Advice for building a back yard cabin, step-by-step...

August 4, 2015 10:59 AM

So, I want to build a 10x12 cabin in my back yard. I've never built anything. I am thinking of a modular approach to help with my learning curve. The site is slightly sloped—the front is about 18" lower than the back. I have a plan...and I'm hoping experienced folks can pick it apart/offer advice on whether this is a good idea...

So, first, I want to build a deck—I am leaning toward a modular composite deck for ease of installation. The idea is that the deck would ultimately become the floor of the cabin.

Question 1 is whether I can use a composite deck as a cabin floor/platform.

Question 2 is how to I raise the front end so it will be level? (Simplest way). Our backyard is steep—so there's a hillside with retaining walls about five feet behind the high end of the cabin site. We live in Los Angeles, so there's not much rain, but when it does rain, there's a lot of water. Our retaining wells were very old and a few failed—not catastrophically—but they've since been secured/replaced.

Question 3 is once the deck is built, what's the best way to do the cabin? From a kit I customize? How would I secure the cabin to the deck?

So, how does this all sound to everyone?

posted by soulbarn to Home & Garden (12 answers total)

Step #1, before doing anything. Check with your local zoning authority and make sure you are aware of all the rules that apply to your location. There is nothing more heartbreaking that pouring hours of work and sweat into building something and then being told it's non-compliant and needs to be removed. By the way, loved your book.
posted by cosmicbandito at 11:03 AM on August 4, 2015 [1 favorite]

Response by poster: city of LA allows a 10x12 shed without permitting. and thanks for the kind words about my book!
posted by soulbarn at 11:11 AM on August 4, 2015

Can you provide a link for the product you are envisioning for the "modular composite deck"?
posted by misterbrandt at 11:17 AM on August 4, 2015

IANAContractor, but I can't think of any compelling reason why a deck shouldn't be an adequate base for a small building. I would suggest sonotube concrete footings as a relatively simple way to deal with your slope - you don't have to get the whole site level, just set the tubes so that the tops are level. There's tons of information out there about tying your frame structure into the posts. Long story short, you sink a bolt into the concrete (or drill and glue in a bolt after the concrete sets) and then connect your timbers to the bolts.
posted by Makwa at 11:18 AM on August 4, 2015

I've built a shed in my backyard of similar dimensions (8x12). My ground wasn't perfectly even either so what I ended up doing was putting in concrete posts, which was probably overkill for a shed but a fun experience. Basically you dig post-holes into the ground. I went about 4 feet deep to try to get below the frost line. You can do this with manual tools (which is what I did) or with a powered augur, either handheld or attached to a bobcat. You then put sonotubes into the holes, backfill any space around the sonotubes and cut the tops so that they are all level. Then you would pour concrete into the tubes up to the top. There are bolts you put into the concrete posts before they set and then your platform/deck would be fastened onto those bolts.

Once you have your platform set up you can make your individual walls and then screw them into the platform and each other. At that point the whole thing will be secure and won't go anywhere. I don't know if you need to get a kit for these as the framing is pretty straightforward.

For my shed I ended up making all my roof trusses on the ground and then raising them onto the top of my shed. All the books I had read on the subject said this was the easier way to go. Depending on what is required in California you may need special fasteners between the roof and rest of the building.
posted by any portmanteau in a storm at 11:19 AM on August 4, 2015 [2 favorites]

Normally you'd build on some sort of pier, and put posts up from the pier to raise that end. This is where it gets interesting and could get expensive. If your retaining walls have failed, you're going to have similar issues with whatever you use for foundation, whether that's grade beams or piers or what-have-you. You might want to engage an engineer to find suggestions for how to anchor that foundation to the slope in a way that isn't going to slip, and this is going to vary based on the specifics of your site.

Worst case it might involve drilling to bedrock and pouring reinforced piers that go all the way to the bedrock (but that deck/shed is never going anywhere...).

For building the cabin, we looked at our local "Tuff Shed" brand dealer, and similar, and decided that for the same price and our own labor we could get a much better wall assembly. This eventually spiraled into a massively overbuilt building with a living roof, but...

Standard stick built construction with 16" on-center studs. Build the walls flat, lift them into place, apply siding (or T-111 or similar). I like a full drainage plane construction: sheathing necessary for wall strength (like 3/8" ply or whatever), firewall material if necessary, moisture barrier, spacers, actual siding, but for a utility shed or other non-habitable structure, just using an appropriate finished sheathing is quick and cheap.

