Hurricane ties

kwerkOctober 10, 2010

I have the siding/sheathing off the bottom of my wall right now and I thought I would take the opportunity to add some hurricane ties. I live under the threat of tornadoes.

The sill plate is bolted to the concrete basement walls, with a rim joist/subfloor/wall on top.

What should I use to tie it together? And should the ties go under or over the new sheathing?

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First, you need to tie the roof rafters to the walls because high winds like to lift roofs. That can be accomplished with Simpson hurricane ties shown in the link below.

Then you should make sure the wall studs and/or structural sheathing is tied to the sill plate on the top of the foundation wall with Simpson strap ties/anchors. If that is not a good connection you can add connectors that attach to the studs and have bolts that go down through the floor framing into the foundation wall.

The next most important feature is to provide exterior walls with structural sheathing, diagonal bracing, or steel moment frames (Simpson) that are capable of transferring the horizontal wind loads from the walls and roof to the foundation.

It won't do much good to have well anchored plates or studs if the rest of the framing system is not adequate to transfer the wind loads to them.

This isn't really a DIY project.

Here is a link that might be useful: hurricane ties

    Bookmark   October 10, 2010 at 8:19PM
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I'm going to replace the sheathing and siding on the rest of the house next year so I have the opportunity to retrofit the whole house then. But right now I only have the wall open where the deck ledger attaches.

    Bookmark   October 12, 2010 at 9:06AM
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To tie the wood elements together, sill plate, rim joist, walls, I would just use sheathing with alot of nails.

To tie the wall/floor system to the foundation, all the options are not a simple add-on. Alot of them are not a remodel item since they need to connect to the footing or to reinforcements in the foundation wall, depending on the foundation type. This is definatelty not an easy "add a strap" problem.

I would also recommend the ties for the rafters/trusses.

    Bookmark   October 12, 2010 at 1:35PM
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Rather than add tie-downs and straps etc. that might be over or under designed for your situation I would hire an engineer to specify exactly what fasteners to buy and how to install them. You would get the most protection for your effort and save money on the hardware. I've been designing renovations for 40 years and I always use an engineer for lateral load resistance.

    Bookmark   October 12, 2010 at 1:43PM
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Look through the Simpson catalog for hold downs and tension ties. There are some that go over the mudsill anchor bolt and then get fastened to the framing. That can take care of the bottom of the wall. I'll use a larger square plate under the anchor bolt nut instead of the "standard" washer fir high wind areas. The larger plate helps prevent tear through of the mud sill.

Simpson has a "High Wind" catalogue.

For the top part, roof to wall, use the hurricane ties already mentioned.

When you re-sheath, if your house is more than one story, have a course of sheathing tie the upper floor studs to the lower floor studs by having it span the rim joist. Same with the first floor platform, have it tie the first floor platform mudsill to the first floor studs.

If you have a gable end wall, same thing there with the sheathing. If the wall isn't balloon framed, try to have the gable sheathing span any rim joists so it ties the upper wall's studs to the lower wall's studs.

The goal is to avoid what some framers do, which is to use a simply ripped strip of ply as an infill on the rim joists.

You can reduce roof wind loading by reframing a simple gable roof into a hip roof.

You can improve wall penetration protection by sheathing the walls with 3/4" ply instead of 1/2" ply. Regardless of what thickness you use, use ring shank nails when fastening and fasten at the proper schedule.

I build in a coastal climate, we'll sometimes run steel strapping all the way from the rafter to the foundation.

Hurricanes are somewhat easy to design for. Winds are generally constant and predictable. Tornadoes can be explosive, with both the wind and the pressure differential ripping the wood structure apart regardless of the quality of the framing.

    Bookmark   October 15, 2010 at 6:16PM
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If you decide to install these, get yourself a palm pounder. Best tool for setting the nails.

The first time I saw one on a home improvement show, I knew I had to have one. Of course, that was a few days after I had just nailed about 45 of those clips on rafters to hold the perlins for a roof of an outbuilding.

    Bookmark   October 18, 2010 at 9:39PM
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I think I will skip the ties then. The current sheathing is some sort of silver faced foam board from the 80s, with diagonal bracing at the corners. I am going to replace it with 7/16 OSB fastened with 8d ring shank nails. I think that should be a big improvement alone.

