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Pergola vs. solar heat gain

Matthew Seabolt | Posted in General Questions on

(Climate zone 4 a) I designed my house to get the max possible solar heat gain by situating the view side south and all my southern windows have high solar heat gain coefficients.  Works great except my siding, as well as decking, is really taking a beating and I am having to restain the siding every two years and the deck paint fails every four or less.  The areas of the deck protected by the sun look as good as the day I painted it nine years ago.

The solution I came up with is to build a pergola on my southern deck which I will use to install some type of opaque plastic roofing during the months I don’t need solar heat gain (approximately april through october) in order to minimize degradation of the siding and the decking.  I am aware that if the pergola itself blocks a significant amount of the sun it will really hinder my shg so I am trying to minimize that via spacing the rafters as far apart as I safely can and making the rafters as tall as reasonably possible.

My main concern is how high I should go up with the rafters at the edge of the deck.  (the deck extends 12′ from the house).  It appears that even if the rafters are 12′ high it will still impact the sun a lot more than I would like during february.  My current plan is to purchase 16′ posts and notch them out a minimum of 3′ max of 4′ so I can anchor them with bolts to the deck, then run beams along the top of the posts (secured with a beam anchor), then the rafters on top of the beams to achieve max height.

Any advice would be greatly appreciated.  Thank you in advance,

Matt

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Replies

  1. Expert Member
    Zephyr7 | | #1

    Consider placing the vertical boards at an angle to help accept or reject more light at certain times of the year. This could include the orientation of the entire pergola, or just the roof slats. You would want the boards aligned parallel to the light in the direction of the sun at the time of the year when you want the most heat gain. At all other times of the year, the board “slats” would obstruct more light. This is a little bit how a sundial works.

    The engineer in me wants to tell you to try hinging the slats and using an actuator to make them adjustable, like blinds. That would be an interesting project, but would be far more mechanically complex than a normal pergola would be.

    You can get smoke color corrugated polycarbonate sheet in Ameriluxe’s coverlite line. Before going that route though, you might want to look into a product commonly used on greenhouses known as “shade cloth”. Shade cloth is a fabric mesh used to cut down on light entering greenhouses and it’s sold based on the percentage of light it lets through (or blocks).

    Since this is a green building site, and you’re in climate zone 4a, you could also think about planting grape vine so that it would grow into your purgola. Grape vine grows fast and in a few years you’d have natural green shading that would automatically block more light in the summer when it leafs out. You can make jam with the grapes too.

    If you were a little further south you could use kudzu, but then in a few years you might not be able to find your house. There are massive walls of the stuff growing between street lights along some of beltway northwest of DC. That vine is almost unbelievable.

    Bill

    1. Matthew Seabolt | | #2

      Thanks Bill! You gave me some additional ideas to keep in mind

  2. Expert Member
    Peter Engle | | #3

    I was also going to suggest growing vines on the pergola. Grapes as discussed above or Wisteria, which also grows quickly and produces beautiful spring flowers. Both vines are cut back to nubs and can be pruned in the fall, allowing full light in winter. Both send new canes out 10' or more in summer and can easily span a framework with 4' or so between major beams. Such an open framework will interrupt very little light in winter. Growing vines will provide a far nicer space in summer than one covered with temporary plastic roofing, and their natural transpiration provides additional cooling. Much greener solution.

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