roof question for an earthbag building

I am building an earthbag house as a charity project in the Philippines, and am a bit unsure of the roof...perhaps someone can tell me if it's ok. Total cost with everything including labour is about $3 to $4k USD...the roof is the most expensive part.

I am trying to figure out if this is safe enough...a simple A frame using 2x4 tubular steel bars spaced at 6 ft intervals with a 2x4 ridge beam at the top and welded to rebars coming out of the concrete tie beam (6 inches thick) with 2x3 C Parlins at 600mm center spacing and long span PBR steel 26 guage roofing at 14ft long each side...the width of the house is 21 ft, the length is 32 ft

a few questions for you, to help guide me:

1.) are you particularly attached to the welded metal truss idea?  Seems odd to have big metal roof on a earthbag building.

2.) what are the load specifications for you region of the phillipines?  I presume that the roof mainly has to  deal with dynamic loads from high winds.  This to me suggests it is more important how the roof is connected to the walls and bond beams.

3.) are there other long-lived buildings in the area that you can look at to gleam an appropriate structural system for the roof?

Ain't an engineer, just a lowly bagger, framer and welder. This isn't a math equation until you state the thickness or guage that you are considering, let alone your loads. If you can't find a local engineer, find you a metal building/carport place and talk their in house engineer, or call local metal roofer. Or run the calcs yourself. Heck, just rafters, there are table charts for that. Are you going bare tin ceiling, vaulted ceiling or dropping a ceiling for attic?

It looks like you are depending on the 32' long sides to hold the roof rafters from spreading. The little ties near the top are not enough to make the 2x4 members into trusses. I don't see the design as shown to be safe; if the walls ever spread a few inches in the middle, the roof could collapse.

What makes wood undesirable for roof framing? I suspect it would be much cheaper than steel, but of course you know the local conditions. Is there a reason for completely avoiding posts in the center of the building? One or two posts to support a ridge would make the structure much more stable, and a failure of one rafter would not propagate across the whole roof. This could be a lifesaver in a hurricane. I agree that the anchoring to the walls would be the most significant factor, after the actual strength of the rafters themselves.

I had the same problem when I built my sheep barn; vast and expensive roof area. I have wood so I combated my problem using a 2x6 rafter, mated to 2x4 studs, braced with 2 x 4's and connected with plywood. Don't let the plywood scare you, a few sheets goes a long ways. These are built as bents on the ground and raised truss style (flipped into place) every 4 feet on center. Total cost was $4450 for a 30 x 48 barn.

But of course this is the USA too and maybe there is not wood in the Philippines???

Lots of good observations.  Generally, your joists need to be 8 inches deep and no more than 4 feet OC using 2x4 purlins (deep, not flat)  Wood is general far more available and easier to fabricate.  Sheathing with 1/2 plywood gets you a better structure and you can use much lighter metal roofing or even tar paper.  Use a fair pitch, as the Philippines gets lots of rain.  Make sure the connection to the walls will resist substantial wind uplift.  You cannot rely of gravity alone to hold the roof in place.  Even 1 x lumber will work in a site built truss at 24 inches OC, fully sheathed.  Seen them standing in Idaho for over 100 years.

As designed, your roof will likely fail.

remember these are metal trusses, should be stronger than wood 2x4s by a lot.  I think the main thing you need to add is collar ties down low, just on top of your bond beam to keep the rafters from spreading, you could also do them every other one, and keep the collar ties up high on every other one.  On your gable ends you are good already because the spreading load just pulls the bond beam end to end (assuming your rafters are tied to the bond beams and the bond beams are well reinforced. once you do collar ties, you can run a vertical post up to the ridge and you've got a simple truss.  i've seen long trusses functioning well made of 1/2 or 5/8" rebar in the 3rd world tropics. you might consider that option if you are having the things custom made.  Dont go wood in philippines, it will be expensive unsustainable hard heavy tropical hardwoods UNLESS you just get the sustainable pollarded or coppiced eucalyptus poles.

