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!
From talking to others, I have heard that the purpose of preventing rats from burrowing under the slab is not to prevent their entry into the building, but to prevent them from excavating burrows in the soil beneath the slab, thereby weakening the grade on which the slab rests. Does this sound reasonable to you?
Jason ~ Every source I have read, describes the issue as one of keeping vermin from entering a building, not as one of structurally weakening the slab. Way before a building site has so many burrowing rodents to begin causing strength issues with a concrete slab, I’d think exterminations would be in order.
I am currently removing an old concrete floor that was de-stablized due to burrowing ground hogs. When I bought the place it was a problem. The barn was full of junk, and only parts have a concrete floor, no rodent wall. I removed 2 full yards of dirt that was excavated by these buggers. The new floor will have a rodent wall. By the way, I live in Michigan.
I should probably take before and afters.
I for one have a lot of burrowing vermits in my ward and they have caused all kinds of problem. Under my patio I have a buried 2X6 all around and 4″ layer of compacted slag and top layer of sand, Then brick pavers on top. I have a low deck next to it which these critters love to hide in. I never thought they could get under the patio but they did and it had to be repaired several times. I would insist on the rat wall or you could have problems under the floor…
A rat wall is a really good idea and I enthusiastically support local ordinances requiring them for basically any freestanding steucture. Whether the vermin enter the building structure is irrelevant. You don’t want vermin tunneling, burrowing, and nesting under your slab and a rat wall prevents that perfectly. You say “well then just exterminate the vermin”. The problem with that is twofold, first it is an ongoing problem. You’ll be exterminating vermin (a lot of which are simply natural local wildlife that just can’t resist the ideal conditions you laid out for them with a nice concrete roof and soft dirt floor). The second problem is that is really hard to enforce. So your lack of a rat wall and subsequent neglect will mean if I live next door to you I’m going to have rats also. If everybody has a rat wall it isn’t an issue. The small extra cost is well worth it.
Lots of options other than pouring concrete as well.
I built an agricultural use pole barn in Michigan ten years ago and the project designer (an employee of the lumber yard) recommended three courses of pressure treated 2X6 T&G skirting below grade to combat burrowing critters. I used a single 2X8 and it was not an effective barrier. It’s not a big deal for the uses of my barn, but I would bury at least 18″ of pressure treated 2X below grade for a building that needed an effective barrier. Each T&G 2X6 only supplies about 4-3/4″ of face, so four courses would be required below grade.
Carl ~ The pressure preservative treated T&G (tongue and groove) lumber should make an effective barrier. In order to meet the requirements of the local ordinance, it would take 24 inches (minimum) of pressure treated lumber below grade. If using 2×6 material (which usually has a net coverage of five inches after milling) this would add five rows below the standard splash plank itself. As always, consult with your local Building Department for approval.
I believe this ” rat wall” requirement came from the Uniform Building Code which was in effect in central and northern parts of Michigan in the early 80’s.till 2000 when the State Code was enacted. It seems like a reasonable request for pole structures that have dirt floors. Other than that it is a cost that should be put into a slab.
My money would be (and is) invested in concrete slab floors as well.
what if you are building a pole barn home and it will have pluming under the slab would a rat wall or foundation be required?
Having plumbing under a concrete slab has nothing to do with the possible requirement of burrowing rodent protection. Some localities have enacted ordinances to protect for burrowing critters (which this article addresses).
If I put a 24 in rat wall in is it going to heave my concert or poles cause they are 42in under grade on a 6in bed of ready mix concrete
Unless you thermally isolate it with insulation there is a strong chance it would frost heave.