Manufactured Homes & Attached Structures Part 1

It is pretty hard to drive through any manufactured home community and find a home that doesn’t have some type of attached patio cover, carport, deck, Florida room or door canopy. In fact, it is so common that it is almost expected by homeowners and routinely overlooked by installers, retailers, community owners and building code officials. I think we should talk about this common practice to be certain we are doing the right thing.

Typical patio cover improperly attached, adds over 1 ton of weight to the manufactured home.

 

A typical manufactured home installation manuals will say something like this:

Install site-built structures such as steps, landings, garages, awnings, carports, breezeways, porches, decks, railings, sheds and utility rooms to the manufacturer’s instructions and according to the following:

  • Construct site-built structures to be structurally independent unless provided for in the design of the home

 (there are additional bullet points in the home installation instructions that we will explore in future posts).

A properly supported door canopy!

 

So why must these common awnings, landings, steps and the like, have to be “structurally independent” of the manufactured home?

It all starts with the Manufactured Home Construction & Safety Standards (HUD Code) and the Model Installation Standards. The HUD Code is no different from other building codes as they all establish the minimum building code requirements.  Building codes cannot anticipate aftermarket construction or additions, and neither can the home producers. So, when we are thinking about attaching patio covers or carports to a manufactured home, we need to focus on the potential impact these added structures can have on the home and the foundation.

Unauthorized site-built additions to a home can impose unintended vertical and horizontal loads beyond what the building code minimum requires. Let‘s start with a look at the vertical load, and save the horizontal (wind) load for the next post.

Most manufactured homes are designed to handle a roof snow load of 20 to 40 pounds per square foot (psf) based on where the home is to be installed. In addition to this snow load, manufacturers must include the actual weight of the home itself (somewhere around 20-25 psf) and the assumed weight of the contents inside of the home (for furnishing, people, pets, etc.). The HUD Code requires that the manufacturers calculate this as an additional 40 psf. Ultimately, a typical manufactured home must be designed to support between 80 and 105 psf with limited deflection of the structure. The foundations are then designed to transfer all of this weight into the ground to assure a stable, plumb and level home.

This improperly supported deck can easily pull free from the home and collapse!

 

Now let’s assume you attach a 20’ x 12’ (240 sq. ft.) patio cover to fascia board of the home.  For a home located in the south roof load zone, that would add 4,800 lbs. (240 sq. ft x 20 psf of snow) of potential snow load beyond what was intended by the manufacturer of the home (not to mention the weight of the patio cover!). About half of that load would be transferred to the columns supporting the front of the patio cover, but the remaining weight (2,400 lbs.) is being transferred back to the fascia board, into the roof trusses and into the structure and foundation of the manufactured home itself.

You have just added over a ton of weight to the home that was not accounted for by the building code or the design of the home.

Carport pulled fascia from the roof trusses

 

I have seen first hand where this added weight has led to ceiling cracks, inoperative windows and doors, floors bowing, and piers cracking, breaking or sinking into the ground and other structural failures.

Broken ceiling panel, due to added structure.

 

There are some other issues to consider as well. The fascia board is not intended to resist the pull of an awning or patio cover. Fascia boards can easily pull free from the home and cause significant damage to the home. The same can be said for decks attached to the floor rim (or band) joist. You run the risk of deck collapse since the rim joist attachment to the floor joists in a manufactured home is not designed for attachment of a deck. The International Residential Code does not allow deck attachment to rim or band joists without a positive attachment such as a deck tension ties. Basically, a threaded rod and bracket that runs through the rim joist and screws to the floor joists.

Simpson Strong-tie deck bracket

See https://www.windsorlocksct.org/site/deck_lat_load.pdf for more details.

So that leaves the installer with two options:

  1. Install columns on both sides of an added structure so the home is not bearing any additional weight.
  2. Ask the home manufacturer to design the home to accept whatever loads will be added. In fact, a few manufacturers already have some designs, but typically they call for attachment to the side wall, not the fascia board.

Ok…we are just starting to explore this issue and there is much more to discuss!  Let’s talk about how added structures impact horizontal (wind) loads in our next post.

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Difficulties With Porches

A few years ago, I was called upon to investigate a complaint filed by a homeowner in a very high end, 55 and older community. The complaint was generated because the homeowner believed that there was mold growing in the crawl space under her home and she was hyper sensitive to air born mold spores.

To paint a better picture, these were all two section homes installed on a masonry (block) crawlspace, on frost protected concrete slabs that were poured 3’ below the surrounding grade.

Once backfilled, the slab will be below the frost line.

The crawl space was properly moisture proofed and the sites were well graded for proper drainage. But the inside of the crawl space was soaking wet even on the nice summer day when I visited.

Notice the wet blocks under the porch area

And sure enough, you could see mold and mildew on the bottom board and in the floor cavity where the access panels were not replaced. The homeowner had a legitimate gripe.

 

As you know, water is needed to support the growth of mold, so we just needed to find the source of the water. The water source was easy to find. The lawn sprinklers!

Every evening, the automatic lawn sprinklers would slightly overspray the grass and water was landing on the recessed porch decking on the end of the home. The porch was decked with composite material that was installed with just enough of a gap for the water to pass through and collect in the crawl space. The ultimate problem was that the crawl space was constructed around the outside of the porch, instead of just around the living area of the home. So, with every lawn sprinkle, rain or snow that landed water on the decking, water was being introduced into the crawl space.

So, I decided to see what the manufacturers installation instructions say about recessed porches. Most installation manuals have two sentences dedicated to this topic, back somewhere around page 95 of most manuals: “Run the skirting along the perimeter of the homes heated, conditioned space. Do not enclose with skirting areas under recessed entries, porches or decks unless the skirting is of the fully vented type and installed as to allow water to freely flow out from under the home”.  And to complicate matters even further, the ground vapor retarder is not to extend under “recessed entries, decks or porches”.

It would be difficult for an installer is to design a crawl space enclosure-skirting that “allows water to freely flow out from under the home”.

Home sits in a “pit” No way for water to escape.

And even if you did, the homeowner would likely do some landscaping that might act as a dam and trap the water in the crawlspace. Easy to see in these two pictures that any water landing on the decking is going straight into the crawl space.

Water gets captured inside skirting.

 

As we are seeing more homes being constructed with these decked porches we need to take a harder look at this issue. Running skirting underneath the home is difficult at best. I have run across one example that is well done. In the picture below you can see the porch area is only enclosed with a vinyl lattice that allows any water to escape. And yes, there is a crawlspace wall (skirting) under that front end wall.

Crawlspace under front end wall, behind lattice work.

 

Maybe some manufacturers have a better idea. But don’t take my word for it, grab an installation manual, turn to the section Complete Exterior Work- Install Skirting. Let me know what you think.