How to line up picket fence?

I'm in the process of installing an 8-ft high wooden picket fence, the type that has 1x8 slats with barely a seam between them, which are fixed unto 4x4 posts set some 10 to 12 feet apart. My question has to do with the easiest way to align the bottom (or the top) of the slats so that they follow a straight line between posts, and not contour the land.
Do I simply tie a string or line to each end post and make sure it's perfectly level, then align the bottom of the slats to the string? Or do I simply line the top of the slats with each support and end posts, mark them with a pencil, and use that for the alignment? Have no experience but I'm willing to experiment until it looks right (and straight).
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Rent or buy a Lazer Level, for perfection.
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(remove_this)prosete" <"(remove_this)prosete wrote:

Align the tops. Irregular picket length will not be noticed through the grass but will be obvious at the top.
Hint:
Buy the pickets. Let them dry (in the garage) with air on all sides for as long as you can stand it (a month is probably adequate). Else when you put them up, they will shrink as they dry leaving a 3/8" gap between the boards.
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You want the top to look straight. Level means nothing. figure on a heighth of your fence measure up from both end posts and string a line for the top of your fence. the bottom can be cut to grade. there is a bazillion vareations to building a fence. I have several fence pics that may give you some ideas
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What length are the 4x4 posts? One rule of thumb I go by is 1/3rd of each post should be underground. The fence described would then need 12' long 4x4s with 4 feet of them in the ground. That's a mighty deep hole...
Spacing the posts 10 or 12 feet apart may be an issue. At 10' separation each post supports 80 sq ft of fence, at 12' separation each post supports 96 sq ft of fence.
Codes in our area (for those windy days) require no more than 32 sq ft of fence per post. YMMV, but I'd be comfortable with no more than 64 sq ft of fence per post, especially if less than 1/3rd of each post is buried.
mikey.

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if you set the crossmember level, you can make a jig that will allow the top of each slat to be set at the same height above that crossmember. no measuring required.
randy
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if this wasnt clear, take a piece of 2x4 (or whatever seems handy) longer than the distance from the top rail to where you want the top of the slats. nail another piece to that so it can be used as a stop for the top of the slats. just set the jig on the rail, push the top of the slat to the stop, and nail it.
randy

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'cept that, if the top rail is a 2 x 4, 10-12' of it will have some sag. Flat wise, anyway. Less on edge.
xrongor wrote:

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just stand em all up to whatever height they stick then take a skill saw or chain saw and whack em to the contour you want or flat top`em
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