Tiny House Building Science: The Roof

** NOTICE **
At least one aspect of the following post differs from that which we implemented
in the end-design and/or the actual construction of our Tiny House.
For the most accurate and up-to-date information please refer to our eBook.

Perhaps the largest single issue in the design of our tiny home has centered on how to construct the building’s envelope.  “Envelope” is just a fancy word that describes the shell; the floor, walls and roof that form the parts one sees when looking at the outside of a structure.

This is the 3rd of 4 posts detailing the methods we have chosen (after much research, consultation and contemplation) to protect our home from the elements and shield it from the negative results of human habitation.  Read the first post in this series to learn more about Building Science.

Building Science & Tiny Houses: The lack of a “cold-roof”

In order to maximize the interior space, our tiny home will feature a cathedral ceiling.  It will also have a standing seam metal roof.

A cathedral ceiling underneath a metal roof, if constructed improperly, can cause all kinds of grief.  Potential problems include condensation underneath the cold metal roofing panels (a result of unchecked interior moisture making its way outwards and coming in contact with cold underside of the roofing), rotting of the sub-roofing/sheathing (caused by the aforementioned condensation), mold issues (also precipitated by condensation), ice dams, and the most noticeable to whomever occupies the space, lots of heat loss (followed by expensive heating bills).

Since we don’t have an attic, we can’t segregate the roof structure from the conditioned/heated part of the house.   With that in mind, we will build the roof in a way to compensate for the fact that our tiny house design does not allow for a “cold-roof”.

The Roof

The following is a sketch of our envelope, specifically the roof,  along with a detailed description, starting from the outermost exterior layer proceeding to the innermost interior layer (from outside to inside).

a

We did a roofing project a little while ago using Fabral Galvulume Standing Seam roofing and we liked the system well enough to use it on our tiny house.  Typical standing seam roofing is fabricated on site; however, Fabral’s comes pre-formed from the factory and is designed to “snap” together.  One nice thing about this style of roofing is that it has hidden fasteners.  This means that no mechanical fasteners, like screws or nails, penetrate the roofing, which is great since no holes means no potential for leaks.  Another great characteristic of this roofing is that it is 100% recyclable.  Finally, the roofing is very substantial.  The gauge of metal it uses is much thicker than your average box store metal roofing.  Any thicker and this stuff would be unmanageable from an installation point of view.  We’ll most likely order the shiny silver Galvulume finish.  I don’t know why they can’t just call it “galvanized”, because that’s what it is.  It should last for at least thirty (and more like 60) years.

b

Immediately under the roofing we’ll lay one layer of Tyvek as a waterproofing layer in lieu of felt paper.  It will keep any condensation that forms on the underside of the metal roofing panels from penetrating the roof structure.

c

Next comes the first layer of ½” CDX plywood.  The hidden clips that hold the roofing down will penetrate the Tyvek and be held fast by this layer of material.  This layer will be attached to the rafters using nails that will penetrate the layers underneath it before being held fast by the rafters.

d

Here we’ll add a layer of 1” thick foil faced Polyisocyanurate insulation which will act as an air barrier & a vapor barrier plus adds an insulative layer (worth R6) underneath the sheathing to reduce thermal bridging and keep moisture generated on the interior from making its way beyond this layer.  If moisture happens to condense against this layer it will wet the layer underneath it which is o.k. since the rest of the roof assembly will allow drying to the inside.  All joints will be carefully taped.

e

Another layer of ½” CDX plywood comes next.  This layer supports all layers above it and will be attached directly to the rafters using construction adhesive and nails.

f

We decided to go with dense pack cellulose in the roof rafter cavities for the same reasons described in our post on buildng our walls.

g

We will attach a single layer of Tyvek directly to the undersides of the rafters mostly as a layer to contain the dense blown cellulose while we blow it in the rafter cavities.  It will allow drying to the inside.  We won’t bother to cover or tape the holes we use to install the cellulose.

h

To match the walls we will use local pine or hemlock tongue and groove boards installed horizontally to finish the interior of the roof/ceiling assembly.  As far as the finish goes, we will likely use the same linseed oil and pigment mixture as described above.

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This is Part 3 in a series of 4 posts in which we examine building science as it pertains to our tiny house.  Our next post in this series will will focus on the construction of our floor.

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