This document discusses the proper methods (and more than one exists) for anchoring manufactured and modular homes with perimeter support foundations, which will be better defined just a little further on. The subject of this document is primarily proper anchoring methods because I frequently find mistakes in these anchoring systems. In my internet searches, I have not come across other publications that specifically address this topic. Building codes and other regulations, such as the Housing and Urban Development's model regulations, and manufacturer installation instructions describe or dictate construction of other foundation elements and the installation of the home; but details on anchoring requirements for these foundation systems is sketchy.
Therefore, I wrote this document to summarize the anchoring methods, and errors in anchoring methods, that I have found while performing over 500 FHA or other inspections on these homes (and all of the photos in this document came from those inspections). This document covers both manufactured and modular homes because some of the home constructions have very similar foundation and anchoring systems. Although Criterium-Cincinnati Engineers does not get many calls for modular home inspections, those I have inspected leads me to believe that the prevalence in anchoring issues are similar to those for manufactured homes.
Note that this article and its contents are only intended as guidance. It is not intended to be used as a design document or to replace any building codes or other applicable rules. Be sure to check local and state building code or other state installation rules for specific installation requirements.
General Guidance for Manufactured and Modular Home Foundation Systems:
Manufactured and modular homes mainly differ visually in that manufactured homes arrive on-site on their own wheels while modular homes are transported on-site via another carrier, such as a flat-bed trailer. For a more detailed description of differences, refer to these web sites:
- http://homebuying.about.com/cs/modulareducation/a/modularhomes.htm.
- http://www.modulartoday.com/modularhomesvsmanufacturedhomes.html.
- http://www.inspectapedia.com/structure/modularterms.htm.
This foundation system uses piers, commonly constructed from concrete blocks (seen in the photo above) for the foundation system. The piers are supposed to be erected on either poured concrete footings or slabs For manufactured and on-frame modular homes that have I-beam structural members that are part of the home structure, the piers are located under and support the I-beams. (For manufactured homes, these beams are commonly called carriage beams because they are used to transport the home to the site. To use common terminology for both manufactured and modular homes, I use "home structural beams" in this document.)
Vinyl skirting or a concrete block perimeter wall is usually installed around underside of the perimeter of the home to enclose the pier foundation system. This skirting or perimeter wall is not part of the foundation system and is not structural. Skirting and perimeter walls have purpose, though, in that they help keep water and vermin from getting under the home, help protect the plumbing from freezing and prevent wind (more particularly high winds) from getting under and lifting the home.
Manufactured homes come in both single and doublewide constructions, while modular homes are usually doublewide construction. Singlewide means that the home is a complete home, constructed so that it can be moved into place as a unit and set on the foundation system. Doublewide structures arrive on-site in two halves that are moved together on a foundation system and the halves are then screwed together. Each half of the doublewide home has comes with a hinged roof. Once the two halves of a doublewide are assembled, the roof is rotated into position and assembled. Although not yet common, complex manufactured and modular homes, such as multiple story structures, are being built. Foundation systems for the more complex structures are similar to the single-story structures, although adjusted for the extra weight.
Both single and double-wide homes would have piers under the home structural beams, while a double-wide home would also have additional piers supporting points along the home's marriage line, as specified by the manufacturer. (The marriage line, which is sometimes also known as the mating line, is where the two halves of a double-wide home meet.) The anchoring systems for pier foundations for manufactured and on-frame modular homes is either a lateral brace system or steel straps with concrete or ground anchors, which are shown in the photos below:
An example of one manufacturer's lateral brace anchored to a concrete slab, in this case.
An example of one manufacturer's steel strap anchored to a concrete slab, in this case.
An example of a manufactured home with a perimeter foundation system that uses foundation beams and concrete block center piers.
