Installing the metal roof, and a note about building science

This post is about several roof related things:

  • Installation of a roof window
  • Installation of the metal roofing panels
  • Some notes on vented vs unvented roof assemblies

Check out the previous post on how we did our roof decking, including a layer of rigid foam. Next step was to add the drip edge trim to the eaves of the house — fairly simple stuff. After that, we installed the roofing underlayment. We chose a peel & stick type, since that seemed easy, and it was high temperature rated for use under metal roofing. Ours was GAF StormGuard; pretty much like Grace Ice & Water Shield, but came at a discount with our roofing panels. It’s not too bad to install… pretty self explanatory.

The hardest thing about roofing was reaching everywhere! We did end up buying that 12′ ladder… sooooo worth it. I don’t know how we would have done it without it… we tried to save money a few months ago by buying the 8′, but it’s just not enough. Spring for the 12′!!

Roof Window Installation

We decided on a large roof window instead of a fixed or venting skylight. The basic difference is that it’s huge and can function as an egress. We had a window just like this in our little apartment in France and LOVED it. We ordered the flashing kit that’s made for the roof window (Fakro brand). I would definitely recommend that. Watch all the videos a couple times. There aren’t any English instructions (or any words at all) — these windows are really popular in Europe, so they just make picture instructions. There’s nothing terribly difficult, just a lot of steps. The hardest part was actually getting the glass back into the hinges after installing the frame — we didn’t pay attention to the mechanism when we took it out, and eventually we had to look up the patent to understand how it fit together. It’s quite a heavy window, so one person had to be supporting it while the other tried to make the damn things fit together. It was super frustrating at first, but now we’re pretty good at it.

The Metal Roofing

The metal roofing panels came next! Actually, it’s kind of required to do some of this in conjunction with the roof window installation, so that all the flashing is properly lapped. You have to do a big of measuring, cutting, and hoping for the best.

As suggested by the instructions, we predrilled our panels on the ground — since we went with an exposed fastener type roof, it looks a lot better if all the holes are lined up neatly. I’ve read good & bad things about exposed fastener, but it definitely has a couple of big advantages — it’s cheaper, and it’s easier to install than standing seam. Although some say it’s more likely to leak, our roof came with a 40 year warranty, and as long as it’s installed properly, it should last. The exposed fasteners are basically wood screws with a neoprene washer to help seal them shut. Additionally, the thick asphalt peel & stick underlayment is self-repairing & provides an additional seal around the screw.  The roofing ended up costing about $600 total, which seemed great to us! It’s a beautiful metallic copper color and just shines in the sun.

As I mentioned, the hardest part was figuring out how to reach everything — the roof is only a 30 degree slope, but that’s a lot on a slippery surface 13 feet in the air. We were able to reach all the fastener locations either from the 12′ ladder, or by carefully crawling out onto the ridge from the roof window.

Finally, putting on the ridge cap was a bit tricky… it had the profile for a vented roof, so it didn’t sit super flat. Since the roof was slippery, Rob couldn’t figure out a way to screw in the fasteners without sitting on the metal… and he kind of ended up squishing it as he worked his way down… so the ridge cap is definitely a little bit derpy. Squished in the middle, then we tried to correct it, and it just got worse & worse. We’ll probably end up replacing it eventually, but for now it’s water-tight, and gives it that “home made” look….

Now that I’m thinking about it again, he should have been sitting on the side that was already screwed in, not the side he was screwing in… seems obvious now… gahh. Anyway, there’s a pro tip for you all.

Unvented  vs Vented Roofs

I wanted to address this topic because of a recent post from Little Yellow Door. I feel really bad for Ella having to tear down her entire ceiling! Of course, it’s a huge bummer when you realize you did something wrong and need to redo it. And I’m absolutely sure she’s not the only one who made an unvented roof assembly — in fact, that’s exactly what we did. Now I’m going to drop some building science on you that explains what makes ours (hopefully) work, and hers fail.

Remember learning about condensation and dew point? No? That’s ok, because you already intuitively know about the topic: Think about your nice cold beer on a hot summer day — there’s water just dripping down the sides of the glass; the beer inside is cold & the temperature outside is hot.  Well, the exact same effect can happen in your roof — if it’s cold outside and nice & toasty inside, water can condense out of the air and cling to the underside of your roof sheathing, resulting in mold. Condensation occurs when the air is colder than the dew point (which varies with pressure & humidity).

Basically, if we keep our roof deck warm, condensation won’t occur (warm is defined as 45 degrees F by buildingscience.com [1]). The easiest way to do this is to install rigid foam over the roof deck. The amount you’ll need does vary by climate — for Florida, we need a measly R-5, just .75″ inches of polyiso. If you live somewhere much colder, you’ll need thicker foam, which could cause trouble for height-limited tiny houses. We happened to be installing rigid foam anyway, as a thermal break for our studs! You can see in our last post, we used .5″ plywood sheathing, .75″ of rigid foam, and then another .5″ plywood as a screw base for our metal roof. If you do go this route, make sure not to add an interior vapor barrier. YMMV.

