Detached Building on Concrete Slab

[JBjunior]

I am a military member currently stationed about 4500 miles away from my property and I am trying to start initial planning for a detached building that I want to build in a few months. The property is in NC (Onslow County) and I am having a hard time understanding the codes and telephone conversations will be long and drawn out, I have already asked some initial questions from them.

There are already two homes on the property, this is being planned as a hang out spot/workshop area.

Here is what I already know: It will have to be on a slab with footers. I am looking at putting a pseudo "yurt", a round building, that is around 1000 sf, that will be heated and cooled. I want it is stout as possible for hurricanes (one of the benefits of the round building). I will have assistance from a professional carpenter, just not someone that is familiar with NC code (family member out of the area).

1) For a round building, how hard is it (cost from a contractor) to do a round concrete pad with footers vs a slightly larger square/rectangle pad to accommodate the building?
2) I plan on hiring someone to do the pad, but what kind of practice should they carry out in NC for vapor barrier, depth, fill material, etc.?
3) What is the preferred method for attaching the frame to the pad? Embedded anchors, drilling. What is preferred within what is allowed within code?

If anyone has any questions, concerns, or advice.... feel free to post.

Basically, something along these lines:

 

[rburke65]

A round building........well there's an idea.......

 

[matt_i]

Concerns, many. Cost being number one. I'm going to guess your 1000sf is going to be 2x as expensive as a rectangular building, I would budget 50k minimum. There are no round concrete forms, no round trusses, no round bottom plates, if there's any permitting you're going to require engineering for your roof structure & walls due to the plating issue. Fitting doors and windows is going to be time consuming, equals more labor.

My take on concrete is around this...pour footer or grade beam to get below frost line, reinforce with rebar 12" to 16" OC. Slab 4" minimum, meaning form with a 2x6, yours would have to be double layer plywood bent to stakes most likely so you could actually rip to 4" tall. 4" washed limestone base 3/4" size, sometimes referred to as #57 stone, plate compacted twice. 10 mil vapor barrier. Set #4 rebar grade 60 on 24" centers, wire tie every other intersection, the rest will be 2" chairs. 4000psi 6 sack mix, pour as a 5-6" slump. You can set J- or L- bolts in the wet concrete assuming you have a wall framing plan ahead of time, that is cheapest, I recommend galvanized versions. Epoxy anchors are an acceptable replacement at 5x the cost. I would not personally anchor a wall with a wedge anchor. Although it complicates things I feel like its excellent practice to pour an 8" wall above the finished floor in order to get wood up away from the ground. Moisture mainly but termites as well as you go south. Absent major calamities or cheating on the specs you have a recipe for a 100 year building

 

[Playwme]

Are you talking true round, or blocky round with a series of flat panel walls?

 

[DougWil]

Rectangular wood framed buildings have simple, well defined load paths for gravity and lateral (hurricane wind loads). Odd ball buildings don't and would require much engineered detailing to do so.

Also almost anything you put into a building is rectangular, vehicle footprint, work benches, welder, cabinets,,,, etc. So you will have a lot of wasted space.

A domed yurt makes sense with cloth skin that can't transfer shear loads with a rigid steel or aluminum frame.

If you really want a domed building, a steel moment resistant frame would be best.
Because of the difficulty of making all kinds of moment resistant wood connections.

 

[wssix99]

Can a moderator move these posts over to the other thread with the picture, or vice versa?

http://www.garagejournal.com/forum/showthread.php?t=324664

 

[JBjunior]

Can a moderator move these posts over to the other thread with the picture, or vice versa?

http://www.garagejournal.com/forum/showthread.php?t=324664

Thanks, not sure how that split off like that. I never got any indication that there was a second post.

This isn't a building that I am "designing" they are already out there and engineered correctly. It isn't truly round, there are panel walls.

 

[wssix99]

Curious - Are you going to buy a kit house for this?

1) For a round building, how hard is it (cost from a contractor) to do a round concrete pad with footers vs a slightly larger square/rectangle pad to accommodate the building?

This part is not going to be easy and you'll probably need to do a lot of shopping and looking at options. If you had a poured rectangular footing, that's a lot easier because its more of a commodity. A round foundation will certainly cost you more.

Contractors typically have rectangular forming systems that unload off of a truck, go up in a day, and then come back down and go back on the truck. For a round, poured footer, you would need custom form work that will drive up the cost. Maybe a block footing would make this less expensive, but that's also more work than a poured rectangular footing. Another option would be a curved ICF foundation, but you'd need a local contractor who knows how to work with that stuff.

