Wood green roof

Hi to all!

I'm planning to build a half-underground garage and I'd like to keep the ground continuous making a green roof. I have two options: Iron beam and concrete or wood from the forest of my land.
Wood is the cheapest way. Sounds interesting, but I'm not sure how long will it last.

I have an idea (see attached document) of the layers I would need:

1.1 Vegetation
1.2 Anti erosion yute layer
1.3 Soil
1.4 Drainage
1.5 Protector and moisture keeper layer
1.6 Wood beam with anti-roots and impermeabilization layer

What do you think about?
Have you built something similar?

Thank you in advance.
Regards

If you decide to go with the wood a liner would be needed to protect the wood.

Folks here on the forum have found that the billboard tarps work well.

Folks might find this of interest:

https://permies.com/w/berm-shed

Thank you for your reply Anne. This is what I was looking for!!! I found a picture of this in the web, but I was not able to find anything with the term 'green roof'. Maybe a mistake due to the traslation.
I'll check all the post.

Hi Raúl , Good luck with your project and please show us pictures when it's done!
You could also try looking for 'living roof' and 'earth sheltered' as well as 'green roof'

I'm thinking of a green roof for a workshop extension (almost invisible to the sky). I can probably get away with quite  a shallow layer of growing medium and plant with sedum and thyme...The soil layer can get heavy with moisture which is something to bear in mind when designing the structure.

Thank you Nancy,

I was thinking about making it horizontal, but I'll probably need to give it a bit of angle.
It's a long-term project, maybe not built this year. I need to finish other constructions previously. .

Nancy Reading wrote:The soil layer can get heavy with moisture which is something to bear in mind when designing the structure.

Using biochar in the soil mix helps trim the weight, since even when it's fully saturated it's still less dense than water (usually about 70%). I supplied about half a cube to a green roof project a couple of years ago, and I should try to get over there one of these days for a few photos....

Ooh biochar - I like that idea!

Raúl Almenar wrote:Hi to all!

I'm planning to build a half-underground garage and I'd like to keep the ground continuous making a green roof. I have two options: Iron beam and concrete or wood from the forest of my land.
Wood is the cheapest way. Sounds interesting, but I'm not sure how long will it last.

I have an idea (see attached document) of the layers I would need:

1.1 Vegetation
1.2 Anti erosion yute layer
1.3 Soil
1.4 Drainage
1.5 Protector and moisture keeper layer
1.6 Wood beam with anti-roots and impermeabilization layer

What do you think about?
Have you built something similar?

Thank you in advance.
Regards

Hola Raúl ! A consideration might be your climate: You didn't tell us where your building is. Do you get devastating rains? Is it super dry? What is your soil like? [A clay soil can retain a lot of water adding a lot to the weight!] do you get snow?
I too would vote for used billboards to protect the structure. [much better than landscaping fabric!] To fasten the billboard to the roof, you will want either roofing nails  with neoprene disk or bolts, but also with some sort of neoprene washer.
Good luck on your project!

Hello Roul,

We have a green roof over our porch and are replacing the roof over our kitchen with a green roof this summer. The kitchen roof lines up and will hopefully just look like it and the porch roof are homogeneous one day. Ours is not flat but has an angle of about 15 degrees.  I'll try to take a picture of it tomorrow, but we're having thunderstorms here regularly right now and I'll need enough sun for it to dry before I climb up onto the roof.

Please excuse me if I don’t manage to explain everything well as I'm trying to come up with words. Ask if you need clarification. My husband put a smooth surface on the under roof, which has a frame around it. He put horizontal slats regularly to prevent the growing surface from sliding down. Then he glued epd, He says that it has to be done in a very specific way to cut it right in the corners, around skylights, drain pipes., etc Then the antii-root barrier went on.We didn't use soil because of the weight with water retention, but volcanic rock with broken clay pottery mixed in. We're happy with this as grass tries to grow on it regularly, but ends up just dying when we don't get rain. So only the fatty plants survive. So we'll never start thinking we somehow need to mow our roof or anything

I'll try to give more details if you want.

Good luck with your plans

May I suggest getting help from an architect who has experience with earth-sheltered buildings in your region? So many important details: Roof weight, water retention and drainage, infiltration, insulation (earth is a poor insulator though effective for heat storage), ventilation, earth to air temperature comparisons, soil types, plantings, weather variability…

More thoughts…
Raúl asked a very important question about his project: “How long will it last?” In my opinion, all one’s thinking about buildings should be informed by the idea that the life of a good building ought to be measured in the hundreds of years. Too often green roof structures seem to aim for much, much less, sometimes mere years, or at best, decades. Wood and billboard tarps obviously fall into the much much less category, and while the appeal of lower initial cost is difficult to shake, it’s wise I think to consider long-term costs, including maintenance, repairs, rebuilds, and potentially even redesigns. (Long term calculations are full of pitfalls, but it’s more than reasonable to give it a good go in the planning stage.) This is not to say that a wood and tarp roof is never a good idea, only that it should, like all building ideas, be subjected to cold, hard analysis.

Last but not least is site consideration. The best place to build an earth-sheltered building is on wasteland, which will be improved by a living roof, as opposed to already biologically active ground, which is almost always diminished by a building, even a successful underground building.

I’ve designed a couple of earth sheltered structures for temperate climates, and for the record, am a fan of insulated concrete forms (ICFs for short) which can be engineered for walls and roofs. Also, notably, though a design professional myself, I was happy for help from more experienced colleagues.

All of this is of course stated out of context, and should be thoughtfully regarded in light of personal circumstance.

Good day Raúl. I'm posting pictures of what I can take pictures. The picture where I pulled back the root barrier is where the connection between the porch roof and kitchen roof will be made, so I could get to it.