I would be wary about how you do the junction between the building and the deck. Normally you embed anchor bolts in the foundation, drill holes in the treated sills, use those sills on the bottom of your wall assembly, and when you lift the wall up you set it up over those bolts and bolt the wall down to the foundation. If the deck extends beyond your wall assembly, you may get seepage in underneath that wall. This might be sub-optimal.

Oh, and: Always use hot-dipped galvanized fasteners with treated lumber, like you'll use for your sill plates.
posted by straw at 11:23 AM on August 4, 2015 [2 favorites]

There are tons of books out there about building sheds. That's probably the place to start. This is basically just like building a tiny little house from a framing perspective. No need to reinvent the wheel.

You can probably use a modular composite deck, but you'll want to look carefully at the load paths to make sure it will support the weight of your cabin properly. Building a normal floor deck on a couple beams is not hard at all and really cheap (you need two beams, maybe 4x6, 10 2x6 10', 2 2x6 12', and 4 sheets of plywood). Nice plywood with some finish on it is a totally fine flooring option. The modular decking also probably is not designed to go together tight, so you'll end up with cracks in your floor that might let in bugs.

You probably want nice deep piers if you've had retaining wall issues. Sonotubes are easy.

On top of the floor deck, you build the walls out of 2x4s, tilt them up and then nail them down. I think earthquake strapping is used in California and probably wouldn't be a bad idea. You can buy trusses from a lumber yard for the roof to keep it simple or just build one wall higher and make a shed roof (like half a normal roof) for a more modern look.
posted by ssg at 12:04 PM on August 4, 2015 [1 favorite]

We have an El Niño coming. I feel like you should know more about how mudslides work given how you talk about your back yard.

There's a house on a hillside that disintegrated the last time we had big rains, maybe 8 to 10 years ago. A bunch of houses washed away that winter, but this one is on a busy street. Finally this year it was rebuilt, just in time for the next round of heavy rains. Every time I drive by I wonder what contractors did differently this time and if it will hold.

I'm pretty sure you should get a consultation from a structural engineer or similar professional. Maybe a different one from whoever fixed your retaining wall? I think you want at least two opinions about your hill and what is holding it together.
posted by jbenben at 12:05 PM on August 4, 2015

Interestingly, I just did the same thing. Here's some photos I posted in a recent AskMe about building a shed. I built the deck one year, framed up the building this year, and next year I will install the windows and insulate.

As others have said, I don't understand what you mean by a "modular composite deck".

1. Not really, but sorta. The decking material itself won't be tight enough to provide a building envelope, unless you use something like 2x6 T&G car decking. What I did was use 2x6 pressure treated with gaps between, and then I glued and screwed 1/2" CDX to the deck for a subfloor when I was ready to proceed.

2. You don't "raise" anything. You just build a level deck on posts (if that's your choice). You run posts up high enough. Then you set your beams and/or joists level and trim off the excess post. Or you cut the tops of the posts to level using a water level and set beam on them.

3. I built mine using standard framing techniques. It's attached by nailing through the sill plate. You could use a kit. It's personal choices at this point.

All this goes without saying that you need to know what you're doing, or know how to find out. Or you need to seek the help of a professional.
posted by humboldt32 at 12:17 PM on August 4, 2015 [1 favorite]

Response by poster: Thanks for all the answers. To clarify, we've had an engineer look at our slope; he said it wasn't coming down, even in an El Nino year. The reason we lost our retaining walls is that they were 40 years old and made of wood—they just rotted away; the amount of dirt that came down was about enough to fill a large trash can (perhaps it isn't right to really call them "retaining walls"—our backyard is terraced and these structures comprised the leading edge of the terraces, so they weren't really retaining much.
posted by soulbarn at 3:55 PM on August 4, 2015

Well, in that case, it sounds like you have a relatively stable site, so you can use all kinds of foundations. Concrete piers cast with sonotubes would still be the gold standard (and what you should use if you have any concern about the stability of the top layer of soil), but you could definitely use precast concrete blocks (like the kind used for decks) and build a little trestle out of 4x4 or use cribbing to get the deck level or even rest the shed on pressure treated skids with a trestle or cribbing.
posted by ssg at 4:15 PM on August 4, 2015

Not sure about the modular composite decking. The platform for my shed was 2x6s with 3/4" tongue and groove plywood along the top for the shed floor.

I dug 4 concrete piers (one for each corner). As far as I'm concerned that is sufficient - the floor joists in my house probably have longer spans than the shed, and bear a lot more weight - but I could see a case being made for 6 as well as we aren't exactly professionals.

Depending on how high you are allowed to build the cabin, you could make use of the area underneath the roof (kind of like its attic) as additional storage.
posted by any portmanteau in a storm at 5:32 PM on August 4, 2015