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We're getting set to build a 32' x 48' shop/garage with doors sized for use with DP's or 5th wheels. While we will be purchasing a 26' TT, I want to size things for use with larger RV's (should we change later and as a resale point). Currently the bid has a 4" thick reinforced concrete slab. I'm wondering about having that increased to 5" and/or specifying concrete with higher psi rating. The shop will be set on pretty darn level pasture ground. Is the 4" reinforced concrete ample for large DP's?

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It's more about surface prep than slab thickness. This varies by region. Either way, nothing better than driving on it (the sub-surface aggregate) for 6 months. It will hopefully crack where the score lines were put in and hopefully not heave with help from the re-bar. You're doing your due diligence so you should be fine.

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I built these buildings but not steel framed, for almost 8 years. We built modern post frame construction with wood frames. I agree with the six months of letting a pad be rained on and run over but we didn't have that time so we build the dirt pad and compacted it well. One thing we did do was use fiber secondary reinforcement. My 24X 30 shop with a 4" 3000psi slab and no rebar or wire has 0 cracks after 12 years of use including having my tractor Ford 601 Workmaster, and my diesel trucks, including my long bed 92 dually back when we first moves here in 2003. I have seen some of the slabs crack and others like mine be perfect. I would use both wire and fiber if you can and 6 inches minimum for the heavier motorhomes. Remember that some fivers have the max 13.5" height almost maxed, but the motorhomes and travel trailers are usually under 13 feet.

 

Another gotcha is folks having their vehicle/RV fit inside, but then realize they have no room to work on the roof of the RV if it has 1 foot or even three feet of clearance. This is where the wooden trusses shine in that they can be stepped over and between when working on the roof of the RV. Lots more to consider but most of the other mistakes can be redone if needed. Remember you can form in a three foot 12" deep section centered on where the normal drive wheels are on average so whenever you drive in straight, the tires are on the thicker section. The rest is usually covered by the local contractor or state local inspectors.

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Here is a resource for modern post frame building. I was the member representative to the NFBA (National Frame Builders Association) for my company and used a lot of their builder member resources for engineering and design. Start here and read through it. http://www.nfba.org/About/default/whatispostframe.html I am no longer in that business because we grew so fast I got tired of the 18 hour days six days a week. Yes I made a lot of money, but I'm too old for 5 am to 9 PM. There are lots of advantages for post frame over steel. And the posts are in the ground in concrete most times, not bolted to the top of the slab footers. So no weight of the building on the slab. Many folks are inexperienced with the engineering for them today.

Edited by RV
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  • 4 weeks later...

The contractor currently doing our master bath remodel is interested in building the shop. He is recommending a monolithic slab, 4 inch, 3000 psi rated, rebar reinforced. He is also recommending conventional frame and truss vs. pole barn. This will use 2x6 exterior walls on 24 inch centers, sheathed with 1/2 OSB, then 29 gauge sheet metal siding and roof. His reasoning is that the cost increase is marginal and this provides a much better, rigid structure which can be easily finished on the interior using conventional fiberglass insulation and sheetrock.

 

One decision I'm still bouncing back and forth about on is side wall height. I really think 16' walls will make the shop look huge near our rambler home - when you also add the 4:12 roof peak. For our immediate needs (TT), 14' walls will be fine. The TT with AC is 133" high. I also see that 13' height doors are available. How many on the forum with smaller class A or C coaches are storing them in garages with 14' high walls?

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I put 17' side walls and 15' tall doors on my RV garage. The doors are just right, but I wish I had gone with 18' side walls now. I have three reasons for this.

 

1. The cost difference would have been minimal.

2. When working on the roof of my RV it is much nicer to be able to walk upright. I can be mostly upright now, but another foot would make it much more comfortable (I have clear span metal beams, no trusses).

3. When siding the building there would have been a lot less cutting of boards. I sided my building in particle board and then hardy panel (concrete board) to match my house. The particle board and hardy panels come in 4x8 and 4x10 standard. With 17' walls, I had to cut a foot off the top of every panel section. With 18' walls there would have been no cutting.

 

In my opinion, 14' walls and 13' doors are not adequate for Class A or 5th wheel storage. Many Class A's and 5ers are in the 13' to 13'6" height range and will not fit in a building with 14' side walls and 13' doors.

Edited by Chad Heiser
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I built my attached garage with 14' walls so the roof matched the house. If I were to do it again it would be taller. Our 5er just fits if I open up the 13' door to absolute maximum. When we bought the latest 5er we had to pass on some because of height. The matching roof looks nice but a couple of feet would be much more pratical and probably look just fine. I also prefer 5.5" to 6" concrete with rebar.

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My NH fifth is 13'3". That is if your approach is level. Mine has some slope which makes the start of entry a little close so I would go at least a 14 " door minimum. I do not know why contractors want to go with 4" concrete. The labor to formup 4 or 6 inch slab is the same just a little more concrete. This is something that should last many years if done right.

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A 14' sidewall with trusses is going to restrict you to at most a 13' entrance. That is not enough for modern RVs.

 

Also, a 4" slab, even properly reinforced is not going to be optimal for heaver RVs, DPs, etc. You need more for that. You can work out the structural load, but 4" is marginal. You could put extra footers in where the RV sits and float the slab on that, though.

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Concrete slab design is heavily dependent on the subgrade material. You can imagine that placing a slab on rock is different than placing a slab on sand or expansive clay. There are places where a 4" reinforced concrete slab is completely adequate, and others where it will fail very quickly.

 

To do this right I would suggest paying a few hundred dollars for some basic soil testing and design advice from a local geotechnical engineering firm. Or at least doing some research on what your local soil types are and making an informed decision based on that.

Edited by mptjelgin
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The contractor currently doing our master bath remodel is interested in building the shop. He is recommending a monolithic slab, 4 inch, 3000 psi rated, rebar reinforced.

 

A guy who remodels baths is not going to do his own concrete work, he's going to sub-contract that to someone else.

 

If I were doing this, I'd go to the nearest concrete plant and get some names of concrete contractors and get some price quotes and advice from some of them. They will KNOW what works and what doesn't in your area, based on the soil and temperatures and so forth.

 

After one of them pours your slab, then a carpenter can put your building up on top of it.

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  • 5 months later...

Finally, the 30' x 40' shop is in progress.  It will have a 12'w x 13'h door and a 10'w x 10'h door.  Chose to keep the wall height at 14'-4" which will provide a large door opening clearance of 12'11" including the 4" concrete pad.  Plenty high for the TT whose max height with roof air is 11' 3".  The 16' high walls and 14' high door was tempting but we just didn't want that large a profile.

IMG_7864.JPG

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Looking good so far.  You will be really happy with it.

I really like my shop.  It is so much nicer to store and work on things inside.  Even as big as mine is (35'x50' with 17' side walls and two 14' wide by 15' tall doors) though, I wish it was bigger.  I don't really need it to be bigger and it fit the space I had to build it, but I still wish it was bigger.  The shop sure gets small inside with the 42' DRV, the Kenworth, the Jeep and my side by side all wedged in there. ;)

 

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