    Bookmark   October 21, 2010 at 1:49PM
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Also I forgot to mention I am not in a hurricane zone but a tornado zone (KS) so the high wind risk is not as likely as on the coast.

    Bookmark   October 21, 2010 at 1:54PM
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The uplift resistance of unbroken sheathing nailed over the framing from sill plate to top plate is entirely dependent on the size and spacing of the nails.

To maximize the performance of the sheathing install all panels in a vertical orientation or if installed horizontally provide solid blocking behind all panel joints (vertical orientation avoids this so it is preferred) and put a lot of nails in the sill and top plates (4" or less) and install a hurricane tie at each rafter.

    Bookmark   October 22, 2010 at 9:51PM
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Hurricanes, tornadoes, and earthquakes all put lateral and uplift forces on your house frame that must be successfully transferred to the foundation without causing the frame to deform and/or separate.

Continuous exterior structural sheathing from foundation plate to roof plate would be the minimum requirement for a low risk area. Diagonal bracing is inadequate unless it is part of a steel frame. For higher risk areas tie-downs at the foundation and more shear resisting walls would be needed.

If you don't understand lateral forces in buildings this is not a good DIY project.

    Bookmark   October 24, 2010 at 8:10AM
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I like your strategy, kwerk, 7/16 osb well-nailed to the sill plate and wall above should provide the connection you need with a modest investment in material and effort. I would probably just use regular 8d common nails, but as long as those ring shank one's your using have the same shank diameter as a common, they will be just as good.

It's my belief that the most common weak point for most homes is the connection between wall and roof, so hopefully you'll get a chance to add some strong-ties there next year. When you see tornado ravaged areas on the news, it usually the entire roof frame (or sections thereof) that are blown down the street. Other than that, the house is often in pretty decent shape. That's why I try to get some kind of positive steel connection from the rafter (or truss cord) all the way down to the studs (or at least to a well-sheathed top plate)

Not that any of this is going to save your house in a F5 tornadic direct hit, but most tornadoes hit relatively few structures directly. Even if some windows get blown out and siding banged up, as long as you roof is mostly intact you can minimize the collateral damage of a storm that usually lasts all of ten minutes.

P.S. If you have an attached garage, the framing and connections around the stub walls are another area where plenty of bracing and uplift resistant connections are often lacking.

    Bookmark   October 27, 2010 at 1:23PM
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Simpson provides a guide for homes.

Here is a link that might be useful: Simpson

    Bookmark   October 27, 2010 at 7:08PM
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APA - Form E510
"Using Wood Structural Panels for Combined Shear and Uplift"

Search the site for "uplift"

Then, under "Publication Search Results" (on the right side) choose item 4.

Here is a link that might be useful: APA

    Bookmark   October 27, 2010 at 7:12PM
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If you rely on the structural panel sheathing for uplift and lateral resistance it will be difficult to transfer the full strength of the panel to the frame even if you space the nails closely (3" oc. or less) and to fully transfer that load to the foundation unless there are several foundation anchor bolts close to the corners. Installing the panels vertically instead of horizontally is stronger and avoids the necessity of adding blocking behind panel joints. Many carpenters insist that horizontal installation is stronger but they are thinking of lateral forces perpendicular to the panel which are not important; the force you are concerned with acts parallel to the face of the panel so panel joints need to be kept to a minimum.

    Bookmark   October 30, 2010 at 11:49AM
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Horizontal sheathing panel placement can be almost as strong as vertical placement if the additional horizontal joints have blocking installed behind them and are nailed the same as the other panel edges but there is no reason to do that extra work.

It is a common assumption that wind and earthquake forces act perpendicular to the face of exterior sheathing but in fact they are transferred through the floors and then act parallel to the face.

The weak point in a plywood or OSB sheathed wall in terms of lateral load resistance is the nails. The nails will always fail before the sheathing therefore the strength of the system is increased by avoiding unnecessary joint nailing. The sheathing is simply stronger than the nails.

A wall with horizontal panels with no blocking is much weaker in resistance to wind and earthquake loads than vertical panels. If there is an advantage to building that way it is not due to structural issues.

    Bookmark   November 10, 2010 at 9:21AM
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