Now i see your truss spacing is 6 feet, I would recommend the extra collar tie at bond beam height and post up to ridge on each truss, also good would be one long diagonal brace welded to the underside of the rafters on both sides of the roof.  The roof metal gives you strength in this dimension also but its light gauge. so extra would help.  And make sure you are well attached at every point to keep the roof from blowing off in big winds.

Christopher Steen states that he "aint and engineer", But I am! and I think that so far that Mr. Steen has given you some of the best advice.   If you want even crude engineering  advice.  Get a digital camera and a ruler.  and send some pictures of the components with the ruler in every picture aligned either vertical or horizontal  or better yet two rulers positioned so sizes can be estimated, e.g. stick the rulers in good places with tape that does not cover up the increments on the ruler.   I still have no credible idea of where and how you are using the rebar.   Take a picture of the end of each structural member with the ruler in the picture so that the cross section of each member can be better understood.   When you say 2x3 "C" purlins, are you talking about hot rolled steel channel or a piece of sheet metal formed into a C.  when you talk about
When you reference the "concrete tie beam" that is 6 inches, I am risking a guess to think that it is what you show in yellow on top of the walls but you have said nothing about how much rebar is in the the concrete and in the picture that yellow thing (if that is what you mean by the "concrete tie beam" ) what is shown is rectangular and both its dimensions for width and thickness would need to be known.  

the 2x4 steel tubes are utterly irrelevant to the strength question without knowing the thickness of the tube walls. and any calculations would have to be made assuming the lowest possible strength unless you can provide a material specification.

If the metal is thin, it will take a pretty good welder to be able to make a good weld  is not prone to crack or tear.  If the members are galvanized they will need to be ground clean or better yet acid stripped (maybe paper towel soaked with acid and then wire brushed then neutralized ) where the welds are to be made to prevent bad welds.  If not properly made the welds may just break apart from metal fatigue due to expansion and contraction from sunshine heating and cooling as well as breezes.  

It sounds like you may have usable materials but how it is all put together can make a huge difference.  Also if this is going to be in a costal location corrosion in any weld joints is a great cause for concern.

Mr. Steens recommendations for how to come up with a workable design may be more within your ability to come up with answers, however I would recommend that at least you take photographs of how it all goes together because you could end up building in a real "Achilles' heel" in what could otherwise have been a strong structure.  

Just a suggestion for your welder.   Weld up one such truss, and then clamp the pieces for all the others to that same truss before welding so that they all have the exact shape. (although any other sort of jig to assure accurate uniformity could work as well.  (but a careful welder would know that anyway).

From what you have provided no ethical engineer would try to ok what you have described  but one might be able to with better information be able to tell you if something definitely looks bad (though still not agreeing that everything is for sure OK.

I find it odd that you have given dimensions in both Metric and U.S. units of measure.  I am wondering if your 2x4's are not 50mm x 100mm

By the way is your steel, scavenged or is it new material?  

If you have asked for assistence on this because you think your level of experience is insufficient   Based on the sketchiness of the information you gave, your instincts are probably right.  and your concern for human safety is commendable.  I have seen disasters created by others who failed to ask.

Best of luck for success.

Lots of info to go through here, but I wanted to say that yes, Kiln dried wood is far more expensive than steel here, probably twice or more...A 20ft rectangular steel tubing of 1.5mm wall thickness, 20 ft long is 750 pesos (about $15 USD). The shanty houses around this little project basically cobble roofs on their houses using coco lumber which rots or has termites within 2 years and galvanised metal sheeting which rusts really quickly as well...termites are a huge problem, hence the use of steel for everything. Steel framed windows, doors, beams, so hardly any wood used. The roof is 4 mil thickness ( I think that is 26 guage?), and C parlins are 1.2mm wall thickness, 20 ft long. The beams will be welded to rebar coming out of the concrete tie beam. I made a modification to the design and will have the same 2x4 steel tubing beam act as a central ridge Using 2 x 20ft lengths welded together, and also using 7 trusses, spaced at 4.5 feet apart. Yes, I want an attic space at the end of the house (about 10 ft x 20ft across), but that will not be hanging from the structure..3/4 inch plywood will sit on the concrete tie beam, with the same 2x4 tubular beams supporting it.

