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Roof ventilation for an L-Shaped (Plan) house with a shed roof

Climate: 4C (near Seattle)

I'm working on a 1-story home with an L-Shaped plan that will have a shed roof (low side of roof at the interior corner of the plan).
The roof will be constructed with trusses where one wing of the house would have a conventional attic but the other wing would have parallel chord trusses (~18" deep).
There will be a continuous soffit vent at the high and low sides of the roof for ventilation.

My question is: at the corner you can't provide balanced ventilation (high and low) because of the geometry. If the bulk of the house is vented properly (1/300 per code) will the be an issue with the corner of the roof (approx. 20'x20') being vented at the high side only (1/150)?

I'm not worried about code but just making sure the roof functions correctly.

Asked by Daniel Stewart
Posted May 13, 2018 8:45 PM ET

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4 Answers

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1.

Daniel,

Unfortunately the part of the roof with no venting doesn't know that the rest has good venting, so it can still encounter moisture problems.

Venting a roof just at the peak doesn't halve the amount of venting it has (1/300 to 1/150), it makes it unvented as there is no airflow.

Are you sure you can't detail the roof so that the parallel chord trusses can't take their ventilation from the fully trussed attic they abut? Perhaps by getting the girder truss that will separate the two roof types set 1 1/2" low and strapping the top?

Is the junction of the L (at 45 degrees) the best spot to change roof types from an architectural point of view? Does it fall over interior walls? Perhaps you could carry the fully-trussed roof around the L before starting the cathedral ceiling.

Answered by Malcolm Taylor
Posted May 13, 2018 10:17 PM ET

2.

Malcolm, thanks. I'd been considering several changes but wanted to check that the venting situation wouldn't work before redoing anything.

I think i'm going to continue the attic assembly through the entire corner and only change to parallel chords when they can be directly vented top and bottom.

Thanks for the response.

Answered by Daniel Stewart
Posted May 13, 2018 11:09 PM ET

3.

Daniel,

25 years ago when I built my own house I didn't pay enough attention to providing adequate venting on a hipped portion, and this summer I will spend my "holiday" tearing it apart.

Good luck with your build!

Answered by Malcolm Taylor
Posted May 13, 2018 11:21 PM ET

4.

Daniel,
I agree with Malcolm. A house with a roof valley is not a good candidate for a vented assembly (unless, of course, the vented cathedral ceiling is located in a portion of the house that is unaffected by the valley).

So you need to either (a) bring the attic around the corner of the L, as Malcolm suggested, (b) double-strap the roof with 2x4s, 24 inches on center, to create generous vent channels above the trusses (this is expensive, and creates problems where you have two types of attic), or (c) switch to an unvented roof assembly using closed-cell spray foam on the interior side of the roof sheathing.

Answered by Martin Holladay
Posted May 14, 2018 6:19 AM ET

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