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Old Yesterday, 06:25 PM   #1
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Roof uplift on open building


This is a new open building garden gazebo I have recently completed as a DIYer at my family home in the Caribbean. I am happy with most aspects of the building, and I believe I have given correct attention to downward forces, outward thrusts, bracing, and bending moments. However, I am not happy with my mitigation of upward forces (uplift due to wind) and this is obviously especially important in the Caribbean where we are prone to hurricanes! So this is why I'm here, hoping someone can advise...

Here is the basic sketchup design (with some rafters etc removed for clarity) and photographs below. Dimensions of the box section are 27ft x 17ft. Eaves overhand a further 18". Ridge height is 12ft from ground. Posts are 8ft tall. Rafters are 3x6, 2ft apart and doubled up at each end. Ridge is 3x12. Pitch is 26 degs (6 in 12). Perimeter wall beams & central rafter tie all 3x10. Posts 6x6. Diagonal braces 3x6 & 3x3 respectively. Roofing 1x6 v-joint covered in stone slates. Wall beams / rafter tie connected to posts with 1/2" s.s. through bolts, posts structurally tied to adequate concrete sub base (7" 4000psi, BRC + 1/2" rebar). Everything else screwed with 10 x 3 1/2" deck screws (6 screws per rafter, 3 at each end)

Sketchup
Roof uplift on open building-gazebo.jpg

Overview
Roof uplift on open building-20190616_160144.jpg

Roof
Roof uplift on open building-20190616_154938.jpg

I'm fairly confident with my structural tie down of the posts, and the wall beams / rafter ties. My concept of the structure was for the ridge timber to essentially just be a ridge board, however since there are only 3 rafter ties (3x10, one at each end plus one in the middle, so they are aprox 13'6" apart) I decided to go for 3x12 ridge timber as I considered it to be a ridge beam (rather than board) between the the rafter ties, so I didn't want it to sag and create outward thrust pushing out the wall beams. Anyway, that all works fine, I fitted a stone roof (aprox 7,000 lbs roof tiles) with zero deflection to wall beams and zero issues. And we don't get snow here!

But I am concerned about uplift forces, which (for the life of me, I can't understand why given that I live in the Caribbean) I haven't given any consideration to...and we are entering hurricane season. I am fitting simpson strong ties x 4 to each rafter (2 at the wall beams, and 2 at the ridge, so both sides at both ends). Shown here:

Hurricane ties at ridge (Simpson H4 left & right pairs)
Roof uplift on open building-20190616_155133.jpg

Hurricane ties at wall beams
Roof uplift on open building-20190616_155145.jpg

That satisfies me at the wall beam end, but I keep looking at ridge and am worried that nothing is holding the ridge beam/board from potentially rising up in a hurricane...

Are the simpson ties at the ridge good enough? If the ridge tries to rise up, the 3 rafter ties will go into compression and prevent this? Or do I need to add collar ties? I'm hoping to avoid collar ties as not to ruin the aesthetics, but if so needed was planning to have some fabricated out of 1/2" steel bar and placing 4 across the building (so just under 8ft apart each) around 1/3 down between ridge and wall beams.

Please let me know your thoughts. And yes, I know...I really should have thought about this and asked BEFORE picking up my tools...but the original project was a lot smaller, I did the whole thing myself, and I probably ended up biting off more than I could chew.
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Old Yesterday, 06:42 PM   #2
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Re: Roof uplift on open building


Nice job with the build. I will let others decide on your actual question.

But the first thing that jumps out at me is framing with deck screws. Sorry, big no-no.
Deck screws are heat treated (hardened) and somewhat brittle and will break in time from fatigue. Lateral forces form winds will help this along.
I wouldn't necessarily pull the screws out, but I would go back and add the required framing nails.
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Old Yesterday, 07:02 PM   #3
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Re: Roof uplift on open building


The three ceiling joists or rafter ties hole the roof straight, uplift in the center is looked after with collar ties in the upper third of the rafter.

