Thread: cantelievered deck question

I am building a deck that I would like to canteliever part of. The lenght of the floor joists will be about 26ft. There are support post at 1ft, 10ft and 20ft. Due to large rocks in the ground I can't put any posts past the 20ft mark. There I have already put in two 8" sona tube 5000psi concrete, steel re-enforced pillars. On top of those I have a 6x8 beam. (3 2x8 with 1/2" acq plywood sandwiched between them). This beam is 10ft long. I would like to canteliever the deck 6ft past this beam so that I can have a view of the creek. The total deck would be 10x26

1. If I use the 1/3 ratio suggested here, I should have enough of the joist to support that 6ft canteliever, correct?
2. what size joist would I need? 2x10, 2x12?
3. I plan on putting the joist 16"oc
4. since the outer part of this deck entends over a 6ft. dropoff (to solid ground below) I could put in angle braces on the box end of the joists, if necessary.

Any thoughts on this, (besides call an engineer)?

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Mike, check section "C4" in this link: http://www.awc.org/Publications/DCA/DCA6/DCA6-09.pdf. According to this guide 18' is the maximum span for overhangs. I don't think you can do the 6' cantilevered even if you had 2x12's on 12" centers.

There are too many unknowns here. Will the deck be attached to the house, and if so, how? What will the decking material be? Will it have steps to the ground? Are the concrete posts half buried, or what do you have for footings? How will the guardrail be constructed (makes a difference for dead load calculations)? It might help if you provide a few photos of what you have so far...although I don't know how helpful the folks here can be in deciding the joist dimensions, when it really is an engineering question. Sounds like the beams will have to be protected from moisture.

Originally Posted by Hank Spinnler

Mike, check section "C4" in this link: http://www.awc.org/Publications/DCA/DCA6/DCA6-09.pdf. According to this guide 18' is the maximum span for overhangs. I don't think you can do the 6' cantilevered even if you had 2x12's on 12" centers.

That is a very useful chart, thanks. Wow it looks like a max 2ft. cantelievered overhang? That does't seem like very much. This is just a 10ft wide deck, not a house.

Originally Posted by Kristi Silber

There are too many unknowns here. Sounds like the beams will have to be protected from moisture.

Here is some more info:
.>the deck will be free standing, 25 ft from the house
>I would use ACQ sothern pine
>I could wrap the beams with alum. flashing
> 3 beams total, 10ft apart
> the outer most beam is already in place. It sits on a 8" reinforced concrete pillars. They are 38" into virgin clay, and are now back filled with an additional 26" of fill. Total 64" in the ground. I can attach a picture tomorrow.
> The deck total would be 10x26 ft. 20ft on solid ground, 6 ft overhang. I could cut that back to a 4ft overhang if I had to.
> 3 sets of concrete colums, 10 apart and 10 ft from each other, with the beams as described. (3 ea. 2x8 with plywood spacer)

> There must be a chart showing joists longer than 18ft. The joists would be attached to all three beams with mechanical fasteners and each joist would of course have to overlap to make them 20ft. long.

Mike,
You might try your lumber supplier. They won't tell you how to build it. But, the yards around here will spec material based on your criteria.

First issue is that of your local building code and their limits to cantilevering.
One county in MD allows 4 feet and the adjoining county only allows 2 feet for the same design deck.
Have you checked your local permit office on allowances/requirements for design?

I doubt you will be able to find any standard or engineered lumber or local codes that would allow a 6' cantilever. That much overhang is steel and/or concrete territory.
This being a free standing deck makes it even worse. Will it be an issue for you your wife, a small table, 2 chairs, coffee and muffins, probably not. However, what happens during a family party and 20+ people are all standing on the cantilever looking at the pretty creek.
Your primary issues aren't the joists breaking. Consider
- Excess weight on the cantilever section lifting the other end of the deck
- twisting of joists, you would definitely need cross bracing between all joists
- cantilever area sway and possibly total deck sway
You might want to look at some FLWright work. He was big into cantilevers. That might help with ideas.
As a carpenter 3'-4' is my max comfort zone.

Yeah, it's gonna take some engineering, but don't give up too easily. Steel or manufactured wood may get you there. Also, if there's enough space for a diagonal kicker back to the 20' line, that can make a huge difference if you've got enough elevation. You're gonna need an AE or SE involved more than likely. The free standing aspect does change things significantly for a cantilevered deck.

