Fence Rails: 2 or 3?

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I notice the old fences around all have 2 horizontal rails for the boards to attach to. I've also noticed that new specs often show 3 rails, without comment. I'm thinking that this might be due to the fence boards being much thinner than they used to be.
I like to place my rails long side vertical, so I don't get excessive sagging over time. But I'm thinking that unless I expect the fence to see hard wind loads or other severe attacks (kicks, etc.), 2 rails might be enough.
Any thoughts?
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Depends on the size of the rails, the size and weight of the fence boards/pickets, the distance between the posts, the material of the rails (vinyl over steel or aluminum, wood species), the aesethetic sense of the the owner/installer, etc.
R
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2x4's. PT Wood. 8 ft post spacing.
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You didn't indicate rather it is a 6' or 8' fence. If you are going with an 8' then use three rails without question. On the 6' fence, you can get away with two but if it were me doing the fence, I would use three rails and a 1x8 or 1x12 kick board at the bottom. The three rails will give better support and keep the pickets from bowing. The kick board (or rot board if you prefer) will keep the pickets out if the mud and water giving them added life. The kick board can easily be replaced as needed.
Also, mound the concrete around the posts to force the water to run off instead of seeping in around the posts. I did that and after a couple of years I added silicone caulking around the posts. Wait a couple of years to allow the wood to dry and shrink. I replaced the pickets on a 20 year old fence a couple of years ago and only had to replace the pickets and a few of the rails. I didn't use the kick board the first time but added it when replacing. The posts were still in good shape and didn't need to be replaced. Oh yes, that was in Houston too.
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It's a 6' privacy fence, but no wood posts. The new arsenic-free PT posts seem to have brown rot issues. I'll keep the fence boards 2 or 3 inches off the ground to keep the dirt off them.
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I've seen fences installed after Huricane Ike hit down here in Houston and the ones with two rails have boards warping and bowing like crazy. I mean every board is going a different direction. This is in more than just one neighborhood so it looks like the quality of fence board in general has gone down. 3 rails (top middle and bottom) may help with this warping issue.
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mook johnson wrote:

My house backs up to a 200' wide power-line easement. The houses on my side of the easement all had their fences installed with metal posts. No a single fence came down during hurricane Yikes! The houses on the other side of the easement all had wooden posts. Almost all of them were flattened.
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On Mon 11 May 2009 06:41:56p, HeyBub told us...

Are you referring to the vertical posts in the ground? What size were the wooden posts? In our area they usually use pressure-treated 4x4's. In almost all cases, whether pre-bilt panels or built in place, there are three rails supporting the vertical pickets.
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Wayne Boatwright wrote:

Yeah, the vertical posts. Best I could tell (from 200' away) they were the standard 4x4s. The laterals were 2x4s, but it didn't make any difference. The wind snapped the vertical posts at ground level.
Truth be told, I suspect the posts were more than fifteen years old, mostly rotted, and required only a puff to tumble them over. Of course galvanized metal posts set in concrete don't rot, so...
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On Mon 11 May 2009 07:25:57p, HeyBub told us...

Yes, they probably had some rot damage. Our pressure-treated 4x4's are set in concrete and we live in the desert, so any type of wet rot would probably not happen. BTW, how do you attach a wooden fence (I'm assuming something like a stockade fence) to metal posts? Curious. Thanks...
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http://www.strongtie.com/products/connectors/PGT.asp
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On Mon 11 May 2009 08:40:00p, SteveBell told us...

Thanks, Steve!
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Wayne Boatwright wrote:

Several ways.
Our fence has holes drilled through the posts with 6" lag bolts attaching the post to the rails.
You can also get clamps that fit around the post and are screwed to the horizontal rails.
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On Tue 12 May 2009 06:23:17a, HeyBub told us...

Thank you!
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On Mon, 11 May 2009 14:18:01 -0700 (PDT), mike

Are they much thinner, or are you only thinking that? Serious quesiton. I can read your sentence either way.

What kind of fence? Rail? Picket? etc.?    
You plan to use 2x4's for rails? When were rails ever that thick? Even if they once were, thinner rails are used all over the place and they are fine.
With a rail fence, three rails will each bear as much load as the two rails would, and the load on the posts will be 50% higher.
Mook, you don't say what kind of fence you're talking about either, but if two rails warp, so will three.
I have a picket fence, and plenty of the rails are 30 years old. They look just like they did 26 year, but a bit greyer. Most are 8 feet long. They are shaped like a parenthesis on one side and straight on the other. They sell new, treated ones at Home Depot that are close enough that even I don't notice that they don't match. The new ones are treated.
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Each? Not quite - each rail in a three rail fence will only have to take 2/3 of the load on a rail in a two rail fence.

The only difference is the weight of the rail and the additional fasteners. Not sure where you get the load difference numbers.
R
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RicodJour wrote:

Nobody else said it, so I will- how tall is the damn fence? For a short fence, 2 rails may be fine, but for a traditional 6-foot privacy fence, we always used 3. We also usually used alternating-side pickets with an overlap, so some wind could still blow through. Plus, that way, no arguments about who gets the pretty side.
-- aem sends...
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6 foot tall privacy fence. All such fences in the area have 2 rails, and seem to be none the worse off for it. The big problems are post rotting and leaning as well as fence sag due to the 2X4 rails being placed with the short dimension vertical.
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On Mon, 11 May 2009 15:14:40 -0700 (PDT), RicodJour

A rail fence, meaning one with no pickets, just rails. I was referring to wind load, that I think someone in another post referred to, and it's the only load the rails have anyhow.

Wind. Some hurricane was mentioned.
But it's interesting that mook assumed a privacy fence, and I was thinking about a 4 foot rail fence never did give an adequate descriptin of the fence. He has to learn to ask questions well.

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I'm talking about standard privacy fence with the dogeared top.
http://images.lowes.com/product/054561/054561106084.jpg
One owed inward the other bowed outward ther other one sideways. Good lord. these look terrible just afew months after installation. Probably cheap wood.
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