The correct phrase is actually “collar beam.” ... A collar tie is a tension tie in the upper third of opposing gable rafters that is intended to resist rafter separation from the ridge beam during periods of unbalanced loads, such as that caused by wind uplift, or unbalanced roof loads from snow.

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Old Yesterday, 07:07 PM   #4
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Re: Roof uplift on open building


Simpson's do make nicer looking hardware for nice building like that.

https://www.strongtie.com/decorative...ctors/category
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Old Yesterday, 07:13 PM   #5
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Re: Roof uplift on open building


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Originally Posted by Yodaman View Post
Nice job with the build. I will let others decide on your actual question.

But the first thing that jumps out at me is framing with deck screws. Sorry, big no-no.
Deck screws are heat treated (hardened) and somewhat brittle and will break in time from fatigue. Lateral forces form winds will help this along.
I wouldn't necessarily pull the screws out, but I would go back and add the required framing nails.
Yikes, just googled that, and you are correct! What about the simpson strong ties, will they not take up the slack? I certainly couldn't now remove the screws, but even getting nails in would be extremely difficult, given that the v-joint is now laid and roof tiles fitted...
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Old Yesterday, 07:15 PM   #6
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Re: Roof uplift on open building


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Originally Posted by Nealtw View Post
Simpson's do make nicer looking hardware for nice building like that.

https://www.strongtie.com/decorative...ctors/category
Thank you...it's all about availability here, not many hardware stores as it is, and generally have to go with what they've got on the shelf... (importing takes weeks and is very costly). I will paint them brown when fitted so as to hide them.
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Old Yesterday, 07:16 PM   #7
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Re: Roof uplift on open building


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Originally Posted by Nealtw View Post
The three ceiling joists or rafter ties hole the roof straight, uplift in the center is looked after with collar ties in the upper third of the rafter.

The correct phrase is actually collar beam. ... A collar tie is a tension tie in the upper third of opposing gable rafters that is intended to resist rafter separation from the ridge beam during periods of unbalanced loads, such as that caused by wind uplift, or unbalanced roof loads from snow.

Thank you - so the question is, do I need to fit some collar beams or will the simpson ties do the job?
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Old Yesterday, 07:23 PM   #8
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Re: Roof uplift on open building


You need something that will hold the rafters together but you can do more than just nailing a board up there.



https://www.pinterest.ca/pin/207095282841778987/
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Old Today, 07:13 AM   #9
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Re: Roof uplift on open building


Quote:
Originally Posted by aquacruise View Post
Yikes, just googled that, and you are correct! What about the simpson strong ties, will they not take up the slack? I certainly couldn't now remove the screws, but even getting nails in would be extremely difficult, given that the v-joint is now laid and roof tiles fitted...

If I understood correctly, you mentioned 6 screws per rafter, 3 at each end.
That really is not a lot. Typically there would be 4 to 6 framing nails on each end, tow nailed into the lower and upper beams. If nailing is not a option, then consider adding structural screws in lieu of the deck screws. And yes the hurricane ties are a must.


There are several on the market, here is just one example https://www.grkfasteners.com/product...ructural-screw
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Old Today, 04:30 PM   #10
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Re: Roof uplift on open building


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Originally Posted by Yodaman View Post
If I understood correctly, you mentioned 6 screws per rafter, 3 at each end.
That really is not a lot. Typically there would be 4 to 6 framing nails on each end, tow nailed into the lower and upper beams. If nailing is not a option, then consider adding structural screws in lieu of the deck screws. And yes the hurricane ties are a must.


There are several on the market, here is just one example https://www.grkfasteners.com/product...ructural-screw
I've taken your advice, today I managed to nail in 4 x 16d simpson galv on each rafter (in a toenail configuration, one each side at around 50 degrees) - 2 at the wall beam end, and 2 and the ridge beam. I've also now installed all the H2 simpson strong ties (one each side of the rafter, and at both ends - so four strong ties per rafter). Each tie is nailed with 8 x 8d simpson galv nails (4 in the rafter and 4 in the beam). Spray painted them brown, you can't even see them! That with the impact screws already installed I think we should be good.