Also, what about the weight of a heavy snow in a once in every 20 year blizzard?

Try looking at this new product from Trex it is a steel deck framing system
Trex Partners » Steel Deck Framing

Or this Lite Steel Beam product. You would have to see if it can be used outdoors though.
LiteSteelbeamâ„¢ - light structural beam with the strength of steel

I'm confused. Where did the 2' max for a cantilever come from? I must have missed it (nice link, Hank!). I see that the max is (joist span/4). Mike's span is about 19', so wouldn't the max overhang be about 4 1/2'? Diagonal bracing would help a lot, as Jim said - and diagonal bracing in the supported part of the deck may also be necessary, since it's free-standing. Laying the decking diagonally would help, too.

"The joists would be attached to all three beams with mechanical fasteners and each joist would of course have to overlap to make them 20ft. long." Are you saying the joists won't actually be 20ft (plus cantilever) long, that they will be made of multiple pieces of lumber? That could change the span calculations if they aren't continuous.

You might look into using Parallam joists, which in some areas are available treated for exterior applications.

I agree, you shouldn't give up. You may not get your 6' cantilever, but 4' doesn't seem too much to safely design for.

Anybody - what is footing thickness (Table 4)? Is that when the footings are continuous (wall-shaped), rather than cylindrical or square?

because this thread is still going...... You might forget about the long (24-26') run on floor joist, and set 2 parallel exterior grade glulams. Think bearing point loads on front and rear peirs, and lift on rear peirs. Then run your joist perpendicular to the glulams. Or if you want the decking to run width instead of length, recess your joist and set sleepers over the top of the joist, but still flush with the top of the glulams. Live load, dead load, tie downs, blah, blah, blah. Proper this that and the other thing. This is just a couple of guys talking options.

You mentioned wrapping the beams in aluminum flashing.
Treated lumber will eat thru the flashing.

"Mike Judson" of "Northwest Ohio":

"On top of those I have a 6x8 beam. (3 2x8 with 1/2" acq plywood sandwiched between them). This beam is 10ft long."

"3 sets of concrete colums, 10 apart and 10 ft from each other, with the beams as described. (3 ea. 2x8 with plywood spacer)"


x8" (eight inch nominal sawn lumber) already in trouble. Field-built five-ply beam/girder not engineered, beyond prescriptives.

I have already put in two 8" sona tube

It sits on a 8" reinforced concrete pillars. They are 38" into virgin clay, and are now back filled with an additional 26" of fill. Total 64" in the ground.

more trouble. Whoops, cart before the horse.

since the outer part of this deck entends over a 6ft. dropoff (to solid ground below)

Yikes! even more trouble.

"Due to large rocks in the ground I can't put any posts past the 20ft mark"

'This beam is 10ft long. I would like to canteliever the deck 6ft past this beam so that I can have a view of the creek"

"The deck total would be 10x26 ft. 20ft on solid ground, 6 ft overhang. I could cut that back to a 4ft overhang if I had to "



No! its not as you have described! and mention of the creek below!

You've built up an 8'2" in 6' unsecured, unretained, untied slope of exposed "clay" and "rocks" (or boulders?), to a 6' in 6' slope of unstable fill and installed insufficient foundation, runoff to waterway.

Below sounds like teh making of a giant spring board.
only canteliever decks I have done over 2feet out have all been done with steel and concrete on commercial work.

Mike

Originally Posted by Mike Judson

I am building a deck that I would like to canteliever part of. The lenght of the floor joists will be about 26ft. There are support post at 1ft, 10ft and 20ft. Due to large rocks in the ground I can't put any posts past the 20ft mark. There I have already put in two 8" sona tube 5000psi concrete, steel re-enforced pillars. On top of those I have a 6x8 beam. (3 2x8 with 1/2" acq plywood sandwiched between them). This beam is 10ft long. I would like to canteliever the deck 6ft past this beam so that I can have a view of the creek. The total deck would be 10x26

1. If I use the 1/3 ratio suggested here, I should have enough of the joist to support that 6ft canteliever, correct?
2. what size joist would I need? 2x10, 2x12?
3. I plan on putting the joist 16"oc
4. since the outer part of this deck entends over a 6ft. dropoff (to solid ground below) I could put in angle braces on the box end of the joists, if necessary.

Any thoughts on this, (besides call an engineer)?