^ I'd expect that all these options and the costs associated with them would vary wildly and will be dependent on which contractors are available in your area and which methods they are comfortable working with.

If you are building a kit home, I would hope that they might have a listing of contractors across the country who might have curved forms that fit their designs. Having such a company travel an extra distance to your property may outweigh the cost of trying to do something custom and local.

 

[Falcon67]

Knowing some contractor types, I'd swag they would ask 2x ~ 3x the cost of a rectangle because of the hassle factor. I would, cutting a 2x6 form board a zillion times every inch or two with a circular saw to get a bend in it is a total PITA. And then is's useless for anything else. Once the boards come off a rectangle, they can be used on the next job or for DIY they can go into the building somewhere.

 

[DougWil]

1) For a round building, how hard is it (cost from a contractor) to do a round concrete pad with footers vs a slightly larger square/rectangle pad to accommodate the building?
2) I plan on hiring someone to do the pad, but what kind of practice should they carry out in NC for vapor barrier, depth, fill material, etc.?
3) What is the preferred method for attaching the frame to the pad? Embedded anchors, drilling. What is preferred within what is allowed within code?

1. Depends on your soil, if you have the type that is almost like cheese it can be excavated and poured w/o forms below grade.
If the soil is more the crumbly, cave in type the footing form would be above and below grade. A thin steel form that can be curved or plywood curved. Isn't that hard but would probably be one use, and never used again for the contractor. So the form cost is yours.

2. Bottom of footing below frost depth. No fill required under the footing unless you have crappy soil or need fill. In which case it should be compacted.
Typically anywhere you want 4" of compacted base before concrete. Sand and vapor barriers tend to be local requirements.
Barriers under slabs can cause issues, so it isn't all pluses.
The engineer and building dept will tell you what is required.
Depending on wind uplift forces a footing may be bigger and heavier than required for gravity loads. The slab my be floating and not help hold the building down, or monolithic and will.
That is something the engineer determines along with reinforcement.

Slab should be min 4", 5 or thicker is what I would recommend.
But here again the engineer will specify that.

3. Anchors are typically embedded because they are far cheaper, but they have to be in the correct location.
Installing epoxy anchors by code, require "Special Inspection", meaning a independent special inspector or the engineer has to stand there while the drilled holes are cleaned up and the embeds are installed. That adds cost.
Sometimes the city will waive that requirement.

If buildings like this are "out there" simply find one you like, find out who the engineer and or architect were and go talk to them.
Much easier if they already have drawing details and calcs and have done one than finding an engineer and starting from scratch.
Bring your checkbook.

Depending on the city requirements and local wind loads a simple rectangular building may not need an engineer at all. Or if it does they already have std details and methods that can be used and have been used hundreds of times.
Have you talked to the city?
What do they say?
It is better that you actually go in their office, wait, hang around and finally talk to someone with authority to give you the lowdown rather than attempting to do so from thousands of miles away.

Finally, you seem concerned about cost, yet want to build something that will be a costly build. That usually doesn't work out so well.

EDIT:
OK, went over to your other thread and see Smiling Yurts.
From their website.
All of our yurt kits have been designed and engineered to 100 m.p.h. wind with 3 second gusts wind rating, 100 p.s.f. snow load and class D-1 seismic ratings.

The kit(s) we build for you will be designed and built to your local site-specific requirements for snow, wind and seismic loads. Any requirements greater than the above design ratings require special engineering.We can design for most wind, snow and seismic requirements.

Typically a kit, or metal building manufacturer will design the building and stamp the plans.
They will supply the anchorage and gravity loads to a engineer you hire locally to design the footing etc.

 

[sublimate]

Looking at you picture, it doesn't look like the structure is truly round, but rather made up of a number of straight panels (like 20-30, I didn't count), probably because each panel is easier to frame like that using normal boards.

If your panels are the same size as standard form boards then it seems like it would be easier for the concrete contractor to pour essentially as (24?) sided slab, instead of a "round" one.

 

[sublimate]

BTW, they pour round slabs all the time for grain silos, you can probably find someone in your area to do one. People even turn them into houses:

http://www.motherearthnews.com/diy/buildings/grain-bins-z10m0gri.aspx
http://www.motherearthnews.com/diy/grain-bin-building.aspx

 

[JBjunior]

1. Depends on your soil, if you have the type that is almost like cheese it can be excavated and poured w/o forms below grade.
If the soil is more the crumbly, cave in type the footing form would be above and below grade. A thin steel form that can be curved or plywood curved. Isn't that hard but would probably be one use, and never used again for the contractor. So the form cost is yours.

