Building for 115 MPH Wind on an Open Lot

TL;DR

1. Twin Falls County publishes a design wind speed of 115 mph (3-second gust, IRC) and 105 mph under the IBC for Risk Category II buildings. That is the number your plans have to work to.
2. The number that changes your build more than the speed is the exposure category. Twin Falls County publishes Exposure C, and an open lot with no meaningful windbreak stays Exposure C.
3. Exposure C means higher design pressures than the sheltered suburban sites most prescriptive code shortcuts assume. Published guidance puts the difference at as much as a 30 percent reduction in uplift design pressure when a site qualifies for Exposure B instead.
4. Wind lifts a house more than it pushes it. The roof is the failure point, and the fix is a continuous load path from the roof sheathing down to the foundation.
5. The IRC's simplest roof connection shortcut, plain toe-nailing under the uplift exception, is written for Exposure B. On an open lot it generally is not available, so you are into rated connectors.
6. Roof overhangs are a real constraint. The prescriptive uplift tables in the IRC top out at a 24 inch overhang. Deeper eaves usually need engineering.
7. Garage doors, gable ends, and large window walls are where open-lot homes actually fail, and they are the items most often value-engineered out of a bid.
8. None of this is exotic or expensive if it is designed in from the start. It is expensive as a change order.

Here is what the 115 mph number actually means, and what it changes about how your house gets put together.

What 115 mph actually means

It is not a weather forecast. It is a design criterion.

Twin Falls County publishes its building design criteria for exactly this reason: so that designers, engineers, and builders all work to the same numbers. The wind figures are a 115 mph ultimate design wind speed measured as a 3-second gust under the International Residential Code, and 105 mph under the International Building Code for Risk Category II structures, which is the category most ordinary buildings fall into.

Two things worth understanding about that number.

First, it is a gust, not a sustained speed. A 3-second gust is a short burst, and structures are designed against bursts because that is what breaks things. A sustained 115 mph wind in the Magic Valley would be a historic event. A 115 mph gust is a design assumption about the worst case over the life of the building.

Second, 115 mph is the threshold the IRC uses to define hurricane-prone regions. We are not in one, because that definition also requires proximity to a hurricane coastline. But it tells you something that our design speed sits right at the line where coastal detailing starts being required elsewhere. The wind here is taken seriously for a reason.

What it does not mean is that your house is guaranteed to survive a 115 mph gust undamaged. Design wind speeds are about structural integrity and life safety. Shingles, siding, gutters, and trim are a different conversation.

Exposure C is the real story

If you only remember one thing from this post, make it this one.

Wind speed is half the equation. The other half is exposure category, which describes how much the ground around your building slows the wind down before it arrives. ASCE 7, the standard the code points to, defines three categories that matter for residential work.

Twin Falls County publishes an exposure coefficient of C in its design criteria. That is the baseline assumption for the county, and on a genuinely open lot it is also the correct one.

Here is why it matters in dollars and hardware. Published industry guidance notes that qualifying a site for Exposure B rather than C can produce as much as a 30 percent reduction in uplift design pressure, and that the difference between assuming B and assuming C when C is actually correct can mean a damage difference of upwards of 50 percent in a real wind event. Same wind speed. Same house. Different ground around it.

This is also where an honest conversation with your builder starts. Exposure category is not a preference. It is determined by the terrain upwind of your building, in each direction, for a defined distance. A lot ringed by mature shelterbelt on every side may genuinely qualify as B. A lot with a shelterbelt on the north side and 400 acres of pivot ground to the west does not, because the wind that matters is the wind from the open direction.

And here is the part that surprises people on new ground: your neighbors are part of your structure. A lot in a half-built subdivision, with foundations where the surrounding houses will eventually be, is Exposure C today. The houses that will eventually shelter it do not exist yet, and the house you are building has to stand up before they do.

Wind lifts houses more than it pushes them

The intuitive picture of wind damage is a wall being pushed over. That is not usually what happens.

Wind moving over a roof behaves like wind moving over an aircraft wing. It accelerates across the surface and creates negative pressure, which is suction, pulling the roof up and away from the walls. At the same time, if wind gets inside the building through a broken window or a failed garage door, it pressurizes the interior and pushes the roof up from below at the same moment it is being sucked up from above.

That combination is how houses come apart, and the sequence is remarkably consistent:

1. An opening fails. Usually a garage door, sometimes a window struck by debris.
2. Internal pressure spikes.
3. The roof sheathing or the roof-to-wall connection lets go.
4. With the roof diaphragm gone, the walls have lost their top bracing and fold.

