I want to build a 14x14 slab with a covered Patio - need advice

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Old 09-08-12, 02:02 PM
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I want to build a 14x14 slab with a covered Patio - need advice

I want to build a covered patio over a 14'x14' slab.
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Myself and my son-in-law are pretty handy. Am I fooling myself to think we can do this project ourselves?

The construction of the covered patio would not deter me at all, but the self-engineering of the slab is bothering me.

The total weight of the covered patio and Spanish tiles will be about 3,500 lbs.

I know I need to dowel into the slab on the side of the house to secure the slab and prevent the patio slab from drifting away from the house.

I know I need to build footings in the slab to support the weight of the structure as imparted through the posts. How do i go about figuring out how deep and wide I should make the footings? What number rebar should i put in the footings?

I plan to have a single expansion joint running from the house through the center of the patio slab. This will help when it comes to floating the slab when the cement is poured.

I plan to slope the patio slab 1" (over 14' run) away from the house. And I plan to slope the slab 1/2" (over 7' run) to the side from the expansion joint. Is this too much? Or is this about right?

Any and all help is appreciated.
 
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Old 09-08-12, 02:30 PM
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Welcome to the forums! I would build it free standing, as opposed to tying it into the existing building. Your vertical supports must have separate footings dug to below frost line. I recommend 12" sonotubes with the concrete terminating at or a little above grade. Of course these posts will be external of your slab. If you want them to be inside, then your sonotube pour will need to be 3 or 4" above grade, depending on the depth of your pour. You will want the top of the sonotube pour to be level with your slab. You will affix your vertical support to the pours using Simpson Strong Tie or equivalent Post bases secured to your poured footings. Less complications if you make the supports external of the footings. You can cut in control joints after the pour is made and cured. The slab will crack. You just have to tell it where to crack.
Others will chime in here with better information, so stay tuned.
 
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Old 09-09-12, 06:43 AM
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Is this what you are suggesting? Shown below are 12" sonotubes with a 4x4 post on center.

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Why would you not attach the back header to the home for structural support?

I actually have to locate the patio exactly as shown due to impervious coverage limits. Also, I cannot go any larger than 200 sqft

In Austin it looks like I am at the 0" frost line. So would 6" depth from grade be OK?

The above 8 sonotubes will support 3500 pounds?
 
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Old 09-09-12, 07:57 AM
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Chandler is more of a pro here, but I like to chime in anyway!

If you tie it to the house you will need to cut into the siding, install flashing, and seal any penetrations to make darn sure water will not get into your house.

I would make at least a 12" deep footing to carry that load, and make them wider at the bottom. Also, you might want to go with 6x6 posts for the same reason.

For the slab, you can hammer drill and install glued in rebar into your existing slab to attach the new slab to the house. I would only install an expansion joint between the house and the new slab. Cut control joints in the centers of the slab 7'x7'. It will look and perform the best.
 
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Old 09-09-12, 08:42 AM
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Do yourself a favor, and contact the local or county building department to find out what their building permit requirements will be. Could save you a lot of grief later. If it were mine (and considering your lack of freezing conditions), I'd forget about sonotubes and just construct a thickened perimeter footing, contiguous with the slab, and continuous with rebar, to support the column loads. Less work, will look and perform better, and be quicker to build.

The slope you plan to use for the finished slab is too flat, meaning rainwater will tend to hang around instead of readily draining off. The slope you specified is less than 1/16" per foot, which isn't enough to move water past minor finishing imperfections. Go with at least 1/8" per foot, and 1/4" per foot would be better yet, and it only needs to be in one direction (away from the house) instead of two as you indicated. And definitely use 6 x 6 timbers for the columns instead of 4 x 4s--the heavier members will definitely look and perform better.
 
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Old 09-09-12, 11:08 AM
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I'd forget about sonotubes and just construct a thickened perimeter footing, contiguous with the slab, and continuous with rebar, to support the column loads. Less work, will look and perform better, and be quicker to build.
Figures BridgeMan would have some better ideas.
 
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Old 09-09-12, 11:17 AM
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BridgeMan45,

Thanks for the slope suggestions. I'll slope away from house accordingly.