Manufactured and modular home perimeter foundations are constructed from either poured concrete or concrete block, similar to site-built homes. Local building codes usually dictate how foundations should be constructed. If the area has no local building codes, construction usually has to adhere to state building codes, which are usually a derivation of the International Building Codes (although the state's current building codes might not be as current as the International Building Codes). Homes installed in areas under the jurisdiction of a building department usually need to have a building department permit and at least a foundation system inspection. Further, installation permitting and inspection requirements can vary from state to state and even from area to area. Be sure to check local building code requirements specific to manufactured and modular homes.
Concrete foundations usually have pockets built in the walls to support the ends of the foundation beams similar to the pockets installed in site-built home foundations to support the ends of these homes' main beams. Concrete block foundations usually have pilasters installed to support the beam ends, and the concrete blocks that comprise the pilasters need to have their holes completely filled with concrete. The concrete fill needs to have rebar reinforcement installed in the concrete. That being said, some concrete block foundations have pockets for the foundation beams instead of pilasters and some poured concrete foundations have pilasters for the foundation beams instead of pockets. Still, even if pockets are installed in a concrete block foundation, pilasters are still usually built in the foundation under the pockets and the blocks are filled with concrete with rebar reinforcement. Examples of foundation beam installations are shown below:
(Top photo) Example of a foundation beam installed in a pocket on top of a pilaster in a concrete block foundation wall.
(Middle photo) Example of a foundation beam installed on a pilaster of a concrete block foundation without a pocket.
(Bottom photo) Example of a foundation beam installed in a pocket in a poured concrete foundation wall.
As the photo below shows, the end of the beam is shimmed. Where the home's structural beams are shimmed, the foundation beams are not normally shimmed. When wood shims are used, they need to be a minimum 4-inch wide hardwood and used in pairs, as shown.
As mentioned previously, while the ends of the home structural beams are shimmed, shims are not usually used between the foundation or pilasters and the foundation beams. The home is usually leveled by installing shims between the foundation beams and the home structural beams because adjustment needed between one foundation beam/structural beam contact point and another can vary significantly. Shims are sometimes used between the middle support piers and the foundation beams. The proper shims between the home structural and foundation beams will be discussed in more detail later in this article.
As the previous and following photos show, the foundation beams are located in nearly the same locations as would be the piers for a pier foundation home. That is, steel beams run the width of the home in similar locations to the piers and support the home's structural beams and marriage line (if needed) in the same way as the piers. The ends of the foundation beams are supported on the home's front and rear perimeter walls.
An example of a perimeter foundation system that uses foundation beams and center support columns.
Depending on the manufacturer requirements, extra piers may be needed to support the marriage line at other locations than above the foundation beams, as shown in the photo below. Installation instructions usually show where marriage line piers are needed and, for newer homes, most manufacturers indicate where support is needed along the marriage line with some kind of mark on the bottom board (the membrane covering the underside of the home). If the manufacturer calls for support of the marriage line at a point less than 2 feet from a foundation beam, then additional support may not be needed.
Homes that have steel columns instead of piers may also need to have support for the marriage line that is farther than 2 feet from the foundation beams. In these cases, additional steel columns can be installed, or if a wall has been installed under the marriage line, for example as part of finishing a basement, the wall can be extended to the marriage line to provide support. However, that wall then becomes a load-bearing wall. Other methods can also be used to support the marriage line between foundation beams; but a licensed engineer or architect should be hired to specify such construction.
Double-wide homes also have blocking installed on top of the foundation beams above all center piers to support the marriage line. Foundation systems that use steel columns instead of piers to support the foundation beams also have blocking is installed on top of the foundation beams above the columns, as shown in the photo below. Blocking is usually wood, although concrete blocks can also be used as long as a cap block is installed on top of the blocks, as visible in the photo above. Blocking needs to be at least the width of the foundation beam flange and at least twice as long as it is wide to assure stability.
For information on the acceptable construction of the piers, please refer to 24 CFR 3285: MODEL MANUFACTURED HOME INSTALLATION STANDARD (http://www.access.gpo.gov/nara/cfr/waisidx_08/24cfr3285_08.html) or your state's current installation standards (which should be a derivation of the Model Manufactured Home Installation Standards). Keep in mind that these standards defer to the manufacturer's installation instructions, if available. Installers should read and follow the manufacturer's installation instructions. Not following the manufacturer's instructions could void the home's warranty or the expose the installer or others to liability issues. The information presented in this article is not intended to replace the installation instructions.
