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Roof weight and structural integrity

Posts: 967
Location: Ohio, USA
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I'm interested in building a wofatti or hybridizing an existing structure to be more wofati-esq. Unfortunately, all I manage to do is run in circles in my mind because of the weight factor. I have this same problem with trying to do a RMH in the house- and, in my opinion this is all pretty important because there are plenty of existing houses for people and it's a whole lot more energy efficient to use them rather than tear them down and re-build a true wofati. The parts of the wofati I want to capture the most in order of importance are: 1.Energy savings, 2. Cost of building/remodel, 3. the Look.

A cubic foot of soil, dry, weighs about 100lbs. Wet, I estimate at double (though this varies by content). Cement dry weight is about 150 lbs. A roof 15ft x 10ft with 1" of soil scattered on top, wet, can be estimated to weigh 2,500 lbs, which is really WAY too much for your average house roof if you factor people animals, and plants might walk, run, or jump on there + snow. And, this is JUST AN INCH OF SOIL!!! So, my question to all those in the wofati building business- what sort of massive structure do you have to compensate for that weight or is there some other trick? Building the structure smaller is not an option, because we are dealing with existing structures.

To get around the soil weight issue I am considering perlite. Perlite might be my most favorite thing now. Perlite, when dry, weighs about 5 lbs/cubic ft, when wet, I read about 25lbs. So, take that 2,500 lbs and divide by 8: now your talking about 300 lbs wet. That's work-able, but an inch isn't much root room and is a pretty poor growing medium. Water weighs 62 lbs, and since peat weighs very little and holds tons of water (when wet), we'll assume it's just 70 lbs/cubic ft. If we add 1/2 an inch to our 1" perlite, we get about 750 lbs roof weight. A walk-out roof SHOULD be able to hold this. shallow-rooted weeds should be able to sprout. If We then add a little extra support (a column in the room), it's pretty certain to remain structurally sound. During dry times you can probably still hold a party out there without fear of falling in. This, however is not a wofati. It is a green roof, which is better than nothing, but it's only about 1.5 inches of insulation (which if your into R-values, that's an R of probably about 4.5). A radiant heat barrier might be a good addition to the structure, adding to temperature normalization without much mass. However, I'm still not satisfied. In this room, no one would notice if the ceiling were lowered and regular rock-wool insulation added to meet an R of 30 above our heads.

Then there's the walls. Straight. Paths on all sides. Property boundaries close. Okay, so I already plan on burying the house as high as I can without compromising the structure on impinging on the neighbors. Because it has a brick exterior, the process is simple. I add a water barrier with water-proofing cement paint (since I don't want any issue) then I unload dirt to less than 6" from the top of the basement. I think I'm averaging 2 ft further underground. That leaves about 8 ft above ground for this one room. The main house is a story for another time. So what then? I was thinking again-radiant barrier on walls. Maybe blowing in insulation into the existing walls. That might get us an R of 18 (except the ginormous window). More pricey would be to re-slope the ceiling interior because round is better at keeping heat than square (and I can stuff the rounded corners full of fluff for Rs exceeding 60) I could then stucco or cobb the ceiling and walls a half inch or inch (now minimum R about 21 for about 6 ft of wall) then place storage shelves with wooden backs against all the walls I can.  On the outside, I keep toying with the idea of enclosing the paths to be more wind-barrier-esq. I think I can do this on one side, I have a pergola on another side where I can also put a firewood or garden tool storage, and I might be able to grow vines up the side of the leeward wall. The last wall is connected to the main house, which is a story for another time. I have also considered stuccoing the outside or soil-cementing it in a pretty cob-like design whereever I can, adding another inch or three at parts.

For the rocket mass, I am considering a mixture of cement and perlite to slow and dissipate the heat exiting the exhaust, then gambling that the thermal mass of a masonry chimney in the center of a house about 30ft tall will take care of the rest. I plan on using uninsulated pipe up the chimney since I want heat to dissipate into the chimney walls.

What you all think? Is there some trick to wofatis that make them bear weight better? Thanks!

Posts: 8850
Location: Northern WI (zone 4)
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I did a quick search for code in my area and what I found is that residential is 40 lbs/sqft and commercial is 60 lbs/sqft.  So for your hypothetical 15x10 building, if built traditionally in my area, it would be able to hold 6000 lbs, if built commercially it would hold 9000 lbs.  I guess I just wanted to interject that 2,500 lbs is not as bad as you might think.  But the roof does need to hold the dirt AND the 40 lb/sqft of snow that will land on it so it needs to be beefier than a "normal" roof system.  But maybe not 10x beefier...
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I always figured gravel (soil) weighs about 3000 pounds per cubic yard for what it is worth.

But here is the thing, when spread out over a broad area, weight per square foot typically is not all that bad. I have built a few concrete counter tops and the first question people ask is; "did you beef up the under-structure?" The answer is "no", because I did not have too. It figures out to around 12 pounds per square foot for a concrete countertop.

I will admit I am having a difficult time picturing what you are trying to do, but if you wanted to do such a thing to an already built structure, I am wondering if the simplest solution would not be to nail on sistering framing on one side, or both of the rafters to make it stronger?

What I find interesting is, in many communities in Maine they are adopting the National Building Code which makes it illegal to use rough sawn lumber. This is a concern to me because I use my own trees, logs and saw mill to make my own building materials, yet because it is unplanned my 2 x 4 is a true 2 inch, by 4 inch two by four, and not 1-1/2 inches by 3-1/2 inches making it 25% stronger. Still it lacks the "stamp" so it is considered "inferior". In your situation, using rough sawn lumber might be enough strength.
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