Tag Archives: rat wall

Avoid Being Driven Crazy With Barndominium Questions Part I

Avoiding Being Driven Crazy With Barndominium Questions Part I

Loyal reader and client GREG in KENTWOOD is planning his new post frame barndominium home and has questions no one else will answer. Mike’s answers are in italics.


Good morning, I hope all is well with you.  

 I have some questions that I would like to understand and it is driving me crazy, because no one other than Hansen Pole Building, it seems will answer my questions and return my calls.  Most the crazy part is about the slab. I find cement workers are not great communicators.  I am 56, a mechanical engineer, have renovated several houses from the studs up, I can do plumbing without butt crack showing, I raced stock cars for 10 years, I tell you this, because I know how to build stuff and am not afraid of hard work. My job requires a detailed list of Bill of Materials, precision drills and reamers and very detailed processes to make fuel injection parts, ABS brake parts and other.  I hope you can help out, as I am ignorant about some simple facts that are driving me crazy.

Mike: We believe good communication is essential to successful completion of most any building. You will find we strive towards written communications in order to minimize (or eliminate) possible miscommunication of important facts and details).

I also would like to say Brenner is doing a fine job.

Mike: Brenner has a passion for post frame buildings and he is not at all afraid to reach out to higher authorities for answers to complex structural questions.

Due to your great level of communication, you so far are my #1 choice to partner with on my new house.  I also would be willing to visit you in MN if you have/think I could see examples of my questions first hand.  

Statements from details I learned at Code Meeting:

  • In Michigan, in my county I need to have a 2’ foam, below grade, with R10 barrier, around the full foundation or slab perimeter. 
  • So my building code staff recommends a Rat Wall 2’ below grade, or a crawl space with footers, or a slab with a wooden wall built to hold the 2’ foam below grade.
  • I don’t really have a problem with the Rat Wall, but it certainly will add more cost to cement.

My questions on this topic are:

    • If using a Rat Wall, it seems like the 6” * 6” poles will then be encapsulated in cement for about 2.5’ at minimum.  I thought the poles were not supposed to be in cement as it causes probably more decay than dirt.
      Mike: Concrete does not cause premature decay of properly pressure
      preservative treated columns.
    • What are your thoughts?
      Personally I would build over a crawl space because my knees are far happier living on wood than concrete.
    • What would you recommend for the 2’ below grade issue regarding the slab foundation?  Mike: With a slab on grade, you can use rigid perimeter insulation without a need for a ‘rat wall’ or wood foundation wall. It can be held in place by backfill on each side. It can be placed running from below base trim on exterior of splash plank, or on inside of splash plank. If on inside of splash plank, it eliminates having to protect it from possible UV degradation.

On attachment pictures in the middle pages show where the 6” X 6” poles go and spacing, listed are my questions:

I am concerned with spacing of 12’ and 14’ of the 6” X 6” posts.  (I’m sure the loading is OK, within code, but see below)
Actually with your 21 foot eave height and Exposure C for wind 6×6 columns will not engineer out. Your building will have glulaminated columns manufactured out of high strength 2×6 or 2×8 depending upon location.

    • One concern is getting wood in today’s supply chain that are straight enough for the girt boards and purlins at 12’ and 14’, should I be concerned? ( I can’t get straight 2” X 4” at 8’ for the walls I have built in last year alone.) Mike: Lumber is obviously organic and we can no longer cut down those old growth trees where one might be able to get straight grained, narrow growth ringed lumber with few or no defects and very little warp, twist or cup. One beauty of steel roofing and siding is it hides a plethora of framing imperfections (like warp), due to high ribs of steel siding.

When using double trusses, at all locations, why not just go to 8’ centers on the poles?   I know it is more 6” X 6” poles, but really is not anymore trusses and may help get straighter girt and purlin boards. 
Mike: Wider spaced columns allow for more flexibility in location of doors and windows and an added advantage of not having to dig as many holes. If you were thinking of using a single truss every eight feet, rather than a double truss every 12, I would discourage it. Double trusses allow for true load sharing and eliminate any possibility of a single truss having a weak point, where under extreme loads (beyond design loads) it may fail and bring down your entire roof.

Come back tomorrow for Part II in Barndominium Questions!

Cabin Design Over a Crawl Space

Loyal readers may recall recent articles guest written by recently retired Hansen Pole Buildings’ Designer Rick Carr as he designed and erected his post frame hunting cabin over a crawl space (his journey began here: https://www.hansenpolebuildings.com/2019/03/development-of-my-cabin-plans/).

Today ROB in DENVER writes:

“Hi Mike.  I was given your contact info by one of your designers; she suggested I contact you with my questions.

 I’m currently looking at options to put up a cabin in Colorado.  The location is up near Leadville at around 10,000’ elevation and the soil (I’m told by local engineering) does not have much clay; it is more a mixture of sand and rock.  Frost level up there is 4’ down.