Anchoring Systems for Perimeter Foundation Systems:
To properly anchor manufactured and on-frame modular homes with perimeter foundations, the foundation beams need to be anchored to the foundation AND that the home structural beams need to be fastened to the foundation beams supporting them. This latter part of the anchoring system seems to be most often forgotten or unknown. No one method exists for anchoring the foundation beams to the foundation; but the anchoring method needs to meet these basic criteria:
- BOTH ends of all foundation beams need to be anchored.
- If the foundation beam ends are not located in pockets that are tight enough to prevent twisting of the beams, both sides of each end of the foundation beams need to be anchored or the full widths of the beams have to be anchored (as shown in the example photos later in this article).
- The anchors have to prevent the foundation beams from being pulled laterally out of the pockets or off the pilasters, as could happen if the foundation wall moves outward.
- The anchors have to prevent the foundation beams from being pushed sideways off of the pilasters.
- The anchors or the foundation pockets have to prevent the ends of the foundation beams from being lifted vertically off of the pilasters or within the pockets.
The photo below shows the end of a foundation beam on top of a pilaster. By far the most common method for anchoring foundation beams to concrete block foundations is using the rebar installed to reinforce the pilaster. As shown, the rebar is extended above the pilaster, and bent over onto and welded to the beam, as shown in the photo. This photo shows the preferred method for attaching the rebar to the beam in that the rebar ends are kept short and at least 2 inches of the rebar is welded solidly to the beam. A similar rebar anchor is on the other side of the beam to prevent the beam from moving sideways on the pilaster. Both sides of each rebar end should be welded to each beam.
The photo below shows another variation of rebar anchor that is acceptable, although not as ideal as the previous photo. In this installation, a similar rebar anchor was installed on the other side of the beam. This anchor would be more ideal if the area around the base of the rebar inside the block were mortared to stiffen the rebar.
The photo below shows another rebar/beam anchor that is not acceptable. As this photo shows, the rebar is much longer than in the previous two photos, which allows the rebar to be bent more easily. However, the longer rebar means that the beam is not restrained from excessive movement. Furthermore, only the very end of the rebar is welded, which could allow the rebar to break loose if the beam moved. A closer pocket around the beam would prevent the beam from moving sideways. Additionally, the rebar on both sides of the beam should be shortened and at least 2 inches of both sides of the rebar should be welded to the beam.
A common issue with the rebar anchor is that the rebar does not get welded properly to the beam, as shown in the photo below. As can be seen in the photo, a gap is present between the end of the rebar and the beam flange even though an attempt was made to weld the rebar to the beam. For this installation, the installer only tack-welded the ends of the rebar to the beam and the rebar pulled loose in several places. If the end of the rod had been more solidly welded, the rebar would likely not have pulled loose. However, as with the previous photo, the rebar should have been shortened first and welded closer to the end of the beam.
As the photo below shows, not all uses of rebar as an anchor are acceptable. In this installation, the rebar attaching the foundation beam to the footer (orange arrow) would not constrain movement of the foundation beam and would therefore not be an acceptable anchor. This photo also shows another issue with the pilaster blocks. In this installation, the gaps in the mortar show that the blocks were not fully mortared, filled with concrete or reinforced with rebar. At least, this installer used a sold 4-inch concrete block on top of the pilaster to properly transfer the beam load fully across the pilaster.
As stated previously, poured concrete foundations offer additional anchoring options as shown in the following photos:
The two photos above show variations of the same type of anchor, except one was installed over the top of the foundation beam and the other under the foundation beam. Both photos show that the anchor is bolted to the foundation and both are wider than the beam flanges. The foundation beams need to be welded fully to the anchors, as can be seen clearly in the above photo. Even if the anchor is installed above the beam, welding is needed to restrict the beam from being pulled out of the pocket. Note also that the beam pockets in both photos are just wide enough to accommodate the beam, which helps prevent beam twist. However, these types of anchors should also help prevent twist.
