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I have had problems with a fence on one side of my garden because of wind in Montreal. The posts end up leaning away after a few years. Part of the problem might be how the posts were put in (perhaps not deep enough), but the wind is a big factor.

EDIT: Current fence design is 2m high with posts roughly every 15 feet, similar to the image below: privacy fence image

I'm wondering if a board-on-board fence design (aka shadow board or shadow box fence) will help much. It seems that there's more wood involved in this design, and so it will cost more. I'm not sure how much it reduces the wind load, and I'm trying to get an idea of the cost/benefit of the design.

Here's an image to give you an idea of what I'm talking about:

shadow box or board-on-board design of fence

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What is your current fence style, that you're comparing the shadow board to? –  Henry Jackson May 10 '13 at 15:03
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Funny how most construction in Montreal seems to fail so quickly :P –  Steven May 10 '13 at 15:10
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Building a fence in Montreal can be tricky. First you have to get permission from the Mob, then you need to bribe a public official. –  Chris Cudmore May 10 '13 at 15:31
    
@HenryJackson See the edit to the question –  Fuhrmanator May 10 '13 at 20:52
    
15 feet is a pretty long post spacing. My privacy fence has posts at 8 ft intervals. –  Doresoom May 10 '13 at 20:56
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4 Answers 4

up vote 5 down vote accepted

This is an interesting read:

http://www.justanswer.com/structural-engineering/6escy-does-shadow-box-fence-significantly-less-windloading.html

The engineer says that there is no reduction in surface area, and the gap between boards isn't significant enough to significantly alter the air flow. He also says that in reality, there may be a minor reduction in the wind loading, but not enough to make it worthwhile.

You may want to consider a Louvred fence (angle board fence). However, these do not provide much privacy in the one direction, so you will want to angle the boards away from any nude sunbathing areas.

enter image description here

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+1 for louvered and the structural engineer answer is what I was looking for –  Fuhrmanator May 10 '13 at 20:57
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I don't see how this would reduce the wind load on the posts. In fact, it might increase it a little since the surface area of the fence is greater.

If the wind force is too great for your posts, you only have 2 options:

  • More posts
  • Stronger posts.
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+1 I'm not sure I agree with your "more surface area" logic, but more/stronger posts makes total sense. –  Fuhrmanator May 10 '13 at 20:59
    
In my mind I was thinking of the original fence as a picket fence with gaps, and you were going to add the extra boards to make it a shadow-style. But now I realize that the question didn't originally say what type of fence was there now. (I see you've edited the question.) –  Henry Jackson May 10 '13 at 22:43
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For sure the new design will help. Basic physics. More force will pass through instead of beating against the wood. Also force (wind) will naturally take the path of least resistance. You have a giant fence with no gaps now. So anything you move to will be better.

Will it be a lot better and help. Maybe, maybe not. Looking at the new design this would help overall but see the best returns for angular wind. If the wind is going straight at it, there are still no straight line gaps. There is no way without trial and error that you will be able to tell if this works ahead of time - unless you have fence wind tunnel data at your hands.

It looks like you want total privacy or close to it. I would move to the new design and pop a support post in between your existing posts - they don't even need to pop over.

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+1 for support post idea, although I think I'll have it redone with more posts overall –  Fuhrmanator May 10 '13 at 21:08
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http://www.iowabeefcenter.org/Beef%20Cattle%20Handbook/Windbreak_Fences.pdf

I was looking for this info as well. Figure 3 of link has a graph that may help.

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I'm not clear about the correlation of a windbreak fence to the OPs shadow board vs solid fence –  HerrBag May 18 '13 at 12:52
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