Should I still plan on 8 posts, or do you think I can span the 14' with an appropriate beam on top of the posts?

You wouldn't have any pictures or diagrams of the footing you suggest, would you?
 
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Old 09-09-12, 12:29 PM
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DO NOT laugh at my drawing. But I think this is what Bridgeman was suggesting, and it may work in your area, not sure. Here again contacting your building authority will help. I don't think it will take the pressure of the load, but he built heavier structures, so I'll concede. I like the sonotube method, even if it means digging a hole deeper than the footing. It places independent load areas for your vertical load and minimizes force in the middle that could cause cracking.
You can use LVL across your span and possibly eliminate your center posts, but you will lose in height, as you will need a doubled 9 1/2" LVL. It will carry the load for 14'.
 
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Old 09-09-12, 01:06 PM
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Larry when I worked in fla, monolithic slabs were pretty common place especially when building houses where the ground wasn't as stable as others. I would assume if you can build a house that way, a small structure would do fine.

The main thing [as the others have said] is to contact your local building dept because it doesn't matter what we say - the inspector is the one that has to approve the construction methods you use!
 
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Old 09-09-12, 05:15 PM
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I agree with building a house, but you are spreading the vertical weight across your bottom plate every 16". In this instance you will have all the weight of the vertical in a space 3 1/2" x 3 1/2". That is why I suggested beefing up the vertical positions. You are right the authority will rest with the inspector.
 
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Old 09-09-12, 07:00 PM
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To deal with any uncertainty some of you may have regarding possible overstressing of the continuous concrete footings, let's crunch the numbers, shall we? First divide the 3500 lb. total load by the 7 columns (assuming we have a uniform distribution, which isn't quite true, but close enough for the girls we go with). That equals 500 lb. per column, spread out over an area of 3.5" times 3.5" (if he uses 4 x 4s), or 12.25 sq. in., which when we do the arithmetic results in a maximum stress applied to the concrete of not quite 41 psi. Should he decide to use 6 x 6s as recommended earlier, the stress works out to less than 19 psi.

If 3000 psi concrete is used, the safety factor using 4 x 4s will be in the neighborhood of 3000 divided by 41, or just a tad more than 73; if 6 x 6s are used, the safety factor is not quite 158. Looking at it another way, the concrete will only be stressed (in round numbers) 1.4% of its total capacity for 4 x 4s (41 divided by 3000), or 0.6% of its capacity when 6 x 6s are used (19 divided by 3000).

Does anyone still think continuous footings won't work?

And chandler's sketch is exactly how I would build it. Go with woven wire fabric in the slab, curled down into all 3 perimeter footings, where longitudinal rebar (a few No. 5s, on chairs or dobies, should do it) will uniformly distribute the point loads from the columns. I'd go with a 5" thick slab, while making the footing bottoms 15" wide and 15" deep, total. Doing so will require a total of just under 6 C.Y. of concrete.
 

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Old 09-09-12, 08:13 PM
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Even a dumb building inspector might see a potential for a screened patio morphing into a 3 season area and actually into an addition, so the enforcement may be close. At that point, it is not not a question of a few hundred psi and here when it comes to life safety an hurricane safety.
Not pointing any fingers, but it is a classic situation that has been seen everywhere, but more common in areas with slab on grade and no storms in the daily forecast.- Especially when the land use and set-backs are specific.

The weight of the roof and clay tile is actually an advantage since it has little impact on any foundation, but the details should be according to normal practices. The weather-tight details between the existing is very critical an attention should me stressed.

Dick
 
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Old 09-10-12, 05:41 AM
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Members,

Here is what I'm leaning towards on the slab. All comments are appreciated.

Footings: 15" wide by 15" deep. Two perimeter runs of #5 rebar. One run 5" from bottom, Second run 10" from bottom

Hammer and cement dowel 6" into existing slab (house) with #5 rebar every 2'

Wire mesh 6" x 6" with #10 guage for patio slab, on 3" chairs.

Patio slab is 6" thick

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Old 09-10-12, 11:33 AM
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skibikegolf,

The skeptics are all quiet this morning, so I guess we satisfied them that your plan will work. As mentioned by several of us, be sure to contact the local AHJ (if there is one) for their blessings on whatever you do.

