How much lift is there on a 12x24 foot 4/12 pitch roof 10’ 2” feet high at the ridge line, in 40, 50 and 60 mph wind? A rough approximate will be more accurate than my guesses.

We are putting up a 12x24 foot roof over a patio. It is hefty wooden posts bolted to a floor of unsecured paving bricks, with a wooden rafter frame and metal roof sheeting. No walls. I am concerned about damage from windfoil lift effect. I saw wind lift the entire roof of a big 40x100 foot hay shed.

On the hay shed, the medium pitch A shaped roof lifted in one unit, like an airplane wing. It flew 25 yards to one side and set down with surprisingly little damage. It was bolted to the tops of telephone poles, but that was not strong enough to hold it down. The main damage was from twisting as it landed.

My grandson thinks I am nuts to be concerned. I have tried online and find only engineering outfits who will calculate lift if you hire them, and a few free sites with complex mathematics and symbols I don't know for input. However, my bumbling calculations so far indicate that the uplift force could range from several hundred to several thousand pounds.

I’m also researching air flow spoilers to kill the lift, however much it is.

https://www.buildingsguide.com/calculators/structural/ASCE705W/

So you got 300 sf. of wing bolted to a unsecured paving brick, why worry?

Does your grandson understand how airplanes fly?

You are setting your poles on papers and bolting them to something I could lift?
While being worried about lift?

I'm a chainsaw and sledge hammer, truck driver, of a builder.

But assuming your name reflects your location,
I would be getting solid support below the frost line as a first step in any building.

Don't know crap about building or even where you are.
But poles would either sit on footings resting below the frost line or be in a concrete filled hole resting below the frost line.


I have been accused of over engineering building projects.
More is always cheaper than nor enough.

Being cheap will cost you 10 times as much. I am dumber for actually responding because you are going to build a kite to save money.

You need a 2’x2’x2’ cube of concrete.I would use steel columns and anchor bolts. I’ve built steel structures and metal buildings for the last 26 years.

Concrete and steel columns shouldn’t be more than $2500. $200 for 4 columns, $1200 for concrete, $150 foot anchor bolts- rest is incidental monies.

Make your dumb grandson dig the piers.

Dont put it anywhere close to something you dont want smashed by flying debris!

He could successfully bracket and bolt wood timbers to the 2x2x2 concrete foundation you suggest.

It'd keep the appearance more aesthetically pleasing.

I agree. However, I like the added strength of anchor bolts tying the concrete and steel better than a bracket and a Tapcon. One could easily wrap the steel column in whatever wood you want. It’s done all the time

But if Lowes or Home Depot don't have it on the shelf it ain't happening for most.

Updated insurance policy.
Mother Nature is very good at destroying things. Some things more susceptible than others.

Osky

Both of those place sell anchor bolts, rebar and concrete last time I checked...

We're talking about steel columns vs wood

Build it and insure it.

I actually had one I built out at my Parent's ranch back in 08 fly off about 75 yards when we got tropical-storm-force winds. It was about the same size as what you're building and had a tin roof, the 4x4s I used were 3ft in the ground but had no cement. I built it out of lumber from part of a house I had demoed so it was no big loss and there was no damage to the travel trailer it was built next to.

Talk with builders that construct post frame buildings in your area. The designers of pole barns and hay sheds have solved the problem of uplift. Specific methods to embed and anchor the treated posts into the ground, and to securely connect the posts to the roof structure to prevent "Liftoff."

Very simple to remove some paver blocks to install properly embedded posts, and then reinstall pavers up to the posts.

Have a 12'x12' gable roof 4 pitch there are 4 steel post supporting the roof they are lag bolted to a concrete slab with a rat wall plate is 6"x6" with lag bolt holding it down....
I live on Lk. St. Clair several time a year we get 60 mph+ winds off the lake ....you can see a little move meant with the wind gusts.....

Actually during the winter I rap the outside to protect the boat inside it seam like the wind effects it less with the outside covered.....we get some heavy storms in the late fall and winter.....

Have you already built it? If not then you need to put concrete posts in the ground and secure your posts on top. You can get a cardboard tube at the building supply for a post form. Just dig a hole. Plumb the tube in the hole and cut it the correct height and pour with reinforced concrete. I would want the concrete post 3 feet in the ground and 18" off the ground here. There's a steel fitting that sets into the concrete post to attach your wood post on top.

If it's already built you need to seriously anchor it to the ground. Mobile home anchors might work.

There's a reason this home remained standing after a severe storm, it wasn't by chance.

The 'absent' homes that use to surround it were built to code..

Thanks for the feedback. Good stuff all. First of all, it ain’t my project though I have helped a bit in building it. There is a reason I mentioned vertical posts merely bolted to unsecured paving blocks and you all confirmed my thinking.

The building is in a small clearing in tall timber, normally quite protected from wind but we get bad swirling gusts about once a year.

Re the original question about lift: Muffin, thanks for the link on how to calculate that. The site you link is the simplest I had found and I’d already spent some hours trying to get it to work for me. It is the simplest site for such calculations and it is still complex and incomprehensible to someone who does not know engineering terms and abbreviations. It starts OK with clear inputs but soon shifts into engineering speak, confusing input terms and by the time it spits out calculations for 13 sections of the roof it expresses them in terms of no use to me. What is wrong with saying, “lift per square inch, or foot.”?