The thickness of the walls are 17 inches wide as is the tie beam concrete, and 6 inches deep. The rebar is 12mm (1/2 inch), which goes down about 4 feet into the wall through the tie beam. The tie beam will have 2 parallel rebars embedded along the length and breadth and they will be tied to the vertical rebars. The steel tubes will be welded to 2 rebars at each point in the wall, plus both rebar connections will be bent over the bar itself

We tend to mix both metric and imperial here - I wish they would go one way or the other, but I am getting used to it now. Most of the time larger measurements are in feet and inches and small measurements in mm (like steel thickness).

I realise I could use a rafter going across the entire width of the structure, and might have to go with that...but would be nice to still keep the cathedral style ceilings..

EDIT...After a bit of discussion, we will probably stick to a skillion shed roof, with a 2ft rise over 20 ft...enough for the rain to come off into the gutters at one side...10 x tubular steel beams at 1 meter spacing, and C parlins at 600 or 900mm spacing. I will keep the more complex stuff for my next project..

Mark,
Please reference the images at the bottom of this post. it is a picture of my tractor shed. It is a prefab metal building commonly sold here in the open ended configuration  as a "car port" and seeing as how it came with the house (which already had a nice garage.  I disassembled it and put it on a concrete slab to be a tractor shed.   It is 18 feet wide and 20 feet long so it appears to be in the same size range as what you are building.  the structure as you can see is 4 preformed arches each consisting of 2 side posts and a single piece arch and a couple of ground rails as many times these are simply installed on a gravel pad.   At the joints between the ground rails and the posts and the posts and the arch, an internal sleeve is welded into one member and simply slides into the other member and is secured from separating with 2 to 4, heavy, self tapping sheet metal screws and at the corners a channel piece is added. Plus an additional channel piece at the top of the arch.  The second image shows the attachment of the 0.020 sheet metal and its ribbing pattern.  The major ribs stand out about 5/8 of an inch and are about 1.5 inches wide.  Just looking at the first picture that the 5 arches in the 20 foot length means that these panels are spanning 4 feet without any purlin support.  The sheet metal is attached with #14 x 7/8 neoprene gasketed self tapping sheet metal roofing screws into the arches and posts.  about 2 weeks ago we had an ice storm and out of curiosity I measured the ice thickness and computed that the roof of the shed was holding up almost exactly 1 ton of ice.

I measured the size of the square tubes (square because they are easier for the manufacture to form than rectangular tubes and much more resistant to twisting)  I believe that this structure bears enough similarity to yours that we can at least tell if you  are in the  ball park.
The square tubes of the posts and arch measure 2.55 inches on a side and have a 0.053 wall thickness (almost the same thickness that you have)

When engineers work with structural members about the two first things we want to know is how strong and how stiff the members are because of their shape.   (both stiffness and strength also depend on the material of the member as well as its shape.

