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Old 10-12-2011, 09:46 AM   #1
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Help Interpreting Town Standard on Concrete Footings


I'm in the design and planning stages of building a patio cover over an existing slab in my back yard. I've already determined I will be spanning 13' and my beams will be 10' apart. Here is a photo of the site, which is california high desert. High wind loading will be a factor, but not so much on the snow. I also do not believe there are any frost depth requirements (I could be wrong!) Because the slab is existing, and I don't want to increase my span past the existing slab, I will have to cut out and pour new footings for my 4x4 posts.



I'm trying to use my towns standard on patio covers as a guide and need some assistance interpreting the minimum footing sizes.

The standard is a pdf file located here . But for ease, here is a photo of the section I have in question with the relevant numbers circled:



I read this as, my end posts anchors need to be embedded in a box of concrete 17" x 17" x 12" and my intermediate posts need to be in 24" x 24" x 12" . Is that the correct interpretation of "Square footing size in inches" ?

Thanks for any insight!

-Guy


Last edited by GuyJ; 10-17-2011 at 10:12 AM. Reason: Chaged Title to reflect thread conctents
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Old 10-12-2011, 12:54 PM   #2
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Help Interpreting Town Standard on Concrete Footings


Hello GuyJ
I would interpret the table as you have. Having said that I think that you should confirm that the 4x4 post meets code as 6x6 is our minimum. Not sure what you are considering for your beam sizes.
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Old 10-12-2011, 01:03 PM   #3
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Help Interpreting Town Standard on Concrete Footings


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Hello GuyJ
I would interpret the table as you have. Having said that I think that you should confirm that the 4x4 post meets code as 6x6 is our minimum. Not sure what you are considering for your beam sizes.
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The 'town standard' references only 4x for both post and beam, so I am going on the assumption that would suffice. I'll be using 4x4 for post and 4x8 for beam.

-Guy
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Old 10-12-2011, 01:14 PM   #4
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Help Interpreting Town Standard on Concrete Footings


I second the suggestion of the 6x6 posts because they will look better and provide more strength even if they are not required. Codes and standards are just worst you caan build, but not necessarily the best way to build.

You might want to consider putting the post outside of the concrete patio.

You could have uplift problems with such a shallow footing. The down loads are probably minimal and an engineer can easily over-ride the inspector and make smaller diameter and deeper sonotubes feasilble that only take a rental auger and eliminate the possibility of cracking the existing slab.

Eventually, you will probably need an engineer for drawings, especially since the "guided missile" (patio roof) must be attached correctly. Going through the tile roof could be tough, but need to bear on the roof for clearance under the patio roof.

As I remember when I was there, the high desert has some nasty winds.

Dick
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Old 10-12-2011, 01:15 PM   #5
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Help Interpreting Town Standard on Concrete Footings


i read the chart the same as you.
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Old 10-12-2011, 01:27 PM   #6
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Help Interpreting Town Standard on Concrete Footings


Just curious but are you planning a flat roof?
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Old 10-12-2011, 01:49 PM   #7
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Originally Posted by concretemasonry View Post

You might want to consider putting the post outside of the concrete patio.

You could have uplift problems with such a shallow footing. The down loads are probably minimal and an engineer can easily over-ride the inspector and make smaller diameter and deeper sonotubes feasilble that only take a rental auger and eliminate the possibility of cracking the existing slab.

Eventually, you will probably need an engineer for drawings, especially since the "guided missile" (patio roof) must be attached correctly. Going through the tile roof could be tough, but need to bear on the roof for clearance under the patio roof.
Thanks for the insight. There is an existing 4x12 header with 6x6 posts in the location of the patio cover (part of the house) A 6x6 post would probably match up better anyways. Right now I am stuck on a 13' span, which complicates the footer, but simplifies (hopefully) the rest of the construction.

My original intent was to place the rafters on top of the existing 4x12 header, cut off the bottom of the existing roof and place the 2x6 patio rafters next to the 2x4 house rafters.. There is only one major flaw in that plan.. cutting off the roof of my house.. This is something I am hoping to avoid.. So, I did the calculations and even mocked it up in a 3d program and found that I could actually tuck the patio cover under the existing roof (with some cutting and trimming of the overhang), then simply use joist hangers on the existing exposed beam..

A flat roof is required to have a minimum .25/12 slope (according to my town standard). Using 2x6 rafters and a 4x8 beam, I need the top of my rafter on the house side to be 8' 1/4" above the ground.. This allows for the rafter to sit on the beam and the bottom of the beam to be the minimum 6'8" required.

So thats the rough plan at this point. Here is a closer look at my house attachment. 8' 1/4" is exactly at the bottom of the (trim?) 1x that is attached to the top of the header. The 4x12 actually extends upwards to the rafters.




