haven't built one properly. Like any other structure, there are good examples and bad examples.

Well-built domes, in general are very practical structures. They are money-saving to build because they enclose more space with less material than conventional square structures; they are highly energy efficient; they are much sturdier and safer (especially in tornado or hurricane-prone areas) than square structures; and if the builder/owner has any imagination or creativity at all, they divide beautifully into much more interesting interiors than square buildings are capable of. Also, building a dome on a riser wall cuts out the headaches of having to place tall furniture against compound curved walls. (One of the other arguments typically touted against domes.)

As to roofing... covering them with plywood WOULD be a waste! Why would anyone do that when there are so many more practical options for roofing and sealing a dome? Properly sealed ferro-cement does not leak, and makes a much better dome covering. (Coating with sodium silicate -- waterglass -- is another, highly durable, permanent option for waterproofing over cement.) There are also a variety of flexible membranes available that work as well inverted over a dome as placed in a hole to make a waterproof pool. If you have enough space available, and deep soil, you can even berm a dome and put a living roof over it.

As for expense comparisons... the argument is not valid. It is like comparing apples and oranges. Saying that the most expensive part of building is the foundation and roof may be true of conventional building but it simply does not apply to alternative structures. Conventional roofing IS expensive. Dome roofing is not.

Deb,
Respectfully, I HAVE built domes. I've lived in them and am NOT green at this.
Yes, a little creativity can solve all of the problems that come with domes. My personal opinion is, why create a list of problems that then need to be solved? To be fair, ALL building methods have their own problems/issues. My criteria runs something like this: does this solution (building method, whatever) solve more problems than it creates, and do the new set of problems outweigh the ones I was trying to solve in the first place? For me, for the most part, domes fail this test.

I find ferro cement to be uncomfortable to live in. It tends to sweat or become a condensation point, doesn't breathe well, has unfortunate acoustic properties and so on. Plastic sheeting (of various types) have their own issues, some similar to cement, some not and who wants to live in a plastic bag?

I HAVE seen/lived in domes that were TOTALLY beautiful, highly efficient, comfortable, dry, well laid out and ALL that. I find them to be the rare exception, NOT the rule.

Lloyd Kahn was pretty much the guy who wrote the books, plural, on domes. He disassembled and sold his dome because of the many issues with it. A dome over cob ignores the fact that cob should have a large overhang to protect it from rains. Just because, mathematically, a dome is stronger than some other structure ignores the fact that polygonal domes, as built by humans, are generally pretty weak because of the actual framing connections. They are nearly impossible to make without extensive waste, and also require high tech materials to make them work. Look into traditional shapes and forms from indigenous peoples - they have innate beauty, and often great practicality for the area they were used in. Wigwams are the traditional domes, but adapting a bent-pole structure into a modern home seems tricky.

Matt River wrote:A dome over cob ignores the fact that cob should have a large overhang to protect it from rains.

In that case, use wood frame & straw bale for the outer walls, and cob for the inner ones. That gives outer walls with a high insulative value to keep it warm in winter and cool in summer, while the internal walls have high thermal mass that will keep the temperature stable.

Tim Crowhurst wrote:

Matt River wrote:A dome over cob ignores the fact that cob should have a large overhang to protect it from rains.

In that case, use wood frame & straw bale for the outer walls, and cob for the inner ones. That gives outer walls with a high insulative value to keep it warm in winter and cool in summer, while the internal walls have high thermal mass that will keep the temperature stable.

Am I wrong (I AM green at this) to say that doesn't solve the problem because shouldn't straw bale have a large overhang as well?

Anyway - I've been mulling this over myself, we're going to be building a 33 foot round hobbit house similar to the Simon Dale house and I was thinking we'd do a reciprocal roundwood-framed roof, but I'm curious about the gable roof. Given that we're going to be berming the house and doing a living roof, it actually makes sense in a way because it would help runoff initially, but the problem is we're planning to "add on" another roundhouse (kinda like phase 2) that's near overlapping just a bit so I'm not sure how we'd do that roof-wise.

Seems like a gable roof would send runoff right to where we're planning on building the 2nd part, moreso than the rounded effect of a reciprocal roof?