The stiffness against bending loads depends on something called "the second moment of area" or sometimes the "area moment of inertia", and is represented by the letter "I "
The strength against bending loads depends on something called the "Section Modulus" and is represented in formulas by the letter "Z" or sometimes "S"

To see if your design is even in the same ball park with my tractor shed I calculated both of these for the structural members you are using and the those used in my tractor shed.  Your roof truses use rectangular members and they have a strong and weak direction for bending and stiffness   the results I came up  were
tractor shed arches were .................................. I = 0.550, Z = .432
Your truss members in the strong direction ...... I = 1.493, Z = 0.747
Your truss members in the weak direction .........I = 0.511, Z = 0.510

So compared to my tractor shed your truss members in the direction of primary load are about 171 % stiffer and 73% stronger

mark vernon wrote:
..... rectangular steel tubing of 1.5mm wall thickness,
.......  4 mil thickness ( I think that is 26 gauge?), ........ C parlins are 1.2mm wall thickness, ........The beams will be welded to rebar coming out of the concrete tie beam. ...... the same 2x4 steel tubing beam act as a central ridge Using 2 x 20ft lengths welded together
.......7 trusses, spaced at 4.5 feet apart.
..........Yes, I want an attic space at the end of the house (about 10 ft x 20ft across),
but that will not be hanging from the structure..3/4 inch plywood will sit on the concrete tie beam, with the same 2x4 tubular beams supporting it.........
The rebar is 12mm (1/2 inch), ........
The steel tubes will be welded to 2 rebars at each point in the wall, plus both rebar connections will be bent over the bar itself

mark vernon wrote:We tend to mix both metric and imperial here - I wish they would go one way or the other,.

 I notice you refer to something as "4 mil" in industrial practice saying "4mil" 0.004" inches or 4 thousandths of an inch, one mil being a thousandths, as in a 4 mil polyethylene sheet.    But I have run across some people who think 4 mil means 4mm which is  0.157 inches !  since you are using a mixed metric imperial system I suggest you be more precise in your specification.

mark vernon wrote:The steel tubes will be welded to 2 rebars at each point in the wall, plus both rebar connections will be bent over the bar itself

Please be very careful with this.  
A. remove all galvanizing before making the weld.
B.  Placing a round bar against a flat and welding it there is a very CRAPPY weld design.  Yes I know that some people do it and get away with it but it is still a crappy design because where the round lays against the flat there is a steadily tapering crevice that makes the weld size very deceptive.  I have seen some heavy steel come apart this way on a walking beam oil pump and kill somebody who was inspecting it.
C. In the case you are describing there is a severe mismatch in the thickness of the two pieces being welded whch means that the thin one will be destroyed or the real thickness of the weld is much thinner than it looks and if the thin piece is not burned through or away the thick piece did not get enough heat to make a good weld.

If this kind of geometry must be welded, (thin flat to thick round) it would be better to put a thick flat between the pieces and make two welds,  E.g. in your case put a 1/4-3/8 in thick flat piece against the thin flat tube wall and weld with a small weld at the edges and end, then put the rebar against the flat piece and put in enough heat to melt well into the rebar.  

Just sticking ther round against the flat side of your tube and tack welding it would be ok for a tack weld.  So if you are bending the rebar around the tube in a way that would hold without the weld and then just tack welding it to prevnt slippage is probably ok.

mark vernon wrote:I realize I could use a rafter going across the entire width of the structure, and might have to go with that...but would be nice to still keep the cathedral style ceilings..

I have some concern for your trusses versus the walls.

As I understand your truss design there is no link between the bottom end of the rafters.  when they are supported at the bottom ends of the rafters and a load is placed atop the roof (e.g. wind load or ice load etc) the bottom ends of the truss will tend to move apart.   even a very slight movement could prove troublesome for the walls.  Even a simple rather small tie rod between those ends would  greatly reduce this. Nor would the tie rod have to be on every truss Such tie rods  could prove useful for suspending a light fixture or ceiling fan.  but if any significant weight is to be hung then the connection should be something sufficient to constitute a ceiling/floor joist

a single rebar dangling from the rafter above and attached to the tie rod could provide a very strong support for a point load and properly painted and done with an eye for appearance could look rather appropriate.   If there is any HVAC unit to be put somewhere this might prove very practical.