2. Bottom of footing below frost depth. No fill required under the footing unless you have crappy soil or need fill. In which case it should be compacted.
Typically anywhere you want 4" of compacted base before concrete. Sand and vapor barriers tend to be local requirements.
Barriers under slabs can cause issues, so it isn't all pluses.
The engineer and building dept will tell you what is required.
Depending on wind uplift forces a footing may be bigger and heavier than required for gravity loads. The slab my be floating and not help hold the building down, or monolithic and will.
That is something the engineer determines along with reinforcement.

Slab should be min 4", 5 or thicker is what I would recommend.
But here again the engineer will specify that.

3. Anchors are typically embedded because they are far cheaper, but they have to be in the correct location.
Installing epoxy anchors by code, require "Special Inspection", meaning a independent special inspector or the engineer has to stand there while the drilled holes are cleaned up and the embeds are installed. That adds cost.
Sometimes the city will waive that requirement.

If buildings like this are "out there" simply find one you like, find out who the engineer and or architect were and go talk to them.
Much easier if they already have drawing details and calcs and have done one than finding an engineer and starting from scratch.
Bring your checkbook.

Depending on the city requirements and local wind loads a simple rectangular building may not need an engineer at all. Or if it does they already have std details and methods that can be used and have been used hundreds of times.
Have you talked to the city?
What do they say?
It is better that you actually go in their office, wait, hang around and finally talk to someone with authority to give you the lowdown rather than attempting to do so from thousands of miles away.

Finally, you seem concerned about cost, yet want to build something that will be a costly build. That usually doesn't work out so well.

EDIT:
OK, went over to your other thread and see Smiling Yurts.
From their website.
All of our yurt kits have been designed and engineered to 100 m.p.h. wind with 3 second gusts wind rating, 100 p.s.f. snow load and class D-1 seismic ratings.

The kit(s) we build for you will be designed and built to your local site-specific requirements for snow, wind and seismic loads. Any requirements greater than the above design ratings require special engineering.We can design for most wind, snow and seismic requirements.

Typically a kit, or metal building manufacturer will design the building and stamp the plans.
They will supply the anchorage and gravity loads to a engineer you hire locally to design the footing etc.

Thank you very much for the information. What have I said that implied that I was concerned about cost? I am concerned about value, not cost. My questions are because I am not at my site and will have a small window of time to have the concrete slab prepared, prior to building. I am trying to get an idea on codes, specifically in NC, for the slab and attachment for framing.

As far as the building itself, I already have inquiries out to several companies for plans, that isn't the concern. The county has said that for an auxiliary building, a simple drawing would suffice.

 

[Bluedodge]

...If anyone has any questions, concerns, or advice.... feel free to post.

Just a head's up -

From my banking days, banks, Fannie Mae, and Freddie Mac hate geodesic dome / A-frame / octagonal structures.

Doing this with cash? No problem.
Need a mortgage or construction loan? Good luck.

A fly-by-night mortgage originator will tell you "yeah yeah yeah", but once your application gets to underwriting you may (will) have issues.

 

[JBjunior]

Just a head's up -

From my banking days, banks, Fannie Mae, and Freddie Mac hate geodesic dome / A-frame / octagonal structures.

Doing this with cash? No problem.
Need a mortgage or construction loan? Good luck.

A fly-by-night mortgage originator will tell you "yeah yeah yeah", but once your application gets to underwriting you may (will) have issues.

Only cash.

 

[Stuart in MN]

Are you sure it has to have footers, and can't be just a floating slab? I suppose it could be a local code requirement to include the footers.

 

[Voi]

Are you sure it has to have footers, and can't be just a floating slab? I suppose it could be a local code requirement to include the footers.

I had the same thought. Would also look into a slab on piers or some type of site-formed, thickened edge slab.

But if it were me I'd design the slab for a rectangular building and put the yurt on it. The extra space would be porch area and it would be more useful if/when the yurt was ever moved.

 

[matt_i]

But if it were me I'd design the slab for a rectangular building and put the yurt on it.

Best idea yet!

 

[JBjunior]

Thank you all, yes it will have between 27 and 32 individual walls around the exterior. Each wall is 4 feet wide. It would definitely make sense for the concrete pad to be made the same way.

The concern that has came up with the rectangular pad is water pooling around the exterior, not sure if that is a valid concern or not.

The grain silo pad thing came up in my searches as well, since this is a farm community I will explore that.

Needing footers came out of my conversation with the code enforcer, it may be a requirement for hurricanes or something, unsure.



As far as buying a kit or not, I am considering it. There are several options: just roof, whole kit, or buying plans. I have inquired about buying plans to source the wood locally.