 Question A):

 I’ve checked out some of the construction options Hansen lists and I like the option of a pole foundation but I’m a little concerned about possible shifting due to temperature changes, etc.  I definitely would want to do a raised/suspended floor with crawl space underneath (rather than building on slab).  The cabin size I’d like to build is about 24×48’ and I don’t want to have to chase it going “caterpillar” downstream since I’d have no way to raise/lower points if they’re locked into poles.

 Do you have any data/literature you could point me to and/or just provide anecdotal background from your experience putting in these types of cabins under these conditions?  I’d actually like to go 2 floors but I’m then concerned about the additional load it might put onto these poles.

Question B):  My other concern is critters tunneling in and making an unwanted home in the crawl space.  If I’m not going with concrete pad and/or block stem wall foundation around the edges, what means are available to prevent animals from digging under any skirting I’d put on and making their home in the warmer space underneath?

Thanks in advance.”

Mike the Pole Barn Guru responds:

Thank you very much for reaching out to me. We are used to dealing with all sorts of rather extreme conditions, especially temperature change, since we can go from -40 to 110 where we are located.

Question A: Temperature changes are not going to perceptibly cause any shifting of columns, unless you had conditions where frost depth would be greater than four feet and there was enough water in your soil to create a frost heave. Given soil with little or no clay, it is probably a non-issue.

Building columns are actually very strong loaded vertically. We have a lakefront home outside of Spokane, Washington where our 22′ x 24′ garage sits on 14 feet of grade change. Soils are similar in condition to what you describe. Our garage has a four inch thick concrete floor on top of a wood framework 14 feet above grade on downhill side. Above garage level we have attic trusses with a bonus room we use as office space. Below the garage level we have a 16′ x 22′ studio apartment. This building is now 30 years old and has performed admirably with no noticeable settling or column movements, even with two SUVs parked inside. I would have no undue concerns of two stories in your setting.

Question B: You could do what I did and stop the siding at the bottom of floor framing, covering underside of floor joists with OSB, plywood, T1-11 or even steel siding. In Michigan we have run into a “rat wall” ordinance on occasion: https://www.hansenpolebuildings.com/2012/06/rat-wall/. Besides solutions offered within the article, one could also use a metal rat wall – constructed out of quarter or half inch mesh (aka hardware cloth). Rolls of mesh are typically three feet wide, so you could dig a trench around your building perimeter two feet deep and a foot in width. Attach top edge to inside of pressure preservative treated splash plank then down and out into trench. Overlap at any splices and use a separate piece on each corner to provide seam corners.

Good Luck! And let me know what you did and how it worked out.

Pole Barn Protection with a Rat Wall

Several months ago one of our clients posed an interesting question to us.

Her building site was to be in a small township in Michigan. When she contacted her Building Department, in regards to building, she was told all buildings required a concrete wall around the perimeter.

She was concerned about the cost involved in having to pour a concrete wall, as well as having to perhaps place her new pole building on top of this wall. After consultation with our engineers, we assured her there was no structural reason for a post frame building to have to be constructed upon a foundation wall. The engineer sealed plans would reflect the pressure preservative columns being embedded into the ground, as well as no concrete foundation wall needed structurally.

Now comes the unusual twist, as well as where I got my lesson for the day.

It turns out her township had enacted a “varmint ordinance” about a dozen years ago. The ordinance requires all buildings to have a four inch wide by two foot deep concrete “rat wall” around the perimeter, with the idea being to prevent rats from burrowing under concrete floors. In reality, the purpose of a rat wall is to prevent burrowing vermin from entering a house, basement or crawlspace under the footers of the foundation. In the case of a building with a concrete slab, burrowing “critters” could not possibly enter the structure.

I had never heard of such a thing, so I started doing my research. My first resource was the International Building Codes. Nowhere in the Code could I find any reference or need for such a rat wall. When questioned, the township Building Official confirmed the requirement is not a part of the Building Code, and had been enacted specifically by their township.

It appears this to be an idea which seems to be limited, for the most part, to Michigan.

Now a rat wall is not a foundation, but simply a non-decaying barrier. Some sources feel there is no need to go to the expense of pouring concrete to meet the requirements of a rat wall. Designing the building with a concrete trench footing, between the embedded columns, meets the requirements, however is just a lot more costly to do it this way. Where a so-called rat wall is required, often any kind of material which will not decay over time can be used. Alternatives may be anything from old sheets of rigid plastic to pressure treated lumber.

My advice, in the event your Building Department has such an ordinance, have a discussion with the Building Official as to the purpose of the rule. If it is indeed to prevent varmints from entering the building and it will have a full concrete slab floor, then the point could be argued as to the slab itself satisfying the requirement. Should the idea be to eliminate burrow under the slab as well, then ask what will be accepted as a material for a rat wall. If told it MUST be concrete, argue the point, as he’s costing you needless money!