Question about load bearing capacity of structure

Hey knowledgeable people! Me and my partner are building a small roundwood post-and-beam cabin in our forest. Up until now we've been mainly playing around because it's fun, but now we've reached the point where we want to make it official. The plan is to put a turf roof on it, which will go all the way down to the ground on three sides. The fourth side (facing south) will be a cordwood wall with windows and door. My question right now is about what kind of load-bearing capacity we can expect from the roof? Will it be enough to carry the weight of the turf?

The sides and back (which are steeper, see pictures) don't really worry me, since more of the weight will be supported by the turf itself. The main roof, however, is less steep (the angle is about 31.5 degrees). I don't know nearly enough about these things to say for certain if it'll be okay. My gut feeling is that it'll work out, but I'm not an engineer and neither are my guts.

The material used is Norway spruce. There will be horizontal beams (which aren't there yet) running perpendicular to the roof beams to distribute the weight, which in turn will be covered by spruce poles and/or split planks. The span between the posts is about 100 cm in the east/west direction. In the north-south direction the span is about 150 cm, but reduced by knee bracings, so longest unsupported span is about half that. The span between the first-layer roof beams is about 70 cm. Span between second-layer horizontal beams yet to be determined.

So, what do you all think? Any input would be very welcome.

What method are you using for connections
Dowels, lag bolts, a mixture of methods?

That roof does not look very load bearing to me.  It looks like it is missing some support beams.

I have read that turf roof weighs a lot.

To support our drywall ceiling we used 4 x 4 support beams.

These might help:

Rico Loma wrote:What method are you using for connections
Dowels, lag bolts, a mixture of methods?

The connections between the posts, main beams and knee bracings are mortise and tenon. Everything else is done with wooden dowels. The construction feels solid, doesn't move at all when we climb around on it, but you don't really need that much turf to weigh the same as a person...

Anne Miller wrote:That roof does not look very load bearing to me.  It looks like it is missing some support beams.

I have read that turf roof weighs a lot.

To support our drywall ceiling we used 4 x 4 support beams.

Yeah, these pictures are not entirely up-to-date. We've added some beams, started the second-layer structure now, but I don't have the pictures of that available at the moment.

Hi Eino;
I'm not a structural engineer, but I have built a lot of sheds and outbuildings over the years.
I have never worked with a turf roof, so I am not familiar with the weight numbers involved.

Here is what I do see.
You and your husband are doing an outstanding job!
Your building is well above grade. (nice stone foundation!)
You have used mortise and tenon with dowels, a proven, solid, long-life connection.
You have purlins connecting the rafters to stop them from spreading.
The upper, less sloped section is relatively short, and the sides will shed snow quickly.
You have added more support that we have not seen yet. The final planking will solidify it together.

Everything I see so far tells me that I would trust this construction to last a lifetime or three.

Unfortunately, the standard that covers log construction/connection, ICC 400, does not have any reference tables and would require an engineer run the calcs and stamp the design.

The IRC has tables on sawn members but the spacing is 24" max and I don't think the dead load table for areas with lots of snow would be something you could compare.

I think the ICC has a section covering green roofs but again, they want you to engage a design professional.

I would trust it if someone with experience in log construction/connection and green roofs looked at it and said it was okay. This person does not need to be a design professional, just someone with experience, maybe someone on here has that skillset/experience.

If not, I would not want to be laying in bed during a heavy rain and wonder if the roof will collapse on me. I would repeat what you have there on 16" centers and sleep well or cover it with metal as is if I could not find an experienced person to advise me.

That is some very nice looking framing!

Josh Hoffman wrote: If not, I would not want to be laying in bed during a heavy rain and wonder if the roof will collapse on me. I would repeat what you have there on 16" centers and sleep well or cover it with metal as is if I could not find an experienced person to advise me.

I am no expert either, but the fact that you have vertical supports within the structure, improves my confidence.

There are other ways to make sure you're safe from a night time collapse - like a traditional medieval bed that is like a cupboard with a roof on it!

Eino Kenttä wrote:

The span between the posts is about 100 cm in the east/west direction. There will be horizontal beams (which aren't there yet) running perpendicular to the roof beams to distribute the weight, which in turn will be covered by spruce poles and/or split planks.

It would be a lot of work, but if you added extra posts in between to reduce that gap from 100 cm to about 50 cm, that would make a big difference.  I don't know how rot resistance spruce is, but with 50 cm centers, there'd be more redundancy if one of the posts went bad.

There are a number of questions I need to ask first?
- what thickness of earth do you wish to support?
- How will you prevent the soil sliding off?
- have you looked at wofti homes?

I have questions also...
Where will the RMH sit?
Will it be a simple J-tube or a longer-burning batch box?
Do you have bricks?
Did you know that once you heat with bricks, you will never go back?

Hey everyone, thank you so much for all the awesome input (and for being awesome in general, of course!) To answer your questions:

John C Daley wrote:There are a number of questions I need to ask first?
- what thickness of earth do you wish to support?
- How will you prevent the soil sliding off?
- have you looked at wofti homes?  