The engineering answer is a continuous load path: an unbroken chain of connections carrying uplift force from the roof sheathing, through the rafters or trusses, into the top plate, down the wall studs, through the sill plate, into the anchor bolts, and out to the foundation and the ground. Every link has to be strong enough, because the chain fails at its weakest connection and the failure is sudden.

A house can be over-built everywhere except the roof-to-wall connection and still fail at the roof-to-wall connection.

Where Exposure C changes the details

This is the practical part, and it is the reason the exposure category is worth arguing about at design time rather than discovering at inspection.

The toe-nailing shortcut generally is not available. The IRC includes an exception in its uplift resistance provisions that lets you use conventional toe-nailing for the rafter or truss to top-plate connection, without rated hardware, under a specific set of conditions. One of those conditions is that the wind exposure category is B. Twin Falls County publishes C. On an open lot, you do not get the shortcut, and the connection needs a rated hardware solution or engineering.

That means connectors, and connectors have published capacities. For scale, a common seismic-and-hurricane tie of the kind used at rafter-to-plate connections carries an uplift capacity in the range of several hundred pounds, while heavier ties in the same family carry a thousand pounds or more, and larger strap-style connectors go higher still. The specific model and capacity is an engineering decision based on your roof geometry, span, and spacing. The point is not the part number. The point is that this is a spec'd, calculated item rather than something the framing crew decides on site.

The prescriptive tables have limits. The IRC's uplift connection tables are bounded. They are limited to a maximum roof overhang of 24 inches and a maximum mean roof height of 33 feet. Step outside those bounds and the table stops applying, which means an engineer has to run the numbers for your specific building.

Anchor bolts and hold-downs matter more. The load path does not stop at the top plate. Uplift travels down the wall and has to be resisted at the bottom. Sill plate anchorage, and in some configurations hold-down hardware at wall ends, is what keeps the bottom of the chain attached.

Sheathing nailing is not a detail. Roof and wall sheathing is a structural diaphragm, not a substrate for shingles. Nail size, spacing, and edge distance are specified, and the edges and corners of the roof, where suction is highest, often call for tighter spacing than the field.

The overhang question nobody expects

Deep eaves look good. They shade south glass in summer, they keep water off siding, and they give a house a grounded, substantial look that a lot of modern farmhouse designs depend on.

They are also wind catchers.

An overhang has wind pressure on the underside pushing up and suction on the top surface pulling up, at the same time, at the most leveraged point of the roof. This is why the prescriptive uplift tables cap out at 24 inches of overhang. Beyond that, the code stops offering a lookup answer.

That does not mean you cannot have a 36 inch eave on an open lot in Twin Falls County. It means the eave becomes an engineered element, with specified framing and connections, and it needs to be in the drawings before the truss package is ordered. Trusses are manufactured to a specification. Changing the overhang after the order is placed is not a field adjustment.

Ask early, decide early. This is a design conversation, not a framing conversation.

Garage doors, gable ends, and the other usual suspects

Three items account for a disproportionate share of wind damage on open sites, and all three are commonly value-engineered.

Garage doors. The largest single opening in most houses, in the wall that most often faces the prevailing wind, made of the lightest material in the building. A garage door failure is the classic entry point for the internal pressurization sequence described above. Wind-rated doors and reinforcement kits exist and cost more than a standard door. On an open lot this is the wrong place to save money.

Gable ends. A gable end wall is a large flat surface with no roof structure tying its top back into the building unless it is specifically braced. Gable end bracing, which ties the top of the gable back into the roof framing, is one of the cheapest structural upgrades available and one of the most commonly skipped.

Large window and door walls. Glass is not the structural issue. Debris is. On an open site the debris source is not palm fronds, it is gravel, farm equipment parts, loose panels off outbuildings, and anything that was not tied down at the neighbor's place. Impact resistance is a real consideration where a wall of glass faces open ground.

Porches, covered patios, and carports. Any roof with open sides gets wind underneath it. These structures need their own uplift resistance, and the connection back to the main building is a frequent weak point.

What this means for how you design

None of the above argues against building on an open lot. Open lots are why people move out here. The view off the rim, the acreage, the not-having-neighbors are the entire point.

It argues for making a handful of decisions early:

Orient the building with the wind in mind. If there is a prevailing direction, the long face and the largest openings do not have to point into it. Garage doors on the sheltered side of a house are a free structural upgrade.

Decide the eave depth during design. Before the truss order, not after.

Budget the hardware honestly. Connectors, sheathing nailing, anchorage, and gable bracing are a real line item, not a rounding error, and they are far cheaper as part of the original framing than as a retrofit.

