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2x4s on rafters for ventilation

pedersjo | Posted in General Questions on


Good Morning,

I read these posts often, and have seen a lot of great information (thanks for that). I signed up to ask a question myself about our cabin. We have a flattish 2/12 pitched roof, the first half has been framed with 2x6s from the ridge to mid-roof then 2x4s from mid-roof to the eaves. This leaves little ventilation and there have been mold issues which we are attempting to resolve. I’d like to replace the 2x4s at the eaves with 2x6s which would give us better ventilation. I’ve read that some have added furring strips to the tops of the rafters to create more space, while others have simply secured 2x4s horizontally across the tops of rafters and below the sheathing to accomplish the same thing. It seems to me that would be more secure, however, I am unsure how far to space the 2x4s, and if in fact, this is a better way to go. Any input would be much appreciated. The roof is off, as well as the fascia and trim, so I am ready to proceed.

Thank you so much,


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  1. Expert Member
    NICK KEENAN | | #1

    A horizontal piece running perpendicular to a rafter like that is called a purlin.

    With any horizontal framing member, you care about two things: span and spacing. Span for a purlin is the same as for a similarly-sized rafter. Rafters always go with the long dimension vertical. Purlins sometimes go that way. If they go flat then the span table doesn't apply, I think 24" is probably the most they can span.

    Spacing is determined by the sheathing and the roof loads. Twenty-four inches is a common rafter spacing.

    1. pedersjo | | #4

      Thank you for that, I was unfamiliar with the term. 24" is what the rafters are spaced at.

      1. Expert Member
        NICK KEENAN | | #7

        Thanks. Sometimes knowing the word you need to Google is half the battle.

  2. Jon_R | | #2

    Be careful about horizontal purlins blocking the eave to ridge airflow. Also think about how much moisture the interior side lets in and then has to be removed by your low slope vent channel. Air sealing is critical.

    I'd seriously consider switching to an unvented design with rigid foam above taped sheathing.

    1. pedersjo | | #5

      Wow, yes that would block the airflow. It won't work. I have thought about going unvented, however, there is an attic, 5' high in some areas, and wouldn't this require an awful lot of insulation? It has been sealed on the ridge before we lived here. I didn't work, of course, no exhaust, have to replace most everything. Maybe I can attach either furring strips or 2x4's on top of the rafters to increase ventilation? Thank you for the input.

  3. Expert Member
    BILL WICHERS | | #3

    Usually people add to the BOTTOM of the rafters to add space for either a vent channel or more insulation. You can only add to the TOP of the rafter if you take off the roof sheathing. If you're going to be opening up the roof from the top, consider putting rigid foam over the top instead of adding a vent channel. This will get you more insulation and may work better than a vent channel. You'll still want to air seal the interior side of the roof assembly, whether that is a cathedral ceiling or an attic floor.


    1. pedersjo | | #6

      Hi Bill, thank you for your input. My first thought was to add to the bottom, however, that would mean lowering the ceiling as there isn't room. I think it would be a lot of work I think cutting the walls down. I do have the sheathing/trim/fascia off now. I'll have a look at ridged insulation. Guessing this lays on top of the rafters, below the sheathing?

      1. Expert Member
        BILL WICHERS | | #8

        Usually rigid foam on a roof goes OVER the sheathing, with another layer of sheathing over the foam to provide a nail bed for shingles. There is more info and a great drawing here:


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