The photo below shows a clever use of the tie-down straps normally used to anchor homes on pier foundation systems. In this case, the straps used to fasten the home structural beams to a concrete footing or pad have been used to anchor this home's structural beams the foundation. Further, the strap has been properly looped around the carriage beam according to the manufacturer's instructions, which firmly secures the strap buckle to the home structural beam. The additional strap between the carriage beam and the end foundation wall should not be needed because the home structural beam's movement is constrained by the foundation and other anchors. If the manufacturer's installation instructions require this strap to be installed, it needs to be installed to prevent voiding warranties. Technically, this installation does not meet the overall guidelines stated earlier because the foundation beams have not been anchored to the foundation or the structural beams have not been fastened to the foundation beams. However, this installation is at least as secure as homes built on pier foundations. Note also that this photo shows an example of an installation with the foundation beam iinstalled near the end of the home strutctural beam instead of a pilaster, pier or column.
In some cases, concrete block is sometimes installed on top of a poured concrete foundation. This kind of change can happen if additional headroom in a basement was desired after the concrete foundation has already been poured. The photo below shows the way one installer, almost acceptably resolved this issue. The installer built pockets in the block for the beams and then constructed angles to anchor the beams to the foundation. While the installer welded the angles to the structural beams, he did not bolt the angles to the foundation, which made this installation not acceptable. The installer also did not fill the blocks surrounding the foundation beams with concrete and install rebar in the concrete to reinforce the block. Additionally, the rebar in the block needs to be anchored into the concrete of the lower part of the foundation.
Up to this point, I have discussed construction of the foundation and anchoring of the foundation beams to the foundation. To complete a proper anchoring system for foundation beam systems, the home structural beams need to be fastened to the foundation beams. Manufacturer installation instructions that I have reviewed require that the structural beams be fastened to the foundation beams at ALL points where they cross. Manufacturers usually allow two fastening methods, either a 1/4"-inch fillet weld or bolting. An example of welding is shown in the following top photo and bolting in the following bottom next two photos.
By far, welding is usually more expedient than bolting and more secure. Before using bolts, installers should check with the manufacturer to assure that drilling holes in the home structural beams will not void the home's warranty. If bolting is allowed, installers should also check for manufacturer's torque requirements for the bolts. Welding installation instructions usually require that both sides of either the home structural or foundation beam be welded; but installers should follow the manufacturer's specific instructions for the home. Installation instructions also normally require that each joint be fully welded across the full width of the structural or foundation beam, as shown in the top photo above. A number of installers us a weld of only 1 or 2 inches, which is normally not acceptable. Alternative methods of fastening the two beams together may exist; but the installer should check with the manufacturer for acceptability. Local building codes should also be checked for requirements for fastening the beams together.
Earlier in this article, I stated that shims are normally installed between the foundation beams and home structural beams rather than between the foundation beams and the foundation. Wood shims, as shown in the top photo below, are not acceptable because they may not support the load and they do not allow proper fastening of the foundation beams to the home structural beams. When the joints between the foundation beams and home structural beams are welded, metal shims need to be installed between the two beams where needed. Additionally, when shimmed joints are welded, the joint needs to be fully welded so that the shims are included in the weld, as shown in the bottom photo below.
Alternative Foundation Systems and Anchoring Methods:
Previously, I mentioned a second general type of manufactured and modular home construction that is supported directly on the perimeter foundations without need for foundation beams. The marriage line of these homes are usually supported on steel columns or concrete block piers. A load-bearing wall could also be used instead of columns; but, the homes I have inspected that had frame walls built under the marriage line also usually had steel columns. (Again, consult the manufacturer's installation instructions or hire a licensed engineer or architect to verify the construction is acceptable if the home foundation system deviates from the manufacturer's instructions.) From the underside, these homes look very similar to site-built homes, as can be seen in the photos below. The first two photos are manufactured homes and the last photo is a modular home:
Note in the previous three photos that the marriage line is the equivalent of the main beam for a site-built home, except one-half of the marriage line "beam" belongs to each side of the home. These homes come in a variety of constructions, and manufacturers are likely to make changes in the future to make the homes look even more like site-built homes.
