Anchorage Requirements on a Manufactured Home. What does HUD require?

Written By: Paul Hayman, P.E.

The September 1996 Permanent Foundation Guide to Manufactured Homes (PFGMH) is not very clear on exactly what it requires by way of anchorage (also referred to as “
tie downs”). There are two kinds of anchorage: vertical anchorage to resist uplifting forces due to wind or seismic events and lateral anchorage to resist side-to-side movement.

The PFGMH has this to say about vertical anchorage (paragraph 100.C): “Vertical stability: 1. Rated anchorage capacity to prevent uplifting and overturning due to wind or seismic forces, whichever controls. Screw in soil anchors are not considered a permanent anchorage.” There are calculations to determine now much vertical anchorage is required. You might think the weight of the unit (which can exceed 80,000 pounds fully loaded) would be enough to resist uplifting in all but the most extreme weather—tornadoes and hurricanes, and you would probably be right.
Just to be clear, the PFGMH does not require anchorage to withstand a tornado or hurricane. In the vast majority of the US, anchorage is required to withstand an 80 mph wind. However, most, but not all, manufacturers, states and federal agencies do require additional anchorage.

The above quote plainly states that ground anchors, the most common type, are not permissible. This requires tie downs be anchored into concrete. This can be accomplished with metal straps (most commonly), welded rebar, cable or proprietary systems. In some states (i.e., AZ) it was, and still is, common to provide anchorage by nailing, or otherwise connecting, the rim joist to a substantive skirting wall.

Lateral anchorage protects the home from side-to-side movement. The PFGMH states: “Lateral stability. Rated anchorage capacity to prevent sliding due to wind or seismic forces, whichever controls, in the transverse and longitudinal directions.” Note that there is no restriction against ground anchors as there is in the vertical section. Very few engineers follow the letter of this requirement—concrete anchors for vertical stability and ground anchors for lateral stability—because it just doesn’t make any sense. Most engineers will either allow ground anchors for all anchorage or require concrete anchors for all anchorage. At least that way they are consistent. Metal strapping, cable, rebar or proprietary systems can supply this lateral anchorage.

The most common proprietary system, manufactured by several companies, consists of a large pan that sits under a pier. There are three arms attached to the pan that are bolted to the I-beams to resist lateral movement—two to the same I-beam as the pier and one to the adjacent I-beam. These systems are very easy to put in after the fact and are the most common upgrade systems. Often in upgrades, these proprietary systems are the only feasible option because the skirting wall prevents access for the equipment to install the ground anchors. The new HUD Model Installation Rules allow ground anchors for all anchorage requirements.

The bottom line is to find an engineer familiar with the nuances of the PFGMH. An engineer unfamiliar with the fine print of the HUD manual can wind up requiring much more substantial (and expensive) upgrades than necessary.

About the Writer. Paul Hayman, P.E., is owner of Hayman Residential Engineering Services, Inc. His company specializes in providing engineering certificates in 49 states. He can be reached at hresanswers@hayman-res.com or www.hayman-res.com.

SOURCE: Published by NAMP Publishing Group, a division of the National Association of Mortgage Processors (http://www.MortgageProcessor.org)