Let’s Talk About Spacers and Shims

A few weeks ago, we looked at pier cap blocks, so now it makes sense to talk about spacers and shims.

Just so we are on the same page, a spacer can be used to fill in the space between the cap block and the frame (chassis) if that space is more than can be shimmed (1″). Sounds simple enough, right??? Well think again.

Double block pier, concrete footing under the ground vapor barrier, double 4″ solid masonry cap blocks, 2″ hardwood spacers and hardwood shims. Nice job!

The Model Manufactured Home Installation Standards describe a spacer as “Hardwood plates no thicker than 2” nominal in thickness or 2” of 4” nominal concrete block…”. (3285.304(c)(3)). But the individual manufacturers’ installation instructions often provide more options, and sometimes confusing details.

Most manufacturers allow 2” x 6” hardwood spacers. One spacer for single stack block pier, two spacers for a double stack block pier (one per cap block). If you stick with this, you will be in good shape.

 A recently revised instruction manual now defines a spacer as “hardwood, southern pine, or Douglas fir larch dimensional lumber 1x or 2x (2 layers maximum)”.  This is the first time I have seen pine in the same category as hardwood, not to mention allowing the spacer to be 1” thick. So, for those of you that like to use 5/4 pressure treated decking boards as spacers this appears to be one of the first DAPIA approvals for this method.

2″ hardwood spacer on a 4″ solid masonry cap block. Too bad the strap is loose.

When checking your installation manual, look at both the table and the text. Many instructions will specify “Nominal 2” thick boards” in the Pier Minimum Specification Table. BUT, if you turn to the text at “Install Shims” generally on the next page, they allow hardwood or concrete, not just 2” thick boards.

spacer text

Pressure treated hardwood dimension lumber is mentioned in a few manuals, but I have yet to see pressure treated hardwood. Maybe it’s a regional thing, but where I come from, hardwood is not pressure treated.

I even saw a few manufacturers’ installation instructions that don’t mention spacers. Just be sure to double check with your manufacturer to be certain that you are following their instructions.

When it comes to the overall size of the spacer, most pier illustrations show a 2” x 6” spacer board. While the installation instructions don’t specify the length of the spacer, the illustrations appear to show them the full length of the pier cap (16”). 

While there appears to be variation in the materials prescribed by different manufacturers, make sure to stay away from plywood or OSB! These materials will delaminate rather quickly and will certainly cause problems.

When it comes to shims, the installation instructions are more straight forward.

Use proper size spacers and hardwood shims! Not cedar shims like this!

Hardwood shims, 4” x 6” x 1” thick, must be used in pairs, and cannot occupy more than 1” total between the cap block (or spacer) and frame (I-beam). And like the spacers and cap blocks, a double block pier would require two sets of shims, one set atop each cap block.

Plastic Shims

Most manufacturers allow for plastic shims of sufficient capacity. I believe that plastic shims are a great choice for piers in porch areas, where water can pass between the decking boards and lead to premature decay of wooden shims. Also, the plastic shims are grooved so that they cannot slip apart. Again, I did see one manufacturer allows southern pine or Douglas fir shims in addition to hardwood. But generally plastic is the only exception to hardwood shims.

Hopefully, this post will encourage you to examine your current practices regarding spacers and shims, and to be certain you are following the manufacturers’ installation instructions. If you have questions, call the manufactuer’s Quality Assurance Manager and ask him to clarify their requirements.

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Let’s talk about: On-Site Completion

In our previous post we talked about Alternative Construction (click here to view it). This is a special authorization from HUD to allow manufacturers to construct specific homes that do not meet a certain aspect of the Manufactured Home Construction & Safety Standards (MHCSS or HUD Code). Tankless water heaters, accessible showers, garage ready, or two-story homes exceeding the allowable path of travel to an exit door are a few examples. These homes may not meet the letter of the code, but will perform just as well or even better than the MHCSS.