Do not plan around future shelter. If the windbreak is not planted and grown, or the neighbors are not built, the house has to stand without them.

Treat the exposure category as a structural fact, not a negotiable assumption. A design that assumes B on a C site is under-designed in a way that will not be visible until the day it matters.

What to ask your builder

1. What wind speed and exposure category are my plans designed to, and where is that stated in the drawings?
2. Is my site Exposure B or C, and what is that determination based on?
3. How is the roof-to-wall connection being made, and what is the specified hardware?
4. Are we within the prescriptive tables, or does any part of this require engineering?
5. What is the overhang dimension, and was the truss package ordered to match?
6. Is the garage door wind-rated, and is the gable end braced?
7. What is the roof sheathing nailing schedule at the edges and corners?

A builder who can answer these without looking anything up has been doing this here for a while. A builder who has to think about it is not necessarily wrong, but you want the answers in writing before framing starts.

Building on an open lot in the Magic Valley?

We build custom homes, additions, ADUs, shops, and remodels across Twin Falls, Jerome, Kimberly, Buhl, Filer, Burley, Rupert, and unincorporated Twin Falls County.

We will tell you which exposure category your site is and why, what that changes about your framing package, and what the structural items cost as a line item rather than burying them. If your plan has a deep eave or a big glass wall facing open ground, we will tell you what that takes before the trusses are ordered.

We are a registered Idaho contractor, RCE-65510, insured and warrantied.

Tell us about your project → or call (208) 731-1729

Structural design for any specific building is the responsibility of the designer of record or engineer for that project. The information here describes code criteria and general practice, not a design for your house.

Frequently asked questions

What is the design wind speed in Twin Falls County? The county publishes a 115 mph ultimate design wind speed as a 3-second gust under the IRC, and 105 mph under the IBC for Risk Category II buildings. Those are design criteria rather than forecasts.

What is a wind exposure category and why does it matter? It describes how much the terrain upwind of your building slows the wind before it reaches you. Exposure B is suburban or wooded, C is open country, D is flat unobstructed terrain near large water. Twin Falls County publishes exposure coefficient C. The category can change design pressures substantially, with published guidance citing as much as a 30 percent reduction in uplift design pressure for sites that qualify as B.

My lot has trees on one side. Is it Exposure B? Probably not. Exposure is evaluated in each direction, and the governing direction is generally the open one. A shelterbelt on one side of an otherwise open parcel does not usually change the category.

Does wind push a house over or lift it? Lift is the bigger issue. Wind flowing over a roof creates suction that pulls it upward, and if an opening fails the interior pressurizes and pushes up at the same time. Most catastrophic residential wind failures start at the roof.

What is a continuous load path? An unbroken chain of connections carrying uplift force from the roof sheathing through the framing, down the walls, and into the foundation. It fails at the weakest link, which is why one under-spec'd connection can undo an otherwise sound building.

Can my roof-to-wall connection just be toe-nailed? Generally not on an open lot here. The IRC exception that allows conventional toe-nailing without rated uplift hardware is conditioned on the wind exposure category being B. Twin Falls County publishes C, so the connection typically needs rated connectors or engineering.

How deep can my roof overhang be? The IRC's prescriptive uplift connection tables are limited to a maximum 24 inch overhang and a 33 foot mean roof height. Deeper eaves are possible but become an engineered element, and the decision has to be made before the truss package is ordered.

Is a wind-rated garage door worth it? On an open site, yes. The garage door is usually the largest opening and the lightest assembly in the building, and its failure is the common first step in the sequence that lifts a roof off.

Do these requirements make an open lot expensive to build on? Not dramatically, if they are designed in. Connectors, nailing schedules, anchorage, and gable bracing are modest costs at framing. They become expensive when they are discovered late, or when a truss order has to be redone because an overhang changed.

Will my house survive a 115 mph gust? Design criteria target structural integrity and life safety, not zero damage. A properly designed and connected house should stay standing and keep its roof on. Shingles, siding, trim, and fencing are a separate matter, and no builder can promise those.

Sources: Twin Falls County published building design criteria, including wind speed and exposure coefficient; 2018 and 2021 International Residential Code provisions for roof uplift resistance, prescriptive connection tables, and hurricane-prone region definitions; ASCE 7 surface roughness and exposure category definitions; published industry and manufacturer guidance on exposure category effects, continuous load path, and connector capacities, accessed September 2026. Connector capacities referenced generally are illustrative of published product ranges rather than a specification for any project. Structural design for a specific building is the responsibility of that project's designer or engineer.