How can I properly vent my tiny house roof (it has cross member framing instead of rafters)?
I have the framing almost complete on my tiny house, but I have run into a stumbling block. Framing the roof with cross members instead of traditional rafters made a lot of sense because the shorter span allowed smaller lumber for more head space and less weight, but upon further research my plan for venting is not going to work (I know – proper planning prevents poor performance). See attached picture for framing structure.
I had initially planned to put 1×3 furring strips along the top of each framing member and leave four gaps about 6″ wide along each cross member to allow venting from the eave to the ridge, but after doing some research it looks like this would not provide nearly enough venting area (based on code ratio for vent area / roof area of 1/300).
I have read many articles on roof venting this week (on this site and others) – it seems like they are endless, but none of them provide a way to vent the weird roof that I have created! I would prefer to vent the roof, so my first question is “is there a good way to achieve proper ventilation with my roof framing?” Please remember that weight, height, and hurricane force winds are all pertinent to the situation. If there isn’t a good way to vent, it looks like I will need to construct an unvented roof, which leads to my second question “If it can’t be vented, can I use some kind of rigid foam sealed around the edges with spray foam or do I need to have it all sprayed in?”
I am in climate zone 6 (SW Colorado with the potential to move to New England in the future). The roof framing is 2×6’s on 16″ centers. I was planning on using Roxul insulation, but I’m flexible on that. The roof will be metal (probably pro panel) with 5/8 plywood sheathing. I was planning on covering the roof with Grace Ice and Water Shield, but I’m wondering if felt roof paper might allow some potential drying if I have to go with an unvented roof (that’s question #3).
Thanks in advance for any advice!
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Replies
Ryan,
You have framed your roof with purlins instead of rafters.
The first question is a structural one: How are these purlins attached to the rake rafters? Did you just nail those purlins to the rake rafters with two 16d nails at each intersection? If so, they won't necessarily support the roof load. Each purlin needs a joist hanger.
Thanks for the quick reply Martin. The purlins are attached to the rake rafters with a #10x3" screw and three 3.25" ring shank nails; the purlins are also bearing on the single top plate, which is attached to the bottom of the rake rafters with a combination of FastenMaster HeadLok screws, angled 3.25" ring shank nails, and construction adhesive. I realize that this would not meet any code specification, but it seems like having the single top plate firmly attached to the bottom of the rake rafters should provide enough support (I am following code for the most part, but one of the advantages of building a house on wheels is that code and permits are not required).
Ryan,
OK, I guess there is no structural emergency. Thanks for the clarification.
If you want to vent the roof, you can cover the roof framing with housewrap, and then install 2x4s on the flat, either 16 inches on center or 24 inches on center, from soffit to ridge. Then install your roof sheathing on top of your 2x4s. This will create ventilation channels that are 1 1/2 inch deep.
Don't forget to install soffit vents and a ridge vent.
Ryan,
I'd build the roof as Martin suggests. Make sure to specify gable and drip edge flashing deep enough to cover the 2"x4"s and extend down onto the fascias you have already built.
Thanks Martin! That sounds like great advice - it will add a little more weight than I was hoping for, but it seems like a better solution than an unvented roof.
And thanks for the input Malcom. I have been worrying about this for days now, so it is really nice to get feedback so quickly.
Ryan,
Just a reminder that the most vulnerable part of your assembly is where the walls meet the roof where there is no overhang. You might want to think through how to build out the fascia to cover the sheathing and siding before proceeding too far with the roof.
Malcom,
You are correct about that weak point. My plan is to attach a 2x4 sub-fascia to the outside of the sheathing along the top of the rake to give a small overhang for the siding and rain screen (the roof sheathing will of course extend over the top of this). I am a big fan of generous overhangs, but there is a balance between interior space and overhang within the DOT width limit for a standard load (8'6") - this is why there will be such a minimal overhang.
One question I have concerning the venting construction is how to terminate the house wrap at the eaves? I don't want any water running down the house wrap to just run into the final fascia at the eave and get stuck, but if I stop it earlier then the water will just drop down into the soffit. Any ideas on this would be greatly appreciated.
Thanks
Ryan,
In this case, the housewrap is an air barrier. If you install roofing, as I assume you will, you won't have any water running down your housewrap.
Martin,
I was thinking that condensation on the underside of the roof sheathing could potentially drip down onto the house wrap, or that snow or any other moisture that found it's way in could do the same. I had assumed that the housewrap was to protect from moisture getting into the roof cavity, so I'm not sure what you mean when you say that it is an air barrier in this case. Can you explain further?
Ryan,
Unless you plan to install metal roofing without any roof sheathing, the chance of condensation is quite low.
If you are worried about condensation, split your fascia. The lower fascia covers the bottom purlin. The housewrap can direct moisture to this fascia if you want. Then overhang your flatways 2x4s, creating a soffit. (That's where your soffit vent goes.) A narrow second fascia can be applied to these flatways 2x4s.