A few suggestions regarding the latest sketch:

1. As drawn, your plan doesn't have any tensile reinforcement tying the slab to the load-carrying footings. You need some steel there, to minimize the likelihood of horizontal cracks developing along the outsides, at the level of the bottom of the slab. That's why I had previously suggested curling the fabric down into the lower portions of the footings, continuous from the top run. If you choose not to do that for whatever reason, throw in some L-bars every 2' or so, tied to the mesh and straight No. 5s with tie wire. Either way allows you to eliminate the top chairs for the longitudinal No. 5s, as they can instead be supported by tie-wiring to the L-bars or fabric.

2. You should "haunch" the inside corners of the subgrade, eliminating the sharp corners shown in your sketch by excavating at a 45-degree angle up from the inside bottom footing corners (as chandler showed in his sketch). Doing so minimizes the tendency for stress-risers wanting to reflect up through and crack the slab along those points. It makes for a smoother (and quite a bit stronger) flow of internal stresses in the concrete. To reinforce the "triangles" of concrete in those areas, you could alternate the L-bars mentioned above with short straight bars parallel to the angled subgrade (maybe bend each end of each at 45-degrees, for tying them to the mesh and bottom No. 5 bars).
 
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Old 09-10-12, 01:04 PM
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Skeptics are working . If you plan on making the footing 15" deep, then you will solve the problem I was trying to get across. Just a slab would not suffice. I think you're good to go with the approval of the AHJ.
 
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Old 09-10-12, 03:33 PM
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Well, I am comfortable with the slab specs. I will slope the interior to 45 degrees of the footing, and add L-bars every 2' or so. Thanks to all or your help.

Now moving on to the posts and beams. I really would like to use 6x6 posts only on the corners.

It has been suggested that I could span the 14 feet with 3.5" x 9.5" LVL. My research looks like that would work.

Are there any structural issues by butting the beam flush with the outside of the 6x6 posts?

Also, does anyone think I'll have a problem with the 14' span supporting approximately 3,500 lbs?

Here is my beam and post configuration:

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Old 09-10-12, 10:56 PM
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Can't answer your question unless we know how you will be framing the structure of the hip roof. Initial guess is that you should be OK, as your total load divided by the net area is fairly low on a unit load basis (PSF). One thing I don't like to see are the chamfered LVL beam corners at the corner columns. Doing that compromises their load carrying ability (compared to square end cuts), and I suspect any LVL manufacturer you use will tell you the same thing.

You would do well to contact the technical hot line people for the manufacturer you buy your LVLs from. All of them (LP, Weyerhaeuser, etc.) have 800 numbers, and will happily answer any specific questions you may have regarding their products.
 
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Old 09-11-12, 04:53 AM
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BridgeMan45,

Thanks for the inputs. As a common hobbyist, I would never have suspected a chamfered join of the beams would reduce the load carrying capability.

I will call the manufacturer to discuss, and I have changed my design for the beam connection.

The roof framing is as shown below. The specs are 2x6 for common and jack rafters, 2x8 for hip rafters, 2x10 for ridge, 1x6 for rafter ties.

All rafters have a fully supported birds mouth cut. The hip rafters birds mouth is larger so the hip rafter will sit lower to allow the roof deck to plane.

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Old 09-11-12, 07:14 AM
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I would only add a couple comments to this discussion.

While I completely agree that the thickened slab is more than sufficient to support the loads it should be noted that no matter where you bear the weight the soil needs to have decent bearing capacity. In some areas the depth of your footing is not based on frost alone but rather getting thru organic soil to good load bearing soil.

Due to the open nature of such a structure the wind uplift in a big storm is much greater than for an enclosed structure. (the wind can get under it and lift if off easier) If this is a concern in your area you may want to invest in some hold down clips on the beams to column connections as well as the rafter to beam connnections. Simpson Strong-Tie has an almost endless selection of options.
 
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Old 09-11-12, 09:41 AM
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Sniper,

Thank you for your suggestions. Here is Austin, Tx, I only need to go down about 12-18" and I hit shale/limestone, so there will not be a problem with the stability of the soil.

I will be using the simpson strongtie adjustable slab to post connectors. I have not yet figured out what type of connectors to use to tie the beams to the posts.
 