If anyone has any suggestions, or considerations I would appreciate it.
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Old 10-14-2011, 09:31 AM   #8
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Help Interpreting Town Standard on Concrete Footings


Based on your description you will be adding additional loads to the existing 4x12 beam which will need to be checked.

If you are not planning on enclosing the porch have you considered constructing it as an isolated structure?
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Old 10-15-2011, 06:59 PM   #9
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Help Interpreting Town Standard on Concrete Footings


this sounds like a can of worms... i know a contractor who did something like this a few years ago... hes known for doing thats that fail inspections and doing jobs without permit which any one with common sense would know would never get approved
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Old 10-15-2011, 08:13 PM   #10
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Help Interpreting Town Standard on Concrete Footings


First possible mistake is thinking that they are talking about 'embedding' the post or anchors at all. They are not.

I hope that was just a figure of speech, and that your anchoring is going to be above ground as shown in the presently existing posts are.

Think about it. Ignoring the fact that embedding any wood in concrete is poor construction and an open invitation to water infiltration and containment and entrapment between the concrete and the post (which causes mold and rot), how could the "Department" know how deeply you were going to embed? They could not.

And that would mean that some of the the most important measurements they gave you would be comprimised.

If you embedded the post 6 inches into a 12 inch footing, all you would have left under the post to support it would be the remaining 6 inches. Not good at all.

Secondly, if you embed a wooden post into the center of a footing, you will in effect, divide the footing into four partial quadrants.... severely weakening it.

Please do not embed the post at all. There are commercial steel brackets sold that can be anchored in the footing as it is poured. This leaves all of the part that attaches to the post up out of the concrete... not embedded in it. The post then attaches to those brackets, and thus stays free from entraped water.
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Old 10-15-2011, 09:28 PM   #11
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Help Interpreting Town Standard on Concrete Footings


As mentioned earlier by someone else you'll need to have an engineer recalculate the loading on the existing beam and the exterior wall of your house. Also don't even consider embedding the wood in concrete (is the base of the existing post concrete or wood?). I actually think it would look better and give you more ceiling height if you do to remove part of your existing roof and set the rafters on top of the beam, it would also give it more slope!
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Old 10-16-2011, 06:30 PM   #12
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Help Interpreting Town Standard on Concrete Footings


Don't know where the notion of embedding the posts themselves into the concrete came from, but no, I will be embedding post anchors into the concrete, they look like "H"s, that the posts themselves bolt to...

So, my first step, after permitting, is going to be to cut the existing slab and dig the five new footings. From everything I have read, this is best dealt with by way of large gas powered self propelled concrete saw.

Does anyone have any experience cutting what effectively will be notches from the edge of an existing slab? I'm concerned that it will be difficult to remove just the 18" square of slab. And after you make the two parallel cuts from the edge of the slab, how difficult is it to make the perpendicular cut that connects the two? It seems like the saw would require quite a bit of overshoot on the surface, to cut all the way through the slab at the bottom.

-Guy
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Old 10-16-2011, 07:36 PM   #13
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Help Interpreting Town Standard on Concrete Footings


Depending on the saws blades diameter you may have to overcut quite a bit to get all the way through the slab into each corner. After you pour the new footers you can caulk these overcuts.

As far as how to remove the section you cut you can either insert a couple of concrete anchors and attach a chain to them so you can get a grip to lift it out. Another option would be to break it up in place so you can remove a small pieces.
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Old 10-16-2011, 07:43 PM   #14
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If you leave any square corners in the opening in the slab, there is a good chance crack back into the slab could start. That is an ideal starting point that usually avoided if possible.

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Old 10-17-2011, 09:23 AM   #15
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Help Interpreting Town Standard on Concrete Footings


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Originally Posted by GuyJ View Post

So, my first step, after permitting, is going to be to cut the existing slab and dig the five new footings. From everything I have read, this is best dealt with by way of large gas powered self propelled concrete saw.

This saw is likely over kill for 4" patio & 5 holes.

Does anyone have any experience cutting what effectively will be notches from the edge of an existing slab? I'm concerned that it will be difficult to remove just the 18" square of slab. And after you make the two parallel cuts from the edge of the slab, how difficult is it to make the perpendicular cut that connects the two? It seems like the saw would require quite a bit of overshoot on the surface, to cut all the way through the slab at the bottom.

-Guy
Sure, I just did a similar installation a week or so ago. The patio was stamped, and only a few months old. No over-cutting allowed. Fortunatly, I own a concrete chainsaw, so I have the ability to saw perfect square w/o overcutting. Drilled a tapcon in the center of each, and simply grabbed the tapcon with the claws of a hammer and pulled it out w/o resistance.

If you don't overcut the corners with a normal concrete saw, you'll be pounding away at the 18" square for a while, and you may very well damage more of the patio than you want to.

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