Let’s Talk About Slabs

In April, 2016, HUD announced that their installation program contractor (SEBA Professional Services, LLC) was preparing a report that would establish the design criteria for frost protected shallow foundations for manufactured homes. When the report was made available last October, it caused quite a stir from the various trade associations, as well as several members of the Manufactured Housing Consensus Committee (adviser to HUD on code and regulatory issues).  The plan was that HUD would use this foundation report as an installation standard that would basically become the bible on design/construction of slab style foundations.

Personally, I am of the opinion that HUD is over stepping its authority and that these types of site specific issues are best addressed by the state or local code enforcement authority. But since the discussion has been started, it might be a good time to take a closer look at a slab type foundations for manufactured homes and how they protect against frost heave.

Typically, in the northeast, we see two types of slab foundations:

1.       A slab on grade, also known as a floating slab (being constructed below). Very likely to shift as a  result of ground freezing. Not recommended!

bad-slab-form

 

2.    A frost protected slab (pictured below) where the site is excavated with the slab poured below the finish grade and a masonry wall constructed around the perimeter which is back filled against. Since the slab is lower than the surrounding grade (to the approximate depth of frost penetration), it is unaffected by frost heave.  The masonry crawl space walls sometimes are used to carry steel cross-beams to support the home, but mostly the home is supported by piers inside the crawl space enclosure.

good-slab-below-grade

I reviewed several different manufactured home installation manuals, and could only find a handful that have designs for a frost protected slab foundations and they were all pretty similar. There are designs that depend on acceptance by the “local authority having jurisdiction”, and designs specific for the state of New York. My concern is that I have never seen a slab foundation being constructed anywhere close to the requirements listed in these designs. This article is intended to get professional installers thinking about frost protected slab design and construction. Here are a few things to consider:

Every foundation design demands that you determine the soil bearing capacity (SBC) at the site.  Most designs require a minimum SBC of 2000 pounds per square foot.   As the installer, you must have a good understanding of the soils in your area. Even though a 2000 SBC is fairly common, make sure that soil bearing capacity is being taken into consideration and documented on your installation check-list.

Next you need to look at the soil moisture content.  Many designs demand some type of determination that the average soil moisture content is less than 25% down to frost depth! Truthfully, I can’t see too many installers having the time or ability to make this assessment.

As you start excavating the site, remember the earth beneath the slab should be turtle backed, just like any other home site. And proper site grading around the perimeter of the home is a necessity (1/2” per foot slope all four sides)!

Next comes a minimum 6” thick gravel bed and 6mm poly vapor barrier under the slab which is an essential requirement common to every manufacturers design I have seen. Often the perimeter of the slab generally is thicker (12”) than slab itself, and reinforcement (rebar) may be required.

Even taking these issues into consideration, there are still concerns of frost heave impacting slab foundations that are poured at grade (floating slabs). My primary concern hinges on the inability to maintain the temperature in the crawl space due to manufactured housing floors being insulated (no heat loss into the crawl space).

The frost protected slab where the site is prepared below grade (talk to your code official for the exact depth), a slab is poured, and a perimeter wall (or crawl space enclosure) is constructed that you can back-fill against seems to make better sense for manufactured housing. 

My experience is such that these “below grade” slabs work great. And even if it is not as deep as the maximum frost penetration, as long as it is close, it appears to works well.

imag0912

One final thought; make sure your homeowners understand how rain water can impact the foundation regardless of its depth. Inform them about the importance of downspout splash blocks to direct rainwater away from the home. Warn them about landscaping timbers or decorative borders that can channel water towards the foundation (as was the case in the photo above). Watch for recessed decks or porches which can also be a huge problem (look for that in an future blog entry).

For more information on this topic, I encourage you to participate in the Pennsylvania Housing Research Center (PHRC) upcoming webinar ” Frost Protected Shallow Foundations in Pennsylvania”. the webinar will be presented on Tuesday, February 14, 2017 at 1:00 PM. If you missed it, the webinars are archived so you can still take advantage of this free educational opportunity. CLICK HERE to access their webinar.

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