Here are a few more resources on the topic:

[1] Unvented roof systems report from buildingscience.com: http://www.buildingscience.com/documents/reports/rr-0108-unvented-roof-systems/

[2] Building Science Insights, Confusion about Diffusion: http://www.buildingscience.com/documents/insights/bsi-049-confusion-about-diffusion

[3] How to Build an Insulated Cathedral Ceiling: http://www.greenbuildingadvisor.com/blogs/dept/musings/how-build-insulated-cathedral-ceiling

[4] 2cycle2gether Tiny House: http://2cycle2gether.com/2011/02/tiny-house-building-science-the-roof/

[5] http://www.buildingscience.com/documents/confpapers/cp-0705-field-performance-unvented-cathedral-ceiling-ucc-in-vancouver

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5 Comments

  1. Just a quick word to the wise: building science is a tricky subject and varies greatly across climates. Roof design in Florida is completely different than that in states from DC to the north (well, probably different than MOST states since Florida is so much warmer/more humid for longer stretches of the year than most places). Also, a lot of the stuff you read about building science and recommended designs assume a certain amount of moisture (i.e. a certain house volume to contain all the humidity-producing activities like washing dishes, showering, even breathing, etc.). But with tiny houses, we still have many of those same functions but in a drastically smaller space…so a lot more moisture/ higher RH. I’m still working through what all this means from a design perspective and hope to use some WUFI software to confirm my thoughts – hopefully I’ll be able to post my findings somewhere. But I guess my main comments here for now are this:

    1) Condensation and mold are real concerns (I have been on several litigation cases/ripped apart several roofs due to it), and even more of a concern for tiny house dwellers. Coincidentally, I remember talking with Ella at a workshop a few years ago about her roof – I should have said something then (maybe I did? I hope i did…) but I knew should was going to have issues with mold (Ella, if you’re reading this, I’m terribly sorry…)
    2) Most of the tiny houses are too “new” for owners to realize they have mold, but I’m sure we’ll hear more about it in the years to come.
    3) Be careful whose plans you buy/whose advice you follow – make sure they are familiar with building science conventions where YOU plan to live because there’s not a cookie-cutter response for everyone. I wish tiny house plans folks would make this more clear to people when they are selling their plans. I feel that there will be some contention over this in the future. Be wary of people who tell you “Well, I did it (this) way and it’s worked for me!” – Unless if they live where you do and have opened up their roof to check a few years down the line, they could be living in a mold farm.
    4) You should become familiar with these ideas (as suggested about)
    a) vented vs. unvented roofs, net free area, ridge and soffit vents, etc.
    b) condensation and relative humidity
    c) vapor barrier vs. vapor permeable material (don’t rely on paint to be your vapor barrier), sheet poly
    d) R-values, split insulation, glass fiber faced polyisocyanurate (“polyiso”) etc.

    I apologize for preaching – This is just a really critical part of construction that I feel gets brushed aside since it takes a while for problems to present themselves. Thanks for reading though!

    -Sara

    • Sarah, thanks a lot for that detailed comment! You’re absolutely right — so much of building science is incredibly location dependent. This has been one of the more difficult things about making design decisions for a mobile tiny house. Right now we live in Florida, which is very hot, humid, and has unusually high wind loads. But in the future, we’ll probably be living somewhere else… it’s just impossible to make a design that’s optimized for all environments. Everyone should do their own research and understand WHY they’re making design decisions. I was hopeful that my post helped explain some of the things that need to be accounted for, and your comment certainly adds to that!

      • Hey Erin,

        Oh yeh! For sure! Your blog post was super informative and I’m really glad you (and others, at this point) are talking about the subject. And you’re right – there doesn’t seem to be a silver bullet design for all weather conditions (gosh – that would be so nice haha). I think when I get around to designing/building my tiny house, while I’ll probably research/model/test all the possibilities to death, I’ll likely just do things the old fashioned way – keep the windows open! haha

        I’m excited to be following your progress – you all look like you’re having fun, and it’s nice to see a different set of material selection. Best of luck to you, and keep up the good work.

        -Sara

  2. Hi Erin and Rob,
    While staying at Deservillers and eating grated “crozets” with cheese after a nice caving day, we suddenly remembered that you were with us the last time we had been in that accommodation. So we decided to google you for news, and we had a good time checking out your tiny house and Rob’s hat. If you manage to pull it out, it will be quite an achievement (I mean the house, not the hat).

    Greetings from the caving club
    (actually, maybe your house could fit in a cave, but Florida isn’t the best place for caves, is it?)

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