Really pleased that you pointed this out by the way - I've posted exact same wording to several forums (inc building code inspectors forum) and you're the only one to have spotted this mistake, so thanks.

I've also managed to find a guy on island to custom fabricate me some collar ties (5/8" steel rod, welded to 1/4" thick mounting plates) which will be bolted every 4ft with 1/2" bolts at the top of the rafters just below the ridge beam - thanks to Nealtw for that, will reply to him with some pics when they're done.

Based on other responses I've had from elsewhere, and a PM from a UK survey / engineer, I'm now fairly happy with everything else - but still happy to take direction if needed from anyone else... Thank you
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Old Today, 04:37 PM   #11
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Re: Roof uplift on open building


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Originally Posted by Nealtw View Post
You need something that will hold the rafters together but you can do more than just nailing a board up there.



https://www.pinterest.ca/pin/207095282841778987/
Many thanks for this. Based on your pic you sent me, I managed to find a guy on island to custom fabricate me some collar ties similar in style (5/8" steel rod, welded to 1/4" thick mounting plates) which will be bolted every 4ft with 1/2" bolts at the top of the rafters just below the ridge beam. Think that should do it. Will reply to you with some pics when done. Thanks again.

P.S. Another on a different forum reckoned that collar ties weren't needed, since they considered it a ridge beam rather than ridge board, and said that as long as I had strong ties connecting the rafters to ridge then it would be ok - since there is no way for the ridge beam to deflect (3x12) therefore it was a fixed triangle? But I looked further into what you said, and I could see that with an unbalanced load (i.e. hurricane winds coming in from one side only) that it could easily pop open the top of the roof on one side - hence why I went to the expense of these collar ties i.e. I think you are probably correct?
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Old Today, 06:12 PM   #12
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Re: Roof uplift on open building


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Originally Posted by aquacruise View Post
Everything else screwed with 10 x 3 1/2" deck screws (6 screws per rafter, 3 at each end)
Never use deck screws in structural applications... use structural screws.

Another thing that concerns me is the cross bracing attachments to the building columns and ledger boards.

How are the columns attached to the slab?

Invest in some steel plates that through bolt and attach the cross bracing to the columns and to the ledger boards.

There is a picnic shelter that folded up near here in a windstorm.... it appears the cross bracing was very similar to yours.

You can see the repaired and upgraded braces with the steel plates in the next to last photo below.
Attached Thumbnails
Roof uplift on open building-picnic-shelter-002.jpg   Roof uplift on open building-picnic-shelter-008.jpg   Roof uplift on open building-picnic-shelter-006.jpg   Roof uplift on open building-picnic-shelter-004.jpg   Roof uplift on open building-picnic-shelter-after-001.jpg  

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Old Today, 07:08 PM   #13
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Re: Roof uplift on open building


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Originally Posted by aquacruise View Post
I've also now installed all the H2 simpson strong ties (one each side of the rafter, and at both ends - so four strong ties per rafter). Each tie is nailed with 8 x 8d simpson galv nails (4 in the rafter and 4 in the beam).

Really pleased that you pointed this out by the way - I've posted exact same wording to several forums (inc building code inspectors forum) and you're the only one to have spotted this mistake, so thanks.

NP, happy to help,
Something else regarding the strong ties. Generally you are required to put a fastener in every hole. It doesn't sound like you are being inspected, but open holes in a bracket is always quick hit by most any inspector.
MFG probably put those holes in there for a reason.
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Old Today, 08:12 PM   #14
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Re: Roof uplift on open building


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NP, happy to help,
Something else regarding the strong ties. Generally you are required to put a fastener in every hole. It doesn't sound like you are being inspected, but open holes in a bracket is always quick hit by most any inspector.
MFG probably put those holes in there for a reason.
Absolutely. Every hole has a simpson 8d galv nail in it (nail at opposing angles as per MFG instructions) so done that. Many thanks.
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Old Today, 08:16 PM   #15
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Re: Roof uplift on open building


Any time we got those big beam hangers with 40 holes, the job always went to the new guy that always asked the same question and always got the same answer, all of them.
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