Tiny House, Tiny Living, The Tiny Life.

Adding The Deck Flashing

After removing the boards for the deck of my house, I then flashed the whole surface of the trailer with galvanized metal flashing.  This will prevent moisture from getting into the insulation and floor framing and prevent mice or road debris from entering into the undercarriage.  I made sure to overlap the seams and then used flashing adhesive to seal it all up.  The sheets then were secured using staples and the vapor barrier was placed on top of that.

It’s worth noting my approach to moisture when it comes to my Tiny House.  I have several layers of redundancy to prevent moisture from becoming and issue.  First is the fact that the trailer is inherently off the ground, this means that there is a good air flow to dry out any moisture that does make its way under the house.  I plan to have a gravel pad to facilitate better drainage under where the house is parked.  From there I have the trailer decking which is pressure treated.  I think it is very unlikely that much water will be able to get up underneath the trailer other than if I were to drive it down a road after/during a rain shower.  From there I have a sealed layer of galvanized metal flashing.  This will prevent any water from entering an because its galvanized, it is well adapt at handling it if it does.  On top of that is a sheet of 6mil vapor barrier.  On top of that is my floor framing and insulation.  The floor framing is also treated and the foam is closed cell so it will not absorb any moisture.  All in all I think moisture isn’t going to be a huge issue because of the air flow, but if it does get in, there are multiple layers to handle it.

First step was to cut a hole for the tiny house deck.  The decking of the actual trailer is treated lumber, to cut the hole I used a sawzall to make the cuts.

Then I attached galvanized flashing to the deck, being sure to overlap the seams and seal them with flashing adhesive.

Then I added a 6mil vapor barrier.

Finally all my floor framing (covered in depth in another post) is all treated lumber.


  1. Hi Ryan,

    Thanks for the post. Well done :)

  2. It’s also a good idea to put some heavy duty plastic UNDER the home on top of the gravel bed to keep ground moisture from damaging the underside. Even with the gravel it will be damp enough to cause issues down the road. It’s the code here in AL now for all mobile homes along with a raised bed of well-drained material for the home to sit on. We also do this with our portable buildings on our homestead.

    Of course this is not necessary in dry land areas.

    • So they apply it to the underside of the trailer of lay it on the ground?

      • Laid on the ground – so that water vapor from the ground is blocked (at least to some degree)from migrating upward and condensing underfloor. The same principal is used in standard houses when a plastic vapor barrier is installed on the ground over a gravel base, and/or underneath a concrete slab before it is poured.

  3. Looks great Ryan, love getting to watch your progress!

  4. Very interested in watching the building progress! Great job!

  5. Did you use galvanized roofing nails to fasten metal to floor?- Or? Thanks much

    • I just used hot dipped galvanized staples to hold it in place. I also used metal flashing glue to do the edges. The flashing is sandwiched between the trailer decking and the floor framing so even without fasteners I don’t think it would go anywhere. the staples were about 1 inch long and 1/3 inch wide. Air powered staple guns are your friends :)

  6. Just curious: Why did you not flash the underside of the support deck?
    You could have fully enclosed the entire wood structure.

  7. Hi as a builder I try to understand what purpose the metal flashing serves. Debris will be stopped by the wooden base floor. Water will be stopped by the vapor sheeting. The glued metal flashing does not pass vapor. On top of that a vapor (damp open) sheeting? So where is the vapor coming from or going to? I think the metal flashing might be overdone, just adding to work, costs and weight.

  8. I’m also a Carpenter/Builder and agree with Joost. The flashing is an unnecessary step, expense, and weight. Flashing is for dealing with running or standing water, like on a roof. Any moisture under your trailer will be easily blocked by Vapor Barrier, that’s exactly what it’s designed to do, be a barrier to water vapor. As for mice and insects, they’ll just find another weak point to get in if they’re intent on setting up shop in your home. Also as someone else above mentioned, VB on the ground, if you’re in an area with moist soil, is worth the investment. putting it on the underside of the trailer will trap moisture between the two layers of VB, not allowing the PT trailer decking to breathe and will encourage rot over time.

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