Kristi,
I referenced the fact that different jurisdictions will set their own limits. Some are 2 feet some are 4 feet others will vary. Which is why Mike needs to check for his location. Engineering is one thing and allowed is another, without a great fight.

Yes, I agree, Garry. I thought of asking if he'd gotten a permit and gone through the necessary rigamarole, but ended up thinking that was his business and didn't directly pertain to his questions. But with a structure like this, it really should be done, and he should inform himself of the rules for his jurisdiction before planning.

With angle braces under the overhang the true cantilevered part should be effectively shortened, no? That's another question altogether, though - figuring the proper dimensions, attachment method, etc.

In the end, I think consulting an engineer who is familiar with code in the jurisdiction is the way to go. Sorry Mike, I know you didn't want to hear that! Better to pay a couple hundred for good advice than to have to rebuild because the structure doesn't pass inspection (or having a deck that is unsafe or vulnerable to the elements). And having an engineer sign off on the plans would give you a much better shot at convincing the powers that be to let you build something beyond the prescribed limits.

Watson:
"You've built up an 8'2" in 6' unsecured, unretained, untied slope of exposed "clay" and "rocks" (or boulders?), to a 6' in 6' slope of unstable fill and installed insufficient foundation, runoff to waterway."

HG, I'm afraid I don't understand your qualms about the footings. Reinforced concrete rated to 5000 psi in undisturbed clay to a depth below the frost line. Can you explain what's wrong please? And what's the "built up an 8'2"" part about?

Kristi,
Mike in the OP stated "...I have already put in two 8" sona tube 5000psi concrete, steel re-enforced pillars. On top of those I have a 6x8 beam. (3 2x8 with 1/2" acq plywood sandwiched between them). This beam is 10ft long...."

He can not have a permit for what he wants to do. If he did it would have been submitted with the permit request. Unless he is now trying to change horses in mid stream with new design.

Originally Posted by Kristi Silber

Watson:
"You've built up an 8'2" in 6' unsecured, unretained, untied slope of exposed "clay" and "rocks" (or boulders?), to a 6' in 6' slope of unstable fill and installed insufficient foundation, runoff to waterway."

HG, I'm afraid I don't understand your qualms about the footings. Reinforced concrete rated to 5000 psi in undisturbed clay to a depth below the frost line. Can you explain what's wrong please? And what's the "built up an 8'2"" part about?"

K.I.S.S. :


<Taking out virtual sledge hammer...> SLOPE!!!



Based solely upon what limited details HAVE BEEN provided by the OP although his characterizations, terms, and conclusionary statements are in conflict with his "details"... (then there is the "pillar" issue which is moot) all of which has been quoted further below.

The 'project' "work" as located and described, is and was a "no-go", "no-way", "no-how" from the "get-go" (before he ever dug the first hole) and went "downhill" (pun intended) from there.

Originally Posted by Mike Judson

I am building a deck that I would like to canteliever part of. The lenght of the floor joists will be about 26ft. There are support post at 1ft, 10ft and 20ft. Due to large rocks in the ground I can't put any posts past the 20ft mark. There I have already put in two 8" sona tube 5000psi concrete, steel re-enforced pillars(!!!). On top of those I have a 6x8 beam. (3 2x8 with 1/2" acq plywood sandwiched between them). This beam is 10ft long. I would like to canteliever the deck 6ft past this beam so that I can have a view of the creek. The total deck would be 10x26
1. If I use the 1/3 ratio suggested here, I should have enough of the joist to support that 6ft canteliever, correct?
2. what size joist would I need? 2x10, 2x12?
3. I plan on putting the joist 16"oc
4. since the outer part of this deck entends over a 6ft. dropoff (to solid ground below) I could put in angle braces on the box end of the joists, if necessary.
Any thoughts on this, (besides call an engineer)?

Originally Posted by Mike Judson

That is a very useful chart, thanks. Wow it looks like a max 2ft. cantelievered overhang? That does't seem like very much. This is just a 10ft wide deck, not a house.

Originally Posted by Mike Judson

Here is some more info:
.>the deck will be free standing, 25 ft from the house
>I would use ACQ sothern pine
>I could wrap the beams with alum. flashing
> 3 beams total, 10ft apart
> the outer most beam is already in place. It sits on a 8" reinforced concrete pillars(!!!). They are 38" into virgin clay, and are now back filled with an additional 26" of fill. Total 64" in the ground. I can attach a picture tomorrow.
> The deck total would be 10x26 ft. 20ft on solid ground (!?!), 6 ft overhang. I could cut that back to a 4ft overhang if I had to.
> 3 sets of concrete colums, 10 apart and 10 ft from each other, with the beams as described. (3 ea. 2x8 with plywood spacer)
> There must be a chart showing joists longer than 18ft. The joists would be attached to all three beams with mechanical fasteners and each joist would of course have to overlap to make them 20ft. long.