On-Site Completion (SC) is different as the manufactured home (after all site work is done) will meet all aspects of the MHCSS. However, there are certain elements of construction that cannot be completed in the factory, so they will need to be completed at the installation site. For example, a home designed for a stucco or brick exterior may be shipped to the installation location where the stucco or brick can then be applied. Maybe a home was designed for roof dormers or roof extensions, again, these would be added at the installation site. Tile and glass shower enclosures, and completion or installation of a fireplace are a few other examples. These homes would all comply with the MHCSS, but the work can’t be completed until after it is transported to the site.

Tiled Shower enclosures are often completed under On-Site Completion.

 

If you are like me, you might be thinking, why isn’t this addressed as a part of installation? Well, one of the big things that occurred when the manufactured housing law was amended in 2000 was that installation work was somehow separated from construction. So, we ended up with two classifications of work: construction (in the factory) and installation or close- up (occurs on site). However, the line that separates construction and installation is often blurry.

Fireplace and hearth extension along marriage line finished at the site, under On-Site Completion.

 

Even though the federal law was amended almost 18 years ago, the On-Site Completion rule only recently took effect (March 7, 2016), so it is still very new. Through time, many things that are addressed under Alternative Construction may be shifting to On-Site Completion. So keep your eyes open.

Here are the things that installers and retailers should know regarding On-Site Completion (with references to the Manufactured Home Procedural and Enforcement Regulation in the event you want more information than I provide here).

  • The letters “SC” will be included in the serial number of the home. Keep in mind, a manufactured home can have both SC and AC (Alternative Construction) features. 3282.605(a)
  • A Consumer Information Notice must be developed by the manufacturer that explains the process and identifies the work to be completed on site. 3282.603(d)(10)
  • The manufacturer must provide a “Consumer Information Notice” and have it prominently on display in the home (often by the Health Notice in the kitchen). 3282.606(b)

Typical Consumer Notice

  • The retailer (or manufacturer) must provide a copy of the Consumer Information Notice to perspective purchasers before they enter into the sales agreement.  3282.606(c).
  • The manufacturer is required to provide all of the designs to be followed and materials necessary to complete the construction outlined under the On-Site Completion provisions. 3282.608
  • If the manufacturer expects their retailer or installer to perform this work at the job site, the manufacturer is to provide authorization before the work begins. “However, the manufacturer is responsible for the adequacy of all On-Site Completion work regardless of who does the work…” 3282.602(b)
  • Prior to occupancy, the manufacturer must assure that the On-Site Completion work is inspected. This may require inspections by the manufacturer and IPIA (2 separate inspections) or the IPIA can accept the manufacturers inspection (which appears to be the most common approach). 3282.605(c).
  • The homeowner and retailer are to receive a final site inspection report and certification of completion after all inspections have been conducted. 3282.608(m)

I hope that both the professional installer and the retailer understand that this means additional paper work and record keeping.

If you are the professional installer and are expected to perform this work, make certain you have been given the written authority from the manufacturer before you start the work. Maintain this paperwork in your file for the home along with copies of the documentation provided by the manufacturer.

As the retailer, have a record that you provided the Consumer Information Notice to the purchaser before the sale. Have them sign and date the notice, and keep a copy in your home file.

If a retailer or installer is going to accept responsibility for any part of the inspection process, they should assure the authorization to conduct the inspection is received from the manufacturer in writing. Also, keep copies of the construction designs and the “On-Site Inspection Report”.

Finally, always keep in mind that the entire On-Site Completion process is the responsibility of the manufacturer. If you ever are unsure or have questions on the SC process, talk to the manufacturer’s Quality Assurance Manager. He is the one with all the answers!

Piers-Part 3-Pier Components

The previous two blog postings have concentrated on pier footings and caps. Now we should look at the pier itself to make sure we understand the backbone of the support pier.

