Replies

1. 
   popawheelie | May 02, 2009 04:21am | #1

   From what I've read here you really need to attach the posts with a footing under them.

   Why don't you want to cut the slab?

   If it is an aesthetic issue I'd get creative with the cut.

   Cut it in a pattern so it looks like you meant to do it.

   Or cut out a circle that would compliment your curved patio.

   "There are three kinds of men: The one that learns by reading, the few who learn by observation and the rest of them have to pee on the electric fence for themselves."
   Will Rogers
2. 
   KFC | May 02, 2009 05:57am | #2

   What do you know about your slab?  Thickness, rebar, etc? 

   k

1. 
   Hat | May 02, 2009 06:34am | #3

   Slab is about 3 to 4 inches. And it has the screen in it. It slips my mind what you call it. And not rebar. I don't have a problem cutting it I just want to do it right.

1. 
   KFC | May 02, 2009 08:39am | #4

   Ok, 3-4" slab with welded wire mesh.  Fairly typical, but not strong enough to support posts from a 15x35 pergola.  If it was a 12" thick driveway with rebar, different story, obviously.

   If you have many posts, the loads will be less on each individual one, so there probably would be a way to get away with it with a whole forest of posts, but it almost certainly would not be worth it.

   You could also try to spread the load from each post out over the top of the slab with a reinforced grade beam or spread footing that set on top of the slab, but again, it probably wouldn't be worth it. 

   You should be able to have clean holes cut in the slab for your footings with no problem.

   A guy called mbaybut posted a thread recently where he wanted to build (and did, by himself, with not too much experience) a pergola on slab.  You may want to try the advanced search option and look through it.  If I can remember what he called it, I'll try to post a link. 

   edit: This may work...   http://forums.taunton.com/tp-breaktime/messages?msg=113370.1

   k

   Edited 5/2/2009 1:57 am by KFC

3. 
   jayzog | May 02, 2009 03:50pm | #5

   The major reason for cutting the slab would be to sink the posts to give them lateral support. If they sit ontop of the slab you will then need some sort of diagonal bracing to keep the thing from falling over, which would be ugly IMO.

4. 
   Piffin | May 02, 2009 03:59pm | #6

   IF
   The soil under this slab is compacted good quality load bearing mineral soil I would not hesitate to place a pergola with posts on it.

   But if it was just slapped down on top of whatever dirt was there, I would be hesitant.

   if you mount without sinking and securing, you will need some knees designed into the upper structure to resist lateral loads from wind, etc..

    

    

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   Taunton University of Knowledge FHB Campus at Breaktime.
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1. 
   KFC | May 02, 2009 08:06pm | #8

   Granted, we don't know the design, but we do know that the span is 15' across the short dimension.

   I have a very hard time imagining any pergola with a 15' span I'd put on a 3" slab.

   k

1. 
   Piffin | May 03, 2009 01:11pm | #9

   You are talking about a point dead load of 500-1000# is all. If a decent slab can't handle that, be carefull walking across it 

    

   Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

1. 
   KFC | May 03, 2009 04:41pm | #10

   Well, you're talking about simple vertical load from the weight of the structure, I guess.  Don't forget to add the vertical load from the overall rotational forces.  And, hundreds of pounds of Wisteria (and carpenter/gardener)...

   All loaded onto 1/9th of a square foot (4x4) or 1/4 sq. ft. (6x6) or more accurately, the 1/2"-5/8" threaded rod.

   And on the issue of the anchor- I don't have the stats in front of me, but I seem to recall most post bases want minimum 3" of anchor in the concrete, usually more, and with some concrete under the bottom of the anchor.

   Could be done, I guess, but it doesn't seem like a very good idea.

   k

2. 
   oops | May 03, 2009 04:47pm | #11

   I agree. Actually, there are thousands and thousands of residential driveways in this country that are no more the 31/2"thk. w/ww mesh, that support 4 to 6 thousand pound vehicles.

   The danger may not be from walking on the slab, but leaning against the post.

5. 
   oops | May 02, 2009 07:02pm | #7

   Since you have not decided what the pergola will look like, it's hard to know what exactly will be needed.

   My best "guess" is the vertical loading will not be the critical factor if it is similar to typical pergolas. I would be more concerned with lateral movement/forces. (wind, seismic, etc.)  You can't just sit the columns on top of the slab and attach it with some kind of angle brackets etc. with out some type of lateral bracing as mentioned by others.

   There is another post today about this very subject. Check it out. By McDesign. This is probably more than what you would like to do, but it takes all of the guess work out of it. It could possibility be simplified/modified to work in you case.

   Like they say, "if it's worth doing, it's worth doing right"& "better safe than sorry". Good luck.