Before we get too deep in the how many anchors are needed to properly tie-down (or stabilize) a manufactured home, let’s make sure we know where manufacturers require tie-down anchors. There’s not one simple answer, but be aware that you may need to provide anchors at the following locations:
End Walls (longitudinal anchors) – some manufacturers only require end wall anchors in Wind Zone 2 or 3, some require them in every wind zone, others only require them for short (less than 52’ or so) homes in wind zone 1. It is critical that an installer know which manufacturers require end wall anchors. A few factories may weld a bracket to the chassis (frame) that might indicate you need longitudinal anchors, but that is hit or miss. The only way you will know for sure is to check the installation instructions.
Marriage Line anchors – again, this is manufacturer specific and there can be various types. In Wind Zone 1, only a few manufacturers require these. Generally, the manufacturer will install straps (sometimes called “G” straps) or brackets along the marriage line for you to attach to a ground anchor. Make certain you install the anchors directly below the bracket or strap before both home sections are set in place.
Shear Wall anchors – like the marriage line anchors above, these are rare in Wind Zone 1, but depending on the design of the home they could be required. Again, you should find brackets or straps. Shear wall anchors could be required on marriage walls or side walls.
Porch Post anchors – again, these are primarily used in Wind Zone 2 and 3, but could be needed in Wind Zone 1. Watch for brackets or coiled up straps.
Side Wall anchors – if you are using anchors and tie down straps to secure the home against a wind storm, every home requires anchors under the side wall, with diagonal straps to the frame for Wind Zone 1. Wind Zone 2 and 3 require both diagonal and vertical (to brackets along the side wall).
To determine how to space the rest of the anchors needed, start digging through the maze of charts provided in the installation instructions. You may need to sort through as many as 21 different spacing charts in a single instruction manual! Make sure you pick the right one! Take a few extra seconds to read the title of the charts to be certain you have it right one!
So how do we determine pier spacing for the remaining tie downs needed to properly anchor the home? You can’t just install an anchor at every pier location. There are too many factors to consider. Let’s assume we are installing a typical two-section home (28′ wide) in Wind Zone 1. The home has 7’ side walls, a standard roof pitch (under 4/12), 99 ½” frame (distance between the I-beams) supported by 12” high piers comprised of one 8” block, a 4” cap block, and a 12” I-beam (8 + 4+ 12=24”).
Looking at this chart, we need anchors spaced every 12’ (don’t forget, the first and last anchors should be within 2’ of the end of the home).
Did you notice the asterisk on the charts if your piers are higher than 25”? That means that the strap angle exceeded 60° and the strap must be attached to the far beam of the chassis.
Confused yet???? I am!
If we add a 5/12 roof pitch, the spacing goes down to 10’4”. Taller piers, higher side wall and steeper roof pitch all impact anchor spacing!
I checked another major manufacturers installation manual for comparison….that was a mistake!
They measure the pier height differently. Only to the top of the I beam, not the floor. PLUS…they require end wall frame anchors for every home they produce.
Maybe the take away here is to find a better solution to ground anchors, tie downs, strap angles and all of these confusing charts! So, let’s explore some of the alternative anchoring systems that are available. But that is a topic for our next post!
As always, check the installation instructions for the specific home you are installing and “Call Before you Dig” or “Drill” as the case may be!!