My ice storm experience with 4 foot spans of well ribbed sheet metal might be cause to question if you even need purlins.  In my case the concrete slab was poured with a 1/4 inch per foot slope for easy cleaning and in this climate that is quite sufficient to run the water off the end of the roof.  by cutting the end of the pulins into tabs they could be butted against the rafters and attached with sheet metal screws.   This might prove more stable.   Also there are reasons to prefer "Z" purlins to "C" purlins.  butted purlins would provide more support to the rafters for more stiffness in the direction of the ridge pole.

mark vernon wrote:After a bit of discussion, we will probably stick to a skillion shed roof, with a 2ft rise over 20 ft...enough for the rain to come off into the gutters at one side...10 x tubular steel beams at 1 meter spacing, and C parlins at 600 or 900mm spacing. I will keep the more complex stuff for my next project..

I think that there are pluses and minuses to such a change but I have not given that much thought so far.

Apologies for typo's I am being rushed.

Two quick comments:

1, Putting an (expensive) traditional roof on a bag wall structure defeats the purpose of the whole "easy simple and low cost" approach of earth bag building. Additionally it makes the building vulnerable to high speed winds which might be an important factor in the Philippines. If it is not too late, it is a good idea to consider a vaulted earthbag structure. If you are interested I will be happy to assist with the design  

2, If you still want to put this roof on the bag walls, you will have to anchor the bond beam to the top three layers of bags with rebars. Also these layers should have a small (5-15% depending on your soil composition) cement in the mix. On the other hand if you mix cement in the top three layers, you stagger the bags or use a continuous tube, reinforce them with the aforementioned rebars, depending on the strength of the dried mix the bond beam may be unnecessary, since those three layers can act as a bond beam.

Good luck and please post some pics!

Here are a few pictures and also the sketchup diagram of the planned roof trusses. Will use 2x2 1/4 inch thick angle bars for top and bottom, with 1.5x1.5 x 1/4th inch for webs. This is what the welders are used to doing here. The cost of 20ft angle bars is $15 and $11 each respectively. They are connected to the walls using 2 pieces of 1/2 inch rebar coming up through the 4 inch thick concrete tie beams, and welded and bent over the truss. On the back wall, it will be more earthbags, and on the ends, we will add glass bottles in lime plaster.

Now that I see the picture you supplied I do see one thing that causes me concern.   I do not have particular experience with earth bag walls or the matter of how much cement you may or may not have in the filling of the bag. But I do see that you hav dug this building into a hillside and that the earth is piled up something like half  or more of the height of the uphill wall.  The fill material against that wall appears to be loose and permeable but there is not indication of what was done for drainage of the fill against the wall.   That same wall also has one of the longest runs without bracing (such as a pilaster).  Water in the soil could put imense pressure against that wall pushing in the wall's weakest direction.   As I say, I am unable to see what was done for drainage of the fill against that wall.  If it is well equipped for drainage there may be no concern at all.   If the slope of the roof is toward the downhill side of the building that will help but if it is toward the uphill side (as shown in your "sketchup") it could prove disastrous. There is an old saying that the first three principles of civil engineering are Drainage, Drainage, and DRAINAGE !!!  Although that is a humorus way to put it, it is not any the less true.  

Just a good coarse rock fill (I.E. crushed rock 1 to 2 inch size (NOT SMOOTH ROUNDED GRAVEL !) with a drain tile dug down below floor level might prove entirely adequate, especially with diversion channels around the ends of the building.  Sloping the roof asto the down hill side would facilitate rainwater catchment.   The catchment could be done with the roof sloping toward the uphill side by digging barrels into the ground at the uphill corners and providing for their over flow and a good gutter system along the roof edge, provided the occupant knows to not skimp on gutter maintenance.

If no such drainage provisions have been made, the retrofit is simple though probably would be labor intensive.