 

[Voi]

The concern that has came up with the rectangular pad is water pooling around the exterior, not sure if that is a valid concern or not.

I had that cross my mind when I posted.

You mentioned value in one of your previous posts (either in this or the other thread) which is what got me thinking about a rectangle. I have seen yurts placed on rectangular decks but I guess they'd shed water between the boards.

My best guess is that if you're somehow required to have continuous footings for your slab then it's going to be quite a bit more expensive.

Is your goal to have the slab insulated and be your finished interior floor?

 

[JBjunior]

I had that cross my mind when I posted.

You mentioned value in one of your previous posts (either in this or the other thread) which is what got me thinking about a rectangle. I have seen yurts placed on rectangular decks but I guess they'd shed water between the boards.

My best guess is that if you're somehow required to have continuous footings for your slab then it's going to be quite a bit more expensive.

Is your goal to have the slab insulated and be your finished interior floor?

Yep, I figured a rectangle would be less expensive since it is more "off the shelf." I am open to having the slab be the interior floor or explore a covering of some kind.

 

[Voi]

Yep, I figured a rectangle would be less expensive since it is more "off the shelf." I am open to having the slab be the interior floor or explore a covering of some kind.

One thought I have is to go with a more straight forward rectangular slab and then have an interior curb poured once the yurt is up or possibly having the entire yurt sit on a curb.

The curb could possible act as a ledge for cabinets to sit on and then front leveling feet could be used to form the toe kick area. I could see this actually helping with cabinet mounting and placement in a yurt.

Here is how I see the advantages.

1) Cost of a curb would likely be much less than continuous footings dug and formed in a circle. So rectangular slab plus circular curb should be less expensive than a circular slab.

2) Curb would help with water pooling which could be a problem regardless of shape of slab. In other words, a curb might be a good idea even with a circular slab.

3) Rectangular slab would provide above ground areas at doorways in and out of the yurt. Plus above ground areas for chairs, etc.

4) Poured curved curbs are common.

5) Curb could be removed if you or a future owner of the land wanted a more traditional building. I could see this as a huge value to you at some point in your future.

 

[JBjunior]

One thought I have is to go with a more straight forward rectangular slab and then have an interior curb poured once the yurt is up or possibly having the entire yurt sit on a curb.

The curb could possible act as a ledge for cabinets to sit on and then front leveling feet could be used to form the toe kick area. I could see this actually helping with cabinet mounting and placement in a yurt.

Here is how I see the advantages.

1) Cost of a curb would likely be much less than continuous footings dug and formed in a circle. So rectangular slab plus circular curb should be less expensive than a circular slab.

2) Curb would help with water pooling which could be a problem regardless of shape of slab. In other words, a curb might be a good idea even with a circular slab.

3) Rectangular slab would provide above ground areas at doorways in and out of the yurt. Plus above ground areas for chairs, etc.

4) Poured curved curbs are common.

5) Curb could be removed if you or a future owner of the land wanted a more traditional building. I could see this as a huge value to you at some point in your future.

6) Curb could be formed with rigid foam for a thermal break where you need it most.

Thanks! One thing to consider is that with a circular pad I would need roughly 1300 sf of concrete and would need 1600 sf for a rectangular/square pad, by my calculations.

40x40 pad would be about 20 yard of concrete for the pad itself at 4" thick and 6 yards of concrete for 12x12 footers - so 26 total.

40 foot circular pad would be around 16 yards of concrete at 4" thick and 5 yards for a 12x12 footer - so 21 total.

Of course, this doesn't take into account what you mentioned about cost of labor and forms for a circular pad.

For the curb, I think I am envisioning what you are talking about but your comment about using it as a ledge for cabinets throws me off.

 

[Tony_F]

Personally, I would abandon the round yurt idea. Contractors and inspectors tend to get squeamish out of their comfort zones, not to mention the other issues you might have as stated with loans, cabinets etc. You can build a regular building wind resistant using strong ties and other methods, I built a large house in tornado alley 3 years ago using Strong Ties, it added about 2 k to the cost.

Go here http://www.safestronghome.com/highwind/?source=topnav
and here to start http://www.strongtie.com/woodconnectors/landing
https://www.depts.ttu.edu/nwi/research/shelters.php

Hope this helps.

 

[Voi]

Of course, this doesn't take into account what you mentioned about cost of labor and forms for a circular pad.

For the curb, I think I am envisioning what you are talking about but your comment about using it as a ledge for cabinets throws me off.

Even if the cost of the extra concrete for the rectangular slab/footings were equal to the extra cost for labor and/or forms for the circular one I still think the rectangular slab has some advantages.

