Overlooked Support Piers

I recently came across another situation where the “pier print” provided by the manufacturer was in error and resulted in a pretty significant problem along the marriage line of the home.

Those of you that I have worked with in the past know that I am not a fan of pier prints. This is primarily because they are not reviewed by the design approval agencies-DAPIA’s. As a result, they often miss some important marriage line and side wall supports for the home. One thing every pier print does contain is a little note telling the installer to always refer to the installation manual. That way, if a problem pops up, it is the installer who takes the blame.

Typical pier print-where is the DAPIA stamp of approval?

 

So, let’s look at a few problem areas that are often overlooked and should be checked against the specific DAPIA approved installation manual. Remember, while installation manuals look alike, there are variations that can lead to big problems if overlooked.

An excerpt from a typical installation manual. But, there are variations that you need to know!

 

Almost every manufacturer’s installation manual requires support piers (with footings) for manufactured homes with fireplaces installed along the side wall or marriage wall. Rarely are these shown on any pier print, and I have not seen them on any actual installations. However, every manual I have checked requires them. Installers would be wise to start including a support pier under the rim joist for homes with fireplaces at the marriage wall or side wall. Maybe an adjustable outrigger? While I have not seen this in the installation manuals, it would be worth asking the manufacturers to approve.

Watch for unusual window configurations!

Everyone knows that openings in the side wall or marriage wall wider than four feet require supports, but often overlooked are multiple windows that are ganged together with mullions. If there are not studs separating each window, it is likely that pier supports will be needed. Be alert for other unusual window configurations that may be 4’ wide, often over a kitchen sink.  These need support as well.

Kitchen window that needs support!

 

Certain manufacturers want you to provide “intermediate supports” at any marriage line span greater than 10’. I have seen these shown on a few pier prints, but not consistently. So double check the manual to see if you need to provide intermediate supports along the marriage line.

Speaking of marriage lines, when it comes to piers and footings under marriage line openings, a good number of manufacturer’s installation manuals have a little note that is overlooked by many installers. The note says that “if the support is shared by spans on both sections of the manufactured home, add the loads together”. In essence, the load shown in the chart is only for one of the two home sections. Therefore, you may need to double the load from the chart when the marriage line opening is on both the A & B sections. This is a manufacturer specific issue.

Another often overlooked area is where the “through-the-rim” crossover ducts penetrate the rim joist at the marriage line (except perimeter frame homes). Quite a few manufacturers call out for these supports in the installation manual, but rarely do they show up on any pier print or even marked under the home as a needed pier location.

Improperly installed G strap and pier.

 

G straps (shear wall straps), are not utilized by many manufacturers, but I see them enough to warrant mention. I don’t recall ever seeing them with the proper pier/footing. Even if you are using an alternative anchoring system, if the home is designed with these G straps, they must be provided with an anchor, strap and often a pier. These G straps have more to do with the transfer of wind load through the structure than anchoring the home to the ground.

Watch for G straps!

 

Ok, I know this can become a little confusing, but it is definitely worth a harder look. As always, refer to the specific installation instructions for the home you are installing. Read all of the notes, and if you are unsure, call the Quality Assurance Manager at the factory.

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Embedment Factors? Cohesive Soils? Let’s talk about it!

Looking over a recently revised manufactured home installation manual, (DAPIA approved in Feb. 2018),  I ran across something that I thought was worth a closer look.

In the section of the manual that talks about footings, I found a chart titled “Foundation Embedment Factors for Cohesive Soils” (there is also a chart for Non-Cohesive Soils).

In case you are wondering, cohesive soils are described as clay, or soil with a high clay content, which has cohesive strength. Cohesive soil does not crumble, can be excavated with vertical sides, and is like plastic when wet. Cohesive soil is hard to break up when dry and sticks together when submerged. Cohesive soils include clayey silt, sandy clay, silty clay, clay. 

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Cohesive Soil

 

On the other hand, non-cohesive soils are loose soils like sand, or sandy soils.

Ultimately, by using these charts and directions, you can increase the maximum load per footing based on the depth that the footing is embedded in the soil.

Let’s try to work through the process.  This manufacturer tells me that a 14’ wide home, in the south (20 psf) roof load zone, with piers spaced 8’ apart, has a pier load of 5640 pounds (per pier).