Two Core Block

Most piers are constructed with dry stacked concrete blocks. As we mentioned in the previous posts, a single stack concrete block pier can typically carry a pier load up to 8,000 lbs. (with an exception made for at least one major manufacturer). I have had installers ask me about the difference between 2 core and 3 core blocks. For our purposes, there is no difference. There is a defined top and bottom to the block, but that is for the laying of block and the application of mortar (the webs of the block are slightly wider at the top to give a bigger surface for the mortar). This doesn’t impact their ability to carry loads should you stack them up-side down.

Look over the typical “Load on Frame Supports for Homes Not Requiring Perimeter Blocking” chart below. There are very few configurations that exceed the 8,000 lbs. where you would need double block piers.

NOTE: this chart is for a typical manufactured home. If you are installing a manufactured home that has features like a steep pitched or hinged roof, high side walls, tray ceilings, terra cotta roof tiles, stone veneer finishes, etc., the pier loads can be significantly higher. Always consult the installation instructions for the home you are installing!

The bigger issue that should concern professional installers is the pier height. In general, if you are using single stack block piers, there are two height limits to keep in mind:

  1. The piers at the corners can be no more than 3 blocks high (approximately 24″ plus the cap block)
  2. Typical piers along the main beam are limited to 36″ high.

Should you exceed either of these limits, you need to use double stack (16″ x 16″) block piers. Unless your pier is 67″ or higher, dry stacking of the blocks is acceptable (mortar is not required between or filling the blocks). If you need to construct a pier higher than 67″ most installation manuals require you consult a professional engineer. I did notice that Clayton Homes provides a design in their installation manual for double stack block reinforced piers up to 108″ high, however the footing and pier require reinforcement and there are some big concerns with anchoring a home that high above grade.

Given that rarely are piers overloaded with our conventional installation methods, I am still surprised at the number of installers that continue install manufactured homes with double block piers. Bigger is not always better, and often it is more difficult to construct.

bad-footing-2

Double block pier not properly positioned on the footing. Note the  crack in the block.

A double block pier that is not properly capped, or not properly centered on the footing, is more prone to failure than a single block pier that is well-constructed.

There are some other pier options available that I think we should take a quick look:

  1. The Steel Support Pier. I haven’t seen a lot of installations that use steel support piers, but maybe it is time to reconsider this option. First of all, these steel piers must be listed and labeled to show they are properly designed, manufactured and can carry the loads. If you are buying from one of the major suppliers, you will likely be fine.

In general, these steel support piers are limited to 6,000 lbs. This is sufficient for pier spacing of 8′ apart supporting a typical 14′ or 28′ wide manufactured home. Also be aware that the adjustable riser (screw) should not extend more than 2″ when finally positioned.

Steel support piers must be painted or otherwise protected from corrosion, and must be stamped with their capacity.

I understand that the material cost compared to concrete block may be slightly more, but I think you will easily save that much in labor.

  1. Pre-cast Concrete. As above, pre-cast concrete piers must be labeled and listed, and here is where I find the problems. Very few of the pre-cast concrete piers I have seen were fabricated in a facility where a quality control program is overseen by an independent inspection agency, with testing or calculations to determine the capacity. To be blunt, if some guy is making pre-cast concrete piers in his barn or garage, it is a good bet that these piers are not labeled or listed. Stay clear!

If you are using any pre-cast concrete piers, ask your supplier to provide evidence that their products are labeled and listed.

  1. Helical Pile Foundations. This could become a more common foundation in the future, especially in areas of deep frost penetration.

    Installation of a helical pile foundation

    This foundation installs quickly, with no concrete, no excavation, and it is removable! BUT, you need to get approval from your manufacturer before you proceed.

Ok, I think we should talk just a little more about supporting manufactured homes with piers, so in a few weeks we will look at pier locations to properly support the manufactured home.

As always, refer to the installation manual before you proceed. If you have any thoughts or images, feel free to send them my way!

Breaking Down Pier Construction-Part 1-Footings

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.

Typical Pier Load Chart

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.

Footing must be smooth and flat! This is certain to fail!

 

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.

mushroom-footing

Poorly constructed footing

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.