So far this month, I have had the chance to work with over 50 professional installers over several different training opportunities. One thing that became evident is that we all need to improve our understanding of the most basic part of a manufactured home foundation, the piers and footings.
When examining our support piers, I think it best to drill down to the bottom and work our way up. And at the bottom of every pier is the footing. Basically, the job of the footing is to spread the pier load in a way that keeps the pier from sinking into the earth (I like to compare a footing to a snow shoe. It keeps you from sinking into the snow by spreading out your body weight over a larger area).
There are three things you need to consider when you are determining what size and type of footing to use:
1. How much weight will the footing have to support?
2. How much weight can the ground under the footing support?
3. Will the footing shift or heave due to frost penetrating under the footing?
To be straight with you, there is a lot of information in our industry that is peppered with wiggle words and qualifiers that can mislead or misguide professional installers. I will do my best to just give you the facts based in the building science that I have examined.
In general, a pier supporting the manufactured home along the frame (you may call it the chassis), spaced every 8’ will need to support approximately 5,500 lbs. in the South roof load zone. If you are installing the home in the north roof load zone, you are looking at up to 6,900 lbs. These loads include the actual weight of the home, the assumed weight of people, furniture, the other contents inside the home (live load), and the anticipated weight of snow on the roof. The pier load charts in the manufacturers installation instructions are where you will find the actual load per pier. Since most of the country is in the South roof load zone, we will assume that each of our piers will need to carry 5,500 lbs.
Unfortunately, too many professional installers don’t put much thought into how much weight the soil can carry (AKA Soil Bearing Capacity). So either they use the default approach, or just do it the way they have for years. The default approach is found in the International Residential Code and referenced in the manufacturers installation instructions. This basically allows you to assume 1,500 lbs. per square foot soil bearing capacity for a decent, typical, well drained site.
So, if my pier load is 5,500 lbs., and my soil can carry 1,500 lbs. per square foot, I need to spread the load out over 3.6 square feet of earth (5,500 ÷ 1,500 =3.6). But you should go to the charts in the installation instructions where the math is done for you. This chart shows a 24” x 24” footing is needed, slightly larger than the 3.6 we calculated.
If your footings are poured in place concrete, they must be at least 6” thick and depending on the load and soil bearing capacity, possibly thicker. The chart above requires our footing to be 8″ thick.
When you order the concrete, make sure the “footing mix” you request is 28-day compressive strength of 3,000 psi. In checking with a few concrete suppliers, there was some variation. Be certain that you finish the concrete to provide a flat, smooth, and level surface for the pier to rest.
If you use pre-cast concrete, they must be at least 4″ thick, meeting with the ASTM standard C90-02a. Patio slabs from the garden center generally do not meet this standard.
Listed and labeled ABS plastic footing pads can be used where protection from frost has been provided, or if the pad is placed below the maximum frost penetration line.
Ok, so we now know that our load on the pier is 5,500 lbs., and our footing will need to be 2 feet square as determined from the chart. But what about the construction of the actual pier? Almost every installation manual states that a single 8” x 16” concrete block can carry 8,000 lbs. (just stay under 36″ high). So, your pier can be constructed with single blocks that can easily carry the 5,500 lbs. load (If you are installing a Clayton product, they limit a dry stack, single block pier to 5,760 lbs.). Make certain you look at the specific installation instructions for the home you are installing.
In the Middle and North roof load zones, the piers loads are a little greater, and remember that not every home is the same! Next week we will talk about frost protection, the cap blocks, shims, and spacers that are needed to complete pier construction.