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Since I like to auger round footings, the chart for circular shape footings says that I need a 28” round footing at a soil bearing capacity of 1,500 PSF. But my auger is only 24” diameter! Well, this is where the “Foundation Embedment Factors for Cohesive Soils” chart might come in handy!


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Since I can only dig 24″ round footings, I need to start there. The chart for circular footings tells me that a 24” round footing can carry 4710 pounds (again 1,500 Soil bearing capacity). I know my 24” round footing will be 36” deep (for example, to get below the frost line) in a clay (cohesive) soil, so I can multiply the 24” footing capacity by the 1.56 as indicated in the cohesive soils chart and my 24” footing works! (4710 pounds x 1.56=7347 pounds, well beyond the 5640 pound load needed to support the home).

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It is not as confusing as it seems. You just need to know if your soil is cohesive (clay) or non-cohesive and know the footing size and depth. Use the chart to determine the embedment factor and multiply the footing capacity by the factor from the chart.

A few important things to consider before you start reducing footing sizes. Currently, this is specific to only a few manufacturers. You need to check with the Quality Control Manager at your factories to see if they allow you to utilize embedment factors. One manufacturer told me that they provide these designs only upon request.

Next, you need to have a very thorough understanding of the soil at the job site. You need to have all of your documents in order and make certain that the building code official (and possibly the purchaser) understand how you are calculating these footings sizes/loads. As always, keep good records for your installation files, including copies of these charts.

While I am not an engineer, I do think that the embedment factor is to be applied only to poured in place concrete footings. But you can verify that with the factory. 

Finally, on the one manual I reviewed, there is a big typographical error, labeling the chart for non-cohesive soils, so be cautious. Stay in touch with Quality Control folks and watch for further changes to the manuals. 

Let’s Talk About Pier Caps

After some recent inspections I thought we should talk a little bit about properly capping concrete block piers.

The function of the pier cap is to evenly distribute the weight of the manufactured home, its contents and the added weight of any potential snow on the roof into the concrete block pier. Ultimately, taking approximately 5,000 to 6,000 pounds from the chassis beam and spreading it out over the top of each concrete block pier in a manner so that the pier will not crack, break, or otherwise fail under the weight.

Properly constructed pier with 4″ solid masonry cap blocks

The first thing to remember is that the cap MUST be the same size as the pier.  A pier constructed of single stacked (8” x 16”) concrete blocks, must have a cap that measures 8” x 16”. If the cap does not fully cover the block, the weight will not be applied evenly over the top surface of the block, and the result will likely be a failure of the pier blocks.

Don’t do this! Cap blocks must cover the entire pier to properly transfer the load!

The biggest problem I see with cap blocks are that installers often use improper materials to cap concrete block piers.  DO NOT USE plywood, OSB, 1” thick lumber, or decking boards! Decking boards would include 5/4” x 6” pressure treated lumber or any composite type of decking.

I checked nine different manufacturers installation manuals, and found that six of the nine specify the same materials for pier caps:

Solid precast masonry 4” thick-8” x 16”, pressure treated lumber 2” x 8” x 16” or ½” thick x 8” x 16” painted steel. I must admit, I never saw anyone use ½” steel, so if you do, please send me a picture!

Check the Pier Material Minimum Specification chart in the current installation manual to be certain you are providing proper pier caps.

 

Two manufacturers don’t mention pressure treated lumber, and only specify 2” thick hardwood as an option to 4” masonry or steel. One manufacturer simply says solid masonry or hardwood with no mention of thickness!

2″ x 8″ x 16″ pressure treated lumber pier cap

 Now is a great time to double check that you are using proper materials as pier caps. As installation manuals are prone to change, make certain that you are using the current manuals for the homes you are receiving. I know that some folks are using ABS (plastic) pier caps. If that is you, be sure to get a DAPIA approved design from the manufacturer for your installer file.

Piers You’ve Probably Overlooked!

I know that most professional installers are very conscientious when it comes to proper pier placement. But with all the variations in the installation instructions, some piers are often missed. Even while the manufacturers are required to identify “point load support areas” many areas are still overlooked.

One example of identifying point load locations.

Let’s take a quick look at some of manufacturers installation instructions to see where they do or do not require “point load” support piers.  