Simple gables solve a lot of problems..
Trying to stage out buildings using reciprocal roofs is gonna be a real bear.

And yes, wide eves are quite important for cob or strawbale buildings.

Kirk (or anyone else who knows),

My wife and I are planning on building a cob home, and I find the task of putting on a roof to be the most daunting. I agree that a gable roof, especially here in Oregon, is my best bet but I have a few questions:

1) What is a good distance b/w rafters and what is an appropriate diameter on the rafters? (If anyone knows of a good book on basic roof construction I would love to know).

2) How far can the beams/rafters span unsupported?

3) Is it necessary to have a deadman for every rafter if you use a shoulder beam?

4) What did you use for sheathing and does it double as the ceiling?

5) How far should the rafters overhang the walls?

6) Does the ridge beam have to be on level or can it be higher on one side (have a lofted side and save on building materials on the other with lower walls), and would this create any additional issues if possible?

Never built before and want to make sure I don't make any more, or serious, mistakes than I already will. Thanks for your patience.

Joseph Davenport wrote:Kirk (or anyone else who knows),

My wife and I are planning on building a cob home, and I find the task of putting on a roof to be the most daunting. I agree that a gable roof, especially here in Oregon, is my best bet but I have a few questions:

1) What is a good distance b/w rafters and what is an appropriate diameter on the rafters? (If anyone knows of a good book on basic roof construction I would love to know).

That depends on the span, what kind of wood you are using and a whole lot of other factors. I like really light roofs and so I often pre-stress them. Using compound curves and forcing planking around those curves lets me build roofs that are incredibly strong with very light materials. 'Course, here I never expect to deal with a snow load..

2) How far can the beams/rafters span unsupported?

Same answer as above.. Round natural pieces, as a general rule are stronger than square of the same dimension, all things else being equal.. Milling wood tends to wreck it's natural strength by cutting into the grain.

3) Is it necessary to have a deadman for every rafter if you use a shoulder beam?

Nope. One of the beauties of the shoulder beam system is that you only need to dead-man in the shoulder beam (and the ridge), which is great because it leaves exact rafter placement open till the end.. I'll often dead-man down several long(ish) pieces on top of a wall, (kind of like a shoulder beam) to get this same effect, this gives for good attachment and support to the rafters AND leaves exact rafter placement flexible. When you place a dead-man for each rafter, you lock down rafter placement at that time, with limited ability to change things later.

4) What did you use for sheathing and does it double as the ceiling?

I've used all manner of things.. One of my favorites lately has been redwood planking that was milled on my property by a local guy, in trade for wood.. It was a great deal, stacking the functions of light to the garden with (free to me) building materials and contributing to a more local, sustainable economy..
Yes it did double as ceiling..

5) How far should the rafters overhang the walls?

Here you get (yet another) "it depends" kind of answer.. What are your needs?? Usually they will be shorter on the south, over your heat and light gain windows, longer everywhere else. they can be extended over doorways to keep the rain off your neck while you fold the umbrella and fumble with keys.. My thumbnail is 2 feet, adjusted from there.. Longer eves can help to support longer internal rafter spans (within limits).

6) Does the ridge beam have to be on level or can it be higher on one side (have a lofted side and save on building materials on the other with lower walls), and would this create any additional issues if possible?

If you are using a pond liner for water proofing, everything can fly around, NOTHING needs to be on the same level.. Thats the beauty of that kind of system, it becomes easy to do some REALLY interesting things.. You can build with extremely curved pieces that draw big arcs, with a large slanted ridge, etc, etc.. The more extreme the shapes, the more flexible your sheeting material needs to be... It can become difficult to keep dirt on top of some of the more interesting shapes but just about anything becomes possible.

Never built before and want to make sure I don't make any more, or serious, mistakes than I already will. Thanks for your patience.

The main thing to remember is to start small.. If you build something VERY small to begin with, with an eye toward expansion later, you will be FAR better off. You can get a good idea of what is involved without getting in over your head too deep, you can have a reasonable expectation of finishing it in one season (or at least a short time) and all of your decisions become MUCH lower in risk level... As a first time builder, with the first building (especially), it's best to reduce the risk level of your choices.. Small buildings REALLY help here, and a whole lot suddenly becomes possible.
I can't stress this one enough. BUILD TINY! Give yourself room to expand, add on, whatever. Over time, you can expand to something much larger. But keep it small for the first go 'round.

