Do Mobile Offices Require Anchors and Tie-Downs?

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As with any building, a mobile office needs a good foundation to be rooted securely to the ground and raised up enough for ramp accessibility, protection against flooding and ground moisture, and proper utility connection. This, combined with the weight of the steel structure and the balanced height-to-width ratio that make the unit topple-resistant, reduces the need for anchors and tie-downs in many cases.


“There’s no need for tie-downs, as the units are heavy and go right onto the ground,” said John Rogers, owner of Affordable Portable Housing in Kailua, Hawaii.


“Mobile offices do not require tie-downs; that is deciphered by site requirements,” said Kristina Weinrich, account manager for ModSpace of Berwyn, Penn. “If a trailer needs to be anchored, we must be notified of what type of ground it’s going on — either asphalt, gravel or dirt. Also, all underground utilities must be marked, and Dig Safe must be called if utilities are close to the area where the trailer will be anchored.”


Dig Safe Systems, Inc. is a Wilmington, Mass.-based nonprofit clearinghouse that aids utility companies in marking underground electric and phone cables, water pipes and gas lines so that digging, drilling and hammering can proceed without damaging utility conduits – a risk anchors and tie-downs pose for a mobile office setup.


On the other hand, the absence of anchors and tie-downs poses the risk of inadequate protection against high winds. Since climate change has made virtually all seaside communities hurricane-vulnerable and many landlocked ones tornado-prone, tie-downs and anchors aren’t a bad idea for that extra insurance against the unexpected.


Foremost Insurance Group of Caledonia, Mich., recommends auger (screw-in) anchors for dirt, gravel or clay terrains and rock or drive (hammer-in or drill-in) anchors for a hard rock ground or concrete foundation (which should be at least 4 inches thick). Therefore, mobile office site preparation must include classification of the ground’s soil type by a qualified building inspector. The tie-down is affixed to the anchor with a tension-adjusting system, which could be a drop-forged galvanized-steel turnbuckle tie. Over-the-top tie-downs should be aligned with the roof rafters and protected with brackets or buffers to prevent damage to both the roof and the tie-down steel cables or straps. Cables must be 3/8 to 1/4 inches in diameter; straps should measure 1 1/4 inches wide by .035 inches thick at a minimum.


Extensive preparation is required before anchoring can begin: mobile office leveling, soil type classification, utility line location and marking, and determination of the required number of tie-downs, according to local wind zones and trailer size, and required types of anchors, according to soil type. Triumph Modular of Littleton, Mass., recommends four tie-downs for 8-foot-wide trailers, six for 10-by-36-foot trailers, eight for 10-by-44 and 10-by-50s, ten for 12-by-60s, and twelve for double-wide trailers. Tie-Down Engineering of Atlanta, Ga., recommends driving a 30-foot or 48-foot cross-drive rock anchor into hard rock, and rod-and-helix anchor combos with stabilizer plates into sand, gravel, silt, clay or alluvial fill. Anchors should be driven in all the way at a 10-degree back-angle tilt away from the unit for maximum stability.


Tie-down requirements and regulations for mobile offices vary from state to state. Some don’t require them at all, while others strictly mandate them and even inspect mobile buildings to make sure they’re tied down properly, according to Foremost. So it’s best to consult with a local building inspector for this information before deciding whether to tie down your mobile office. Make sure you know which way the wind is blowing, too.