It didn't post the picture of the epdm and root barrier. I'm trying again.

OT - Tiffaney, I'm intrigued by the metal springs - a mattress in the windows?

With what David said,  you can choose more expensive materials and have something that lasts, or go for cheaper or even free options and have something that doesn't last. For our house, we chose to spend the money for quality.

There is a plastic material that wraps around the basement or foundation of houses that we used on our yurt, when the canvas wore out. I'm including two pictures of it. A yurt is movable, so we chose by the price tag on it. We know of two others who chose this material (I'm sorry but I don't have a name for it) on a house and a garage. The house had a slant to it and the person in this case did not put cross bars on it, nor properly attach it. It ended up sliding down, giving the roof a very short life. The garage had a slope of 5 degrees at most. The person who did this did properly sealed it up to not have any leaks. Once the material is covered and is not exposed to light, it does last. But it breaks up if it's exposed to light. The garage is doing really well after probably fifteen years now. And our yurt roof is doing good while approaching ten years. But it would probably fail pretty quickly if it was moved and exposed to the sun.

We recuperated what I assume is similar to the advertising canvas that someone spoke about for lining a duck pond. It was never able to be filled up with water, so was a complete waste of time.

Nancy Reading wrote:OT - Tiffaney, I'm intrigued by the metal springs - a mattress in the windows?

Yes. We have a very clever cat who really likes sleeping on the roof. When we open the windows, she would climb up onto it, only she could never figure out how to get down. In her opinion, not a problem if she needs to relieve herself as its an elevated yard. She only thought it was a problem to get to the cat food again. Her going up there was a big problem for us because we would need to clean up after her, which is not as easy as cleaning up a litter box. (And it had to be well cleaned since recuperate water from the roof.) So we got rid of the problem with springs from an old mattress.  Light, air and insects still pass through the windows,  but not the cat.

Tiffaney Dex wrote:There is a plastic material that wraps around the basement or foundation of houses that we used on our yurt, when the canvas wore out.

Looks like it is called (wait for it) a dimple mat, or dimple sheet!


source

I like the idea of the dimples holding water like little swales to keep the roof watered in drier weather. I know it isn't much but must help a bit. They seem to be mainly made of polythene, so durability will depend a lot on the material specification, but as you say buried out of the sunshine it will last a lot longer. A quick look shows a 1m x 20m roll is just £30 or so, so an affordable option I would say.

I'm wondering if it would help to add native tree chip mulch to the soil to help get a mycelium network throughout the soil layer to help keep that soil from sliding/washing down over time. You might need to add some form of nitrogen at first since the mulch breaking down uses nitrogen, but that's easy to accomplish. I've never done it though.

Nancy Reading wrote:
Looks like it is called (wait for it) a dimple mat, or dimple sheet!

LOL!  A rather obvious name. Thanks for giving it to me.

As far as price, the friend who built the almost flat roofed garage said that it was the easiest and cheapest roof he had ever done.

Charolett Knapic wrote:I'm wondering if it would help to add native tree chip mulch to the soil to help get a mycelium network throughout the soil layer to help keep that soil from sliding/washing down over time. You might need to add some form of nitrogen at first since the mulch breaking down uses nitrogen, but that's easy to accomplish. I've never done it though.

My hunch is this would not help long term. In my experience, fungus that eat wood chips are not permanent - they eat the wood chips and the chips break down and the fungus eventually is gone (eaten by something else).

Mycelium used to make structure is usually grown on the substrate and then dried so it stays in that one form.

Wood chip eating mycelium is different from soil mycelium that are connected with living plants and could add living structure for a very long time. I think these are typically different species of fungus.

I am intrigued by the idea of using a lot of charcoal/biochar in the soil layer as that could give you the most lightweight and drought-tolerant form of living roof as opposed to a soil that is mostly sand/clay (i.e. rock).

Flat roofs are easy to build, but the 5-10 year maintenance will bite you if you’re not careful. That is if you don’t have to worry about snow load. If you regularly get snow, I HIGHLY suggest you take a second look. The green roof of already equivalent to one snow load, so you really need to build to about double the snow load capacity for your area.

From bottom up for a wooden living roof:

Rafters/trusses built for roughly twice the snow load for your area and roof slope.
Decking heavy enough for load. Probably at least 3/4 sheeting or 2x car siding
Waterproof membrane
Dimple mat. Make sure to leave a weep drainage so anything that gets between the membrane and the dimple can drain
Anti slump layer. This could be wooden runners or geomesh material, something to keep the soil from washing away in a downpour. Mudslides on the roof are bad.
Lightweight soil/media. The right mix really depends on your climate, slope, and available materials, but you need something that holds enough water to get between rains without holding too much on the downpours and draining without washing away.

Your green roof scheme for a semi-underground garage is feasible in concept, but the following points should be noted:

Wood structure: Although cheap, wood is prone to rot in a long-term humid environment. It is recommended to use antiseptic wood (such as antiseptic hardwood or antiseptic treated pine), and add a root barrier and waterproof layer (such as asphalt membrane or EPDM membrane).
Structural layer design: Your layering idea is basically reasonable. It is recommended to use a drainage board + filter layer for the drainage layer to avoid excessive water accumulation.
Load-bearing problem: The wet weight of a green roof is usually 100-300kg/m². It is necessary to confirm whether the wood beams are load-bearing enough, and if necessary, they can be combined with steel or concrete structures.
Service life: Concrete and steel have a long life and less maintenance; if wood is selected, regular maintenance and inspection are required.

There are precedents for similar projects. The key is to ensure waterproofing, drainage and structural safety. If you need specific suggestions, you are also welcome to continue to communicate.