Other potential issues that I see in examining the photos include:

-- The fact that there is a great deal of slope at the building site and the immediate surroundings.   The ground looks to be permeable to rainfall and and that raises concerns as to retaining walls.  Doing retaining wall correctly is not particularly difficult but doing a retaining wall incorrectly is also very easy and having built a few of these I am well aware of the potential for failure and sometimes disasterous failure.  With a proper retaining wall it might have been far better to obtain a level building site by setting a retining wall and filling behind it rather than just digging into the hill side.  such site preparation could be done in several ways depending on material availabilities.  
-- Just looking at that uphill wall and the long span of it, I am concerned to know why that portion of the wall has a sag to the top line of it, Particularly just past the "stub-wall"/doorway.  If it is subsidence I would want to understand why it happened, but if it is thoroughly stable, then careful casting of your "reinforced concrete tie beam" may provide a sufficient corrective.  If your earthbag wall proves to be subject to unequal settling there had better be adequate reinforcing in the tie beam to deal with cracking.  That Tie beam will proove to be the "load spreading plate" along the top of the wall, and skimping on it would be unwise.

I do have a point of curiosity about what that thing is in the foreground where there is a pipe of some sort sticking up out of a bucket filled withconcrete.  Is that some portion of the structure or is it just some sort of "tamper" for the earth bags.

Please keep us up to date with more pictures as well as text. It is a most interesting project and perhaps some of us can help and all of us can learn.   And just because I am as an engineer as I previouly stated does not mean that others such as those with field experience in this kind of work may not have more valuable advice.   I have not done such a structure before and learning by doing often trumps a PhD

The curve in the back wall is actually the guys not laying the wall very straight...

We did actually dig down and out of the hillside and then filled it afterwards. There is also drainage at the base of that wall. I have posted some pictures from early on, a few weeks ago. We started clearing and digging 6 weeks ago.

There is 10 mil PE sheeting on that back and sides of the wall where it is bermed..extra rebar through the walls and a french drain around 3 sides of the building, with sharp gravel below and above it, and that is laid on PE sheeting as well. The earth has a high level of clay in it, and we have already been through one typhoon and the rainy season when we were building the trenches and no problems with water coming down the hill. The old couple has lived on that spot for many years and they say no problems with water coming down the hill. However, I have added a lot of drainage options there just in case. In the middle of the wall there, I may add another buttress inside, but that was partly the function of the built in seats for the dining area.

The thing in the foreground is actually a homemade tamper, one of three we made from concrete.

Any advice most welcome..I have done a lot of research and reading but this is my first practical project...the entire project to move in, will be around $3,500. Labour is 4 guys at $7 per day plus food (normal rate is $6 per day). The welder with his welding machine and a helper is $14 a day. Currently experimenting with soil/sand mix to go as first coat on the walls, before adding 3 layers of lime plaster. The lime putty has been sitting in large bins for the last 2 months...

Just some extra pictures...with the french drains, the 4 inch perforated pipes go all the way around both sides, and will drain off to a culvert.

I like the built-in bench with backrest. That alleviates a lot of the concern about the stability of the long wall.

Your drainage work also looks reassuring.

I may have missed it, but what is the fill in the bags? In the picture it looks like loose lumpy stuff. Is there something holding the wall together other than the bags themselves? Is the fill inside the bags something that will bond into bag-sized units, or just loose fill? Does your region have high earthquake risk?

The bags are full of earth, with a fairly higher clay content and tamped down. Each course has 2 strands of barbed wire along the lengths and in certain places there are 4 ft lengths of 1/2 inch rebar. The top of the walls now has a 6 inch tie beam, because Philippines does have an earthquake risk.. The building has about 100 tons of earth in the walls. This is what it looks at the moment - we are getting 5 trusses made from 2x2 angle bar, and these will be laid on top of the tie bems and the rebar rods are bent over the trusses and welded to them, then C Parlins laid aross and finally the steel roofing is screwed to the C Parlins...

I thought I should add a few more pictures of the building...after 2 years it has no issues, and already been through 2 earthquakes and a few typhoons..