I inspected a house about a month ago where the cantilever on the deck was 5' and the results were not good. The cantilever sagged and it pulled the corners of the deck up at the outside corners. I put a 2' cantilever on the deck I built on my own house last year and it is very solid even with three people sitting on the cantilevered area. 2x8 pressure treated spaced 16" OC.

I wouldn't go beyond 2 feet for a cantilever.

Asking a bunch of Home Inspectors for free deck-designing advice will result in at least one sure thing--"you get what you pay for." Think about it.

There are valid reasons for decks collapsing (although you only hear about the ones involving fatalities), and I suspect faulty design and construction are at the top of the reasons list.

This guy's probably long gone by now. Interesting project, but I doubt we'll ever know how it turned out.

This

Joists should be cantilevered no more than one-quarter of the joist length or three times the joist width (nominal depth), whichever is smaller.

from Inspecting a Deck, Illustrated - InterNACHI

1. If I use the 1/3 ratio suggested here, I should have enough of the joist to support that 6ft canteliever, correct?
2. what size joist would I need? 2x10, 2x12?
3. I plan on putting the joist 16"oc
4. since the outer part of this deck entends over a 6ft. dropoff (to solid ground below) I could put in angle braces on the box end of the joists, if necessary.

1. No, not correct.
2. 2x24 or engineered lumber.
4. Then your cantilever would be less than 6' and so return to formula at the beginning of my post.

BridgeMan,
Have to agree in poor choice for his questions. Even though there are many here that have the building experience and knowledge that could answer the questions. But to layout an engineered design on a forum may be asking a little to much. I just do not understand why he had not found a good contractor to answer his questions. Many would help him out for little or nothing. It pays to have friends in the building trades.

Jim R.,
It may turn up in the news or back here when he goes to sell the house and the Inspector posts the pict of the excessive cant. that did not have a permit to build.

This pic that Lisa Endaz posted is exactly what I want to build, except there will be 3 sets of concrete columns/posts. Thanks Lisa The lower ones will be about 5ft high, but the other two sets will be almost ground level and below frost. .

Lots of good advice here, lots of it contradictory (but that's what opinions are !). I plan to put the project on hold until I can get to the county building permit people and get some more info. It looks like 6ft cantilever is out of the question. I'm not a pro, but I have built a few decks, and this is really just a simple platform except for the overhang part.

I will let you know what I plan to do, but it may wait until Fall now.Thanks everyone for all the advice.

Originally Posted by Mike Judson

This pic that Lisa Endaz posted is exactly what I want to build, except there will be 3 sets of concrete columns/posts. Thanks Lisa The lower ones will be about 5ft high, but the other two sets will be almost ground level and below frost. .

Lots of good advice here, lots of it contradictory (but that's what opinions are !). I plan to put the project on hold until I can get to the county building permit people and get some more info. It looks like 6ft cantilever is out of the question. I'm not a pro, but I have built a few decks, and this is really just a simple platform except for the overhang part.

I will let you know what I plan to do, but it may wait until Fall now.Thanks everyone for all the advice.

And you've already filled in on the slope 26" at that set, and it is implied the toe of the slope is far below (creek, etc.). Consideration to the overall topography and where the TOE (bottom) of the OVERALL slope is relative to the overall height.

Even at that point and according to the numbers you provided before post-fill you're at 5 ft 4-1/2" there in 10 feet. It was 7 ft-6.5" before you filled in 26" depth there.

Even as you have altered you're well beyond 33% grade and BEYOND prescriptives in the IRC and per IBC require engineering application of IBC.

Your "pillars" are unfooted, unrestrained, and in unstable (fill on clay) slope in a watershed (to creek/stream) and as described further subject to washout. They are not piers, neither are they prescriptive piles. As you describe they are 10'8" x 8" round, 64" of which is in ground, 38" hole in clay & 26" of fill in a the top or mid-SLOPE which was 7.5' in 10' and you've "filled" some to have created a 5'4" in 10.

Your built up beam is beyond perscriptives, is not engineered, and is not sufficient.