-We're thinking 10 or 15 cm on the main (less steep) roof, thicker on the sides and back where the turf will support more of its own weight.
-The turf will go all the way down to the ground on three of the sides, and on the sides and back we plan to lay the turf sheets horizontally and "staple" them together by pushing sticks through each layer into the one below, to stabilize it until the grass roots have had time to do their thing. So the idea is that the lower turf cover stops the upper turf cover from sliding, much like on a natural hill.
-I'm a little bit unclear on what distinguishes a wofati from other types of green-roof buildings. If I've got it right, the layer of soil would be quite a bit thicker than on traditional turf roofs? Given how much it sometimes rains in our area, I don't think I fancy trying to build a wooden structure sturdy enough to hold up a meter of soil...

thomas rubino wrote:
I have questions also...
Where will the RMH sit?
Will it be a simple J-tube or a longer-burning batch box?
Do you have bricks?
Did you know that once you heat with bricks, you will never go back?

-It'll be in the back right-hand/middle part of the "main roof" area as seen from the front, the chimney will exit just to the left of the peak of the roof.
-Right now we're planning for a first-generation batchbox, but still open on this.
-We do have bricks, not quite enough yet, but we'll get more. We also have clay on the property, so planning a double-skin bell with an outer skin of cob.
-So I've heard, and really looking forward to proving it right

Jay Angler wrote:
I am no expert either, but the fact that you have vertical supports within the structure, improves my confidence.

There are other ways to make sure you're safe from a night time collapse - like a traditional medieval bed that is like a cupboard with a roof on it!

It would be a lot of work, but if you added extra posts in between to reduce that gap from 100 cm to about 50 cm, that would make a big difference.  I don't know how rot resistance spruce is, but with 50 cm centers, there'd be more redundancy if one of the posts went bad.

We're planning to put the bed in the back, where the roof is way steeper. The back row of posts is also a bit thicker than the rest, so we feel reasonably safe. Also, we won't sleep there the first time it's subjected to heavy rain or wet snow. We'll observe it carefully for a while after finishing and see how it holds up.

We are considering extra posts along the sides just to make sure, but like you say it's a lot of work. We also have a few other methods for increasing stability in mind. We'll see how it turns out in the end.

Josh Hoffman wrote:
Unfortunately, the standard that covers log construction/connection, ICC 400, does not have any reference tables and would require an engineer run the calcs and stamp the design.

The IRC has tables on sawn members but the spacing is 24" max and I don't think the dead load table for areas with lots of snow would be something you could compare.

I think the ICC has a section covering green roofs but again, they want you to engage a design professional.

I would trust it if someone with experience in log construction/connection and green roofs looked at it and said it was okay. This person does not need to be a design professional, just someone with experience, maybe someone on here has that skillset/experience.  

We're in contact with the municipality people to get it approved. It seems like there won't be a big issue, they're giving us some very good suggestions. Also, we do know some people with experience in this, and we'll definitely ask them as soon as we can get them to come out here. That might be a little while, since our place is a tad remote, but hopefully before it's time to lay the turf.

this pdf might be of some use

Grab some of the soil that you are going to use for the roof, make it some standard unit size (for me, I would use one square foot), and as thick as you plan it to be. Water it to saturation (until the soil can hold no more water) and weigh it.

That number, when multiplied over the total area of your roof, will tell you how much weight your roof will have to hold.

Wood is pretty good in compression. The thing you have to watch out for is buckling; that is, the post starts to deflect (bend out of true), and then rapidly fails if the post is too long for how thin it is. So holding structural members together to restrain deflection is very important.

Do you have stats on load bearing for Norway Spruce round wood poles and what is the loading x  sq. ft. (worst-case  snow-ice-rain scenario (or the reverse) event in the zone? Looking at what you have so far, it's not overbuilt. But I haven't but a few winters under my belt in a similar zone. -Rick

You could contact this guy :



and check out his channel.

First time posting. Been creating a 40 acre farm with 10 acres brings a traditional farm using permaculture practices whenever I can and very slowly turning the other 30 acres of existing old age (not that old it’s Illinois) into a food forest. Started by planting 1000 fruit trees randomly around it.
To my question. My next build isn’t traditional like my barns and house have been. I’m building a root cellar probably out soil bags to hold the walls in and attaching a cob studio guest house coming out of the hill side using cob.
The root cellar will likely have to have a stronger roof because it will have a fair amount of heavy clay soil above it but I am concerned about the roof for my cob building. The walls will be fine but I’m unsure just how beefy the ceiling will need to be. I can calculate the dirt load for my living roof and I can separately calculate the snow load but is the snow shed going to act different because it is shedding off of soil and vegetation instead of traditional roof material.
Is it going to melt a lot faster and waterlog my living roof instead of shedding like normal and resulting in extremely heavy wet dirt with a snow load on top?
Also kind of concerned that because I’m making the cob building shoot out of the hill of my root cellar is the snow going to shed down the hill onto my roof creating a build up spot?  I won’t always be around to knock snow off the roof and need to make sure it can handle worst case scenario.
Thanks in advance
Adam

Hi Adam,

Welcome to Permies.