Besides differences in the marriage line for these homes compared to the homes that have foundation beams, these homes are anchored around the perimeter to a sole plate that sits on top of the foundation. To fully anchor these homes, the sole plate must be anchored to the foundation AND the home needs to be anchored to the sole plate.
Two methods for anchoring the sole plate are shown in the following two photos. The top photo shows a sole plate that has been fastened to an anchor installed in the foundation similar to methods used for site-built homes. The bottom photo shows a sole plate attached to the foundation using metal straps imbedded in the foundation. Some installers have also used concrete anchors and bolts to fasten the sole plate to the foundation. Local or state building codes, the International Building Code or the manufacturer instructions should be consulted to determine acceptable methods for how the sole plate is fastened to the foundation. Keep in mind that the weaker the fastening method, the more anchors might be needed. For example, I would expect fewer anchors being needed for the anchor bolt fasteners in the top photo than for the strap anchors in the bottom photo.
I have observed two variations in home construction that determine how the home is anchored to the sole plate. For homes with wood joists, the wood joists can be fastened to the sole plate directly, as shown in the photo below. In the installation, the sole plate was oversized and lag screws were used to screw joists to the sole plate.
Homes with steel joists may have a sole plate attached to the bottom of the joists, as shown in the photo below. As can be seen in this photo, this sole plate is then screwed to the foundation sole plate.
Some homes have rim joists that extend below the floor joists, which can be seen in the photo above. For these homes, the rim joist can be fastened to the sole plate from the exterior side of the rim joist prior to siding being installed. On the other hand, some manufacturers extend the exterior sheathing below the floor level and attach the joists to the sheathing, which appears to be the case in the photo above where a strap anchor has been used. Manufacturers may have specified methods for anchoring the home in the installation instructions. If the instructions do not specify an anchoring method, refer to local or state building codes or contact the manufacturer or a licensed engineer or architect for recommendations.
Although manufacturers might allow anchoring the home using the rim joist or exterior sheathing, I do not favor this method of anchoring. One major issue with this anchoring method is that the a gap is left between the exterior sheathing or rim joist and the sole plate because of an error or inaccuracy in the construction. This gap could be sizeable. For one home I inspected, a gap of nearly 1 inch was present in some places around the home's perimeter. The strength in this anchoring method comes mainly from the contact between the sheathing or rim joist and sole plate woods when the fastener pulls them together. If a gap is present, the fasteners takes all of the load, which weakens the anchor. Additionally, verifying whether the home has been properly anchored is difficult to impossible one the siding has been installed. Due to the number of issues with this method of anchoring the home, I recommend that an alternate method that anchors the home's joists to the sole plate be used.
The Wrap:
I have tried to be as thorough as possible in writing this article; however, I have likely missed some important points. Further, manufacturers periodically change designs that could change installation requirements. Further, so much variation exists currently in home designs that a particular home's foundation system and anchoring method may be different than presented in this article, but still be acceptable.
Another important point is that recent HUD rules require states to develop installation rules based on the minimum standards listed near the beginning of this article. HUD further required that new manufactured home sets be inspected according to each state's rules. For installations not covered in the model and state rules, such as perimeter foundation systems, HUD rules and likely many state rules defer to the manufacturer's installation instructions or a licensed engineer or architect designs. If other state rules are similar to those in Ohio, this information must be obtained prior to obtaining a permit to begin construction. Make sure you are aware of your state's requirements so that a potentially stiff fine or reconstruction costs are avoided.
Please be sure to contact us with questions or comments or to request a PDF version of this article. I hope to revise this article based on those comments and questions.
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