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Old 09-11-12, 10:00 AM
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Will you be installing any kind of sheathing on the structure's underside (sheetrock, plywood, etc.)? The framing diagrams shown would indicate not, since you don't show joists in a horizontal plane tying the rafter ends together.

A possible nuisance problem resulting from going that route will be a tendency for birds and bats wanting to nest up in the structure framework. Just something to consider.
 
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Old 09-11-12, 10:15 AM
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Bridgeman45,

I do intend to install sheathing up to the base of the rafter ties and then flush against the rafter bases in both the open main area and the soffit.

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Old 09-11-12, 10:27 AM
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I think I've settled on the simpson strongtie connectors. I'm wondering what the pro's and con's are between using the simpson screws or through bolts to attach the beams.

I think I will use either of the following for the post caps.

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Can anyone help me understand the load chart below. The weight of the roof materials (rafters, rafter ties, ridge, roof deck, paper, spanish tiles) is about 3,500 lbs.

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Old 09-11-12, 09:57 PM
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Your collar ties (what you call rafter ties) are too skimpy for attaching sheathing to, as the 1 x 6 stock will split when you nail or screw into it in the narrow dimension. Splurge a bit, and use 2 x 6 material.
 
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Old 09-12-12, 04:44 AM
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Bridgeman45,

Another good suggestion on the "collar ties" - thanks.
 
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Old 09-13-12, 07:09 AM
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I'm getting ready to order my beams, and my lumber desk (Lowe's) says that LVL's in my situation (exterior and exposed but with a covered roof over the beams) would work.

Additionally, they want to sell 1 3/4" x 9 1/2" beams that can be doubled up to give me the 3.5" width that I want.

The cost for 8 14' beams would be $437.

I priced out 3.5" x 9.5" x 14' PSL that are treated, and 4 of those beams would run $1,200

Austin is not a particularly rainy place, and the humidity is tolerable. Do I need to be concered about the LVL's and go with the PSL's, or can I save a few bucks and go with the LVL's in this situation.

For those new to this thread, this project is an exterior covered patio. A picture of the project is in the first post to this thread.

Looking for comments.

Thanks
 
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Old 09-13-12, 10:35 AM
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For a dependable answer to your question, skip the counter guy at Lowes and talk directly to the manufacturer's tech people. I strongly suspect they'll tell you to go the PSL route, not only to cover the company's reputation but also to avoid having to deal with a disappointed customer should your untreated LVLs start doing weird things when they change dimensionally (grow and shrink) with changes in humidity. I've heard a few horror stories (and personally experienced one very major one) when built-up members intended strictly for interior use are used in exterior applications. The "very major one" involved multiple, large interior gluelam beams used outdoors, and cost the DOT I worked for upwards of $100,000 to correct.

Saving money initially could be costing you more in the future. If I were in your position, it would be a no-brainer--PSL all the way. They are designed and built to be stronger and more dimensionally stable than LVL, and the wolmanizing means there will never be performance problems associated with changes in moisture or humidity.

On a side note, I'm curious why you think you need a beam on the house side? It really doesn't serve any purpose, structurally, as no rafters above it are in contact with it. Maybe you like the looks of it there?
 
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Old 09-13-12, 12:39 PM
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BridgeMan45,

Thanks, I suspected you would suggest going with the PSL, and it makes sense.

Regarding the back beam, it supports a post that supports the ridge.

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Old 09-13-12, 01:58 PM
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Minimal and temporary ridge support only is needed, as the rafters will provide permanent support. A connection bracket on the house exterior wall would work just fine. Nothing is supporting the opposite end of the ridge, other than rafters. And the collar ties you've included will tend to prevent rafter spread under load, meaning the beam along the house wouldn't be needed for that, either.
 
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Old 10-01-12, 12:15 PM
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Save using LVL by protecting them

Another option not mentioned here is doing overkill on protecting the LVL option with several coats of oil base primer and paint or Stain and then wrap with aluminum. This may giive long term moisture protection. Have you asked the LVL manufacturer if paint and stain is adequate for an under roof outdoors application.

I know that all the LVL's sold by the local 84 lumber are not stored indoors BTW.
 
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