  

I started with the new Clayton Installation Manual to see exactly where they require point load support. They want supports at each side of exterior doors in the side wall. No support is needed if the door is on the end wall supported by a “steel header”. So, do we need supports if the door is at the tail end of the home if it doesn’t have a header? They go further and say no support in needed for doors in the side wall if the chassis I-beam spacing is 112”.

A support pier may be needed at through-the-rim crossover locations.

Support is also required at locations where through-the-rim joist heat duct penetrate the floor rim-joist.  I have seen this requirement in a few installation manuals, but not much further direction. I don’t know exactly where to position this pier, and I have never seen these areas identified as point load support areas on the underside of any home. 

A support is needed at each side of a factory installed fireplace when located along the side wall or marriage wall, (again with the exception of fireplaces supported by the front chassis crossmember). 

Adjustable outrigger at patio door location. Note the white paint marking the location.

The Clayton Installation Manual does allow adjustable outriggers to replace piers at the fireplace locations and door locations (less than 48”) along the marriage wall or side wall. The Clayton Manual doesn’t mention the use of an adjustable outrigger at the “through-the-rim” heat duct location. (check out page 21 on the Clayton Installation Manual).

So, just for fun, I decided to compare a few other installation manuals, starting with Champion.  

Window configuration creates side wall opening greater than 4′.

When it comes to doors, Champion is pretty clear (page 16). They require a support at each point load including: both sides of doors in the side wall. If the door is less than 48”, adjustable outriggers may be used in place of door piers. “Blocking” is not required for doors in non-load bearing end-walls.  

Here is an interesting one: Champion requires support “Under heavy (400 lbs or greater) items, such as heavy furniture, waterbeds, fireplaces and large fish tanks”. I better have a few extra blocks ready for the next visit from my mother-in-law!

Typical porch support.

What about Commodore/Colony you say? Ok, turn to page 15 in their installation manual. They want support at both sides of exterior doors at the side wall, but not at doors in the end walls. Porch posts always require support. They also want a support at through-the-rim heat crossover ducts, and under heavy items like waterbeds, fireplaces, and mothers-in-laws.  

Here is the curve ball: “…where marriage line openings are greater than 10 feet, intermediate supports must be placed at maximum 10 feet on center”. Some other manufacturers have this same requirement. Others only want these intermediate supports if the home has perimeter supports (evenly spaced under the side walls).

T Brace (pier saver).

I think it’s only fair to say, that Commodore/Colony was the first manufacturer I was aware of to introduce “Pier Savers” for support of patio and other exterior doors! I am a big fan of pier savers.  Look up their “Alternate T Brace” addendum A-7.

Adjustable Outrigger

Skyline requires a support at exterior doors on side walls (not end walls), typical 4’ marriage line and side wall openings, through-the-rim crossover ducts, porch posts, heavy furniture, fireplaces, etc. BUT..Skyline utilizes the adjustable outriggers more liberally than most. Basically you can use an adjustable outrigger to replace a support with a load up to 1,700 lbs. Go to their charts on page 20 of their installation manual for span loads. Some Skyline plants provide the adjustable outriggers with the homes. Make sure you get a copy of the “Addendum to Installation Instructions for Installation of Adjustable Outriggers”   

Shear wall strap needs pier support.

Finally, I checked out Fleetwood, pretty similar to the others, except they want support at “labeled G-2 strap locations” (see page 20 in their installation manual)

At this point you might be thinking, “just follow the pier print”. Well, I checked out a handful of “pier prints”, and most of the locations mentioned above are NOT identified on the pier prints. None showed the piers at the through-the-rim crossovers, or at any fireplaces. Also, the “intermediate” supports at the marriage line is missed by most. One pier print I noticed shows piers that defied any reasoning!

OK, here are the take-aways:

There is no “One Size Fits All”. Make sure you take 30 seconds, and open the installation manual and look under the heading “Install Footings” where you “Determine Locations”. Make sure you know each particular manufacturer’s variations.

Don’t trust the pier prints! Never, Ever! Follow only DAPIA stamped (approved) designs and instructions.

Investigate pier savers and adjustable outriggers! Some manufacturers already approve their use. If yours doesn’t, start asking for approval! The squeaky wheel always gets the grease.

And lastly, don’t invite my mother-in-law to your house!

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.