Tag Archives: rat wall

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!