I don't know much about yurts, but for example, if any entry/exit doors the yurt had were lined up with corners you'd have an above ground slab to step onto instead of ground. Likewise, those corners are a good place to add a covered entryway if you need to kick off muddy shoes or whatever.

I also suspect you'll have more contractors willing to take on the job.

And again, a future buyer of the land might find the rectangular pad useful or at the very least not something they'll need to tear out or expand with yet even more footings.

 

[joes169]

As a concrete contractor, I'd take that on as a project. Just a guess, but probably 40-70% more than a simple square pad with 12" grade beam footings.

If I did it, I wouldn't pour a perfect circle, as you're going to have an inch or 2 of additional concrete sticking out at each segmented wall section. I'd form a square first, and start sectoring the square off into a octogan, and then into 16 walls, and finally into 32 individual walls, if that's what it ends up being.

I would plan on wet-set anchor bolts, their going to be the best value, as long as the pre-built walls need a treated plate set first.

 

[DougWil]

What have I said that implied that I was concerned about cost? I am concerned about value, not cost.

Well there was this in your OP.

1) For a round building, how hard is it (cost from a contractor) to do a round concrete pad with footers vs a slightly larger square/rectangle pad to accommodate the building?

So if you aren't concerned about cost, why are you asking about cost?

And no one can give you specifics about the footing or slab because the yurt hasn't been designed yet, nor the loads calculated.
And the local engineer that designs all of that will have the final say along with the governing authority.

Personally, I think they are going to have a hard time designing something like that for hurricane winds if that is what is required.

But anyhow, 1st step is to get the yurt designed and the loads passed on to the engineer, through the plan review and approval process.
You can't shorten the process by doing anything till then.

And anything you may do like excavating may have to be done again if it isn't big enough, wide enough etc.

 

[JBjunior]

Well there was this in your OP.


So if you aren't concerned about cost, why are you asking about cost?

And no one can give you specifics about the footing or slab because the yurt hasn't been designed yet, nor the loads calculated.
And the local engineer that designs all of that will have the final say along with the governing authority.

Personally, I think they are going to have a hard time designing something like that for hurricane winds if that is what is required.

But anyhow, 1st step is to get the yurt designed and the loads passed on to the engineer, through the plan review and approval process.
You can't shorten the process by doing anything till then.

And anything you may do like excavating may have to be done again if it isn't big enough, wide enough etc.

Thanks for the information.

I mentioned cost as an indication of difficulty/time/availability. Although, if a rectangular slab is $3000 and a circular slab is $6000, but both would adequately do the job, it wouldn't make sense from a value perspective to go with the higher priced option. I didn't ask specifically about cost, simply in everyone's opinion how much more difficult would it be to do one over the other.

There are MANY more practical alternatives, if cost was a driving factor.

 

[JBjunior]

As a concrete contractor, I'd take that on as a project. Just a guess, but probably 40-70% more than a simple square pad with 12" grade beam footings.

If I did it, I wouldn't pour a perfect circle, as you're going to have an inch or 2 of additional concrete sticking out at each segmented wall section. I'd form a square first, and start sectoring the square off into a octogan, and then into 16 walls, and finally into 32 individual walls, if that's what it ends up being.

I would plan on wet-set anchor bolts, their going to be the best value, as long as the pre-built walls need a treated plate set first.

Thanks!

 

[NUTTSGT]

Threads merged.

OP, the first and second post may be identical but I'm not sure. If you can cut/paste any information from the second post and edit into the first post, I will deleted the second one and clean the thread up a little bit.

 

[In2toys]

BTW, they pour round slabs all the time for grain silos, you can probably find someone in your area to do one. People even turn them into houses:

http://www.motherearthnews.com/diy/buildings/grain-bins-z10m0gri.aspx
http://www.motherearthnews.com/diy/grain-bin-building.aspx

beat me to it

 

[JBjunior]

Threads merged.

OP, the first and second post may be identical but I'm not sure. If you can cut/paste any information from the second post and edit into the first post, I will deleted the second one and clean the thread up a little bit.

Thanks! I just "deleted" the second post but if you want you can delete it completely.

 

[Onewolf]

I still think you should rethink the whole round building thing. There are construction/materials/techniques for rectangular buildings that far exceed any sort of windstorm rating needed for NC. Every time you try to do something productive in your round building you're going to wish it had some nice long straight walls. My $0.02.

 

[Pointbock]

I say stick with the yurt style. This isn't the first one ever made but many people are uncomfortable with "different." I'd spend more time on the research end of things, and find a few other forums where folks have more experience with this type of structure. Good Luck!