Haven't seen a recent post from Cactusdan but am curious if someone in South Carolina actually has been able to obtain a permit to build a cob home? Know several people wanting to build them in western S.C. near Greenville but have found no precedents with the permitting department. VERY DIFFICULT!

I have never heard of a reciprocal roof before so after doing a fast search I found lots of info and they look amazing specially if the beams are exposed on the inside. Would look great on a cob or strawbale home, or any project that requires a self supporting roof system. Here is one site that I found that shows how to build from start using temporary supports.
http://simondale.net/house/frame.htm

Seems like the best, most efficient, easiest to build roof for a round house would be a geodesic dome made out of emt (extruded metal tubing) google "desert domes" or check out kits from "pacific domes". A dome would more than double your interior volume, be super strong, easy for two people to build, have no need of interior load bearing walls, plus the dome shape naturally circulates air.

This is the exact same size and shape (circular) abode that I intend to build. I've done a ton of research on this, built spreadsheet models, CAD drawings, etc. A two-story, twelve-foot radius roundhouse with a loft breaks the 1000 square foot barrier for house-versus-cabin. If you have a center post (one only), not only will you find it FAR easier to build a roof with something to pin to, but in addition you will reduce your horizontal clear spans to twelve feet, a very important consideration in putting in second stories or lofts.

People have lived in circular houses and teepees for ages and have yet to find themselves desperately seeking square angles, like they had a gene for that or something. You build-in everything on the walls . . . not like Americans, who stack their furniture in front of the walls. The walls are rock-hard, you use them, and you don't build sideways, you build up. You put planters around the central column. You build-in a rocket mass heater, heating a couch, run it halfway around the circumference, exit with a stove pipe and put your stove right there, using the same exit stovepipe. Stairs go the other way around. There is plenty of perimeter to work with. Use those walls, put shelves and pegs everywhere above eye level, for instance.

As for the roof, go with a 3-4-5 triangle (53 degrees, or rise of 4, for run of 3, for cant of 5) because that is the traditional roof pitch for all ancient roundhouses, for many and various excellent reasons. (Mainly, if you don't do it right in the first place, do you have the money and time to do it all over again?) DON'T be so stupid as to make a roof with a big hole in it.

Let the roof overhang the walls by at least a yard to protect against rain and give it a dry, high foundation. Slope the ground away from the building and dig ditches to carry away any water. Never use concrete finishes like stucco, or oils, or paint, or sealants on it. They trap water inside. That or any roof leak will ruin it.

What you use on your roof depends on your climate. Shingles or roof tiles are okay for temperate climes, to keep rain and the occasional snow out. Yurt roofs are fine providing you repair or replace the canvas and felt yearly -- and people also lift these roofs up, slide inside and steal your belongings. For Alaska, I would definitely consider two-foot thick thatch and an overbuilt roof to handle heavy, thick snow load. Don't plan on snow just sliding off. Does it just slide off your car windows?

You said you were going to build your roundhouse to house a water tank? Are you kidding? You can't tar it or plaster it?

i built a roundhouse spring 2012 with a reciprocal roof here in west tennessee... it was straight-forward design and roof construction built up by my self in a day... the entire home was constructed using only hand tools with the single exception of a battery powered drill... the roof rafters are 6" diameter on average at base and 18 1/2 feet long... this gave me a nice 4' diameter skylight and little over 3' eaves... the roundhouse is not a 'mudhouse'... after reading much about alternative construction material (cob, cordwood, bales, tires, etc) i decided to go with 4' x 8' wall panels... 20 panels will give a diameter of about 25 1/2'... i chose reciprocal over the compression ring used for yurt roofs mainly because of the band needed for yurts to hold it together vs the self-supporting feature of a reciprocal roof... the ONLY problem i had with the recip was trying to use bale wire to tie together... found out quick that the poles want to slide down... working by myself, it was simple and much better to drill thru the overlaying rafter and the one under and drop a 10" bolt to pin them...

I did a thatched roof on a composting toilet...the Spiral Chamber, a wattle and daub spiral. It is pretty sweet.
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