It has been a curious exercise. Airplane engineers post such statements as,”...a 747 wing can generate one pound of lift per square inch.” That is clear, useful info, and obviously over simplified. Roof engineers apparently cannot bring themselves to compose a parallel sentence such as , “A 4/12 shed roof with a 50mph wind at 90 degrees to the ridgeline can generate x pounds of lift per square x.”

Damned timing !

Betting Cash, could answer this !

Why??? Problems were addressed in replys 2,3, and 4. With the definitive answer in post 4 from me.

But whatever- Someone always has a better answer here😂

Put a working cupula, a good ridge vent, or some turbines in the roof. Let that air pressure out

Don't get ya panties in a wad, Nancy !

Not being a structural engineer, how in the fuct would I know that you had, possibly given the definitive answer ?

As to my above post, I know because he & I traded favours. I helped him with some auto stuff & he helped me with some load bearing & shear construction stuff.

Get over yourself, tool !

You were Post #5
Pretty good answer.
Pier footings for the posts would be the way.
Tube steel columns are nice but could be done with 6x6 wood posts for the size of the structure.

Simpson CB66 type cast in place post bases and through-bolted to posts.



Then the same class of hardware for the overhead work :



Then Hurricane clips for all the rafters to complete the Continuous Load Path all the way up the structure.
H1 clip is a nice clip

You're both wrong.

The OP merely asked for mathematical formulas to help strengthen his view point with his grandson.

The OP isn't doing the construction.

My sarcasm in referencing wings and airplanes may have inadvertently lead him to the information he needed to inforce his stance.

OK hero, put up the formula.

Dig holes and put in the ground with concrete. Then get hurricane clips to attach each rafter or truss to the beam. Sounds like a problem if you bolt the posts to paving stones

Have you emailed a company that sells building fasteners? Simpson strong tie perhaps

a 747 wing can generate one pound of lift per square inch.

12x24 roof @ 4/12 pitch = roughly 300sf.

300sf = 3600 square inches.

3600 square inchs = 3600lbs of potential lift.

How much lift can your recommend Simpson H1 clip bare?*

























* 363lb

You’re a diipsheit. I’ve been doing this for 26 years. However, I had my 23 year old Civil Engineer daughter run calculations for local codes and she said I was overkill by 1/3 yard per pier.

No one on this website can answer the question with precisely the exact answer. Wanna know why dummy?

OP didn’t provide where he lives and his design criteria- which should’ve included IBC Building Code, reactions on each column, seismic stability factors, wind speed design, collateral loads, and location factor correction values.

All of which I knew to ask, but Emma (23) wanted you to know Karen.

Go dig his piers with his dumbass grandson dippy.

LOL

I would appreciate it if the 2 diipsheit diggers could "neat dig" the piers so we don't have to do a bunch of formwork.

Great pics and advice, DirectDrive.

You'll notice, if you read either or both of my replies, that I did not imply any knowledge, what so ever.

I only offered a potential solution !

Say hi & thank you, to your daughter.

Have a nice day, arsehole.

I do this for a living. Licensed PE. There isn't a place in our lower 48 that has a design wind load of only 60mph - and there's no need to design to a lesser load. Use worst case loading. I think the lowest is 90mph...and that depends on what code you are using. Generally we use what's called a 3-second gust value and most design loads are 3-sec gust values. The basic wind pressure formula is 0.00256 x (wind speed in mph)^2. So a 90mph wind has a pressure of about 20.7psf. The exact wind calcs are PIA (which is why no one will do them for free) and the pressures/uplifts vary on different parts of the roof, e.g. corners, edges, center. Suffice to say, if you use 30psf as your uplift value, that should cover about 90% of the USA's wind loads and roof designs.

That uplift load is distributed based on tributary area of the structural elements. I don't know your rafter spacing, but assuming four equally spaced posts, you'll have 2160lbs. of uplift at each post. (288sqft x 30psf) / 4 = 2160lbs. Concrete is 150lbs/ft^3 so you'll need 14.4 cubic feet of concrete at each corner and a bracket at the top and bottom of each post that is good for 2160lbs. in tension.

Mountain10MM,
THANK YOU!

Yours is the first and only reply to what I was asking though others provided good info about other factors. I did not expect a precise calculation, just a ball park figure from a knowledgeable person to give us a idea of the force we are dealing with. By this time my main interest is pure curiosity at how much lift the airfoil shape of a gable roof with no walls can produce. You nailed it, in plain English.

You are welcome Okanagan.

I hope you're not employed in a position that requires much in the way of math skills. Your calculation is off by a factor of 12. There are 144 square inches in a square foot, not 12. The correct product of the equation is 43,200 square inches, not 3600 square inches. It does make a difference.

Sorry if the intended humor of my replying to a question that was asked with the OPs early findings went over your head.
It was asked with a wink and a smile and replied to likewise.

Good info, but would probably be worth a few hundred bucks to have a local structural engineer analyze and provide details for your patio cover. While at it he could analyze shear, diaphragm, verify any beams, columns, connections, post sizes, etc. Do it right once and don't worry about it again.

Nice try at deflecting. It didn’t work any better than your calculator.