The H/3 rule applies to slopes such as yours (H is H of overall slope not just a portion of same). Lisa's document "7ft footing to daylight rule" is a non-existant "rule" and is not prescriptive, and doesn't meet prescriptives for slopes or even part of overall slopes, at this grade.

45 degrees is 100 percent (1 in 1). 1 in 3 is 33.3%. 1 in 2 (or 5 in 10 as your changed measurements after altering grade with "fill" in most recent post indicate) is 50% (which is greater than 33.3%, and beyond prescriptive exceptions to engineering for the freestanding deck or other accessory structure by size, in the IRC).

Geotechnical professional likely necessary applying U.S. Geo & Army Corps info, waterway, watershed status and on-site review/survey including soil and sub statbility.

Then there's the flood zones, the zone is one of substantial damage/deterioration of concrete, etc.

Only after that information is generated would a design professional design a foundation system and structure design.

Did I miss a photo of the site or something? Where are you getting all this data, HG? He said he wanted a view of the creek, he didn't say he wanted to be hanging over it - it could be 100 ft away, behind some trees for all we know. Where is it "implied" the toe of the slope is far below?

Everywhere is part of some watershed, it doesn't mean it's in the flood zone. (Whoops! stated a fact. I forgot I couldn't possibly comprehend anything in this forum, right Watson?)

At least you finally answered my question about why you didn't like the concrete thingys, whatever they are. I'm not very good at guessing what's going on in your imagination - "slope" wasn't quite descriptive enough.

That said, you may very well be right about stability problems. Maybe Mike will provide a photo of the site.

Originally Posted by Kristi Silber

Did I miss a photo of the site or something? Where are you getting all this data, HG? He said he wanted a view of the creek, he didn't say he wanted to be hanging over it - it could be 100 ft away, behind some trees for all we know. Where is it "implied" the toe of the slope is far below?

Everywhere is part of some watershed, it doesn't mean it's in the flood zone. (Whoops! stated a fact. I forgot I couldn't possibly comprehend anything in this forum, right Watson?)

At least you finally answered my question about why you didn't like the concrete thingys, whatever they are. I'm not very good at guessing what's going on in your imagination - "slope" wasn't quite descriptive enough.

That said, you may very well be right about stability problems. Maybe Mike will provide a photo of the site.

Good idea. I can provide a photo. Meanwhile:

.yes the deck will only provide a VIEW of the creek, not be in the creek. It goes like this.
house > 50 ft of backyard> 6ft slope to a lower area (which the deck will span (10ft in the backyard, 10ft over the slope) that lower area which in 40 ft wide >25 ft slope > creek) So the deck will anchored in the backyard and extend over that 6ft slope and allow me to see the lower area and the creek below that.

I'll take some pics.

Wow this is all useful information but the people that will have the final word on how this deck should be constructed and the design criteria for this would be your local Building Dept. Plan check, Building Offical, Engineer and they may recommend a Geotech.

Originally Posted by Mike Judson

Good idea. I can provide a photo. Meanwhile:

.yes the deck will only provide a VIEW of the creek, not be in the creek. It goes like this.
house > 50 ft of backyard> 6ft slope to a lower area (which the deck will span (10ft in the backyard, 10ft over the slope) that lower area which in 40 ft wide >25 ft slope > creek) So the deck will anchored in the backyard and extend over that 6ft slope and allow me to see the lower area and the creek below that.

I'll take some pics.

Lets see....deck 26', starts 25' out from house wall, overhangs 1 ft from deck edge towards home, support at 1', 10', and 20', with 6' projection desired at 6' elev. from slope due to "rocks" (errosion control & bank stability) in slope, and you've filled over same.

Slope starts at about 35' out from house, did drop 8'2" in 10, you filled (pink) to 5'4" in 10 and 6' in 20. Slope continues another 40' out to water line, 25' drop. Total distance from home to waterline is 31 ft drop in 91 feet.

Lets see if your recent 'word picture' is communicating...see attached sketch.



Unknown depth of "creek", or any particulars of the watershed/waterway (navagitable or not)/tributary, etc., geography, and no geotechnical analysis, nor survey. No Army Corps or NR info, no spillway, floodway, 100 yr or 500 yr flood data.

Looking forward to the promised (week ago?) pictures.

Mike,
You need to get together with someone from CA. They love to hang things on the side of hills and cantilever at every opportunity. Of course they do tend to slide down the hill.