Wind Uplift and the Continuous Load Path

Why this matters

A house does not blow apart because the wind is strong. It comes apart at the weakest connection between the roof and the ground, and on most houses that hit by high wind, the roof lifts first. Every connection in that chain is a few nails and a strip of steel that nobody sees once the drywall is up. Electricians, plumbers, HVAC installers, solar crews and remodelers routinely cut, bend, notch or remove one of those links to get a pipe, a duct or a cable through, and nothing about the house changes on a calm day. In a high-wind area, that one link is where the roof starts to peel.

How wind actually loads a house

Most people picture wind pushing a house sideways. That is part of it. The larger danger to a residential roof is uplift: wind flowing over the roof creates suction on its surface, the way air over a wing lifts it.

• Suction is not even. It is strongest at the corners, along the edges and the ridge, and on overhangs. An overhang is the worst case of all: suction pulls from above while wind pushes up from below at the eave.
• An opening multiplies it. If a garage door, window or door fails on the windward side, wind pressurizes the inside of the house. That inside pressure pushes up on the roof from below while suction pulls from above, and the two add. This is why an unreinforced garage door is often where a roof failure begins. See related: Garage Doors and Openings in a Wind-Rated Envelope.
• Walls get pushed and pulled. The windward wall is pushed in, the leeward and side walls are sucked out, and the whole structure is pushed sideways (racking), which the wall sheathing and its nailing resist.

How strong this gets at a given house is set by the design wind speed on the jurisdiction's design criteria table; the sibling card on design events explains that number. Pressure goes with the square of wind speed, so a house in a 150 mph zone is designed for about a third more pressure than one in a 130 mph zone, not 15 percent more: 150 squared over 130 squared is about 1.33.

The continuous load path

The load path is the chain of members and connections that carries a load from where it is applied to the ground. For uplift, it starts at the roof covering and ends at the foundation, and it is continuous only if every link is connected to the next one with enough capacity. The house is as strong as its weakest link, and the weakest link is almost always a connection, not a piece of lumber.

LOAD PATH FOR UPLIFT, roof to ground

  suction on roof surface
  ROOF COVERING -> underlayment -> SHEATHING
       |  sheathing nails into framing
  RAFTER or TRUSS            (cut webs? never)
       |  hurricane clip or strap
  DOUBLE TOP PLATE           (bore over 50%? tie)
       |  stud-to-plate strap or wall sheathing
  STUDS                      (notch or bore limits)
       |  strap or sheathing lapped onto sill
  SILL PLATE                 (cut for a drain?)
       |  anchor bolt, washer and nut
  FOUNDATION                 weight holds it down

The labels on the right are where trade work most often breaks a link. Each one is a place the next section returns to.

What each link is and what the code expects

The IRC, as adopted by your jurisdiction, sets a prescriptive minimum for each link; houses in high-wind regions often have engineered connections that exceed it, shown on the original plans.

• Sheathing to framing. The nailing schedule for roof and wall sheathing tightens in higher wind zones and at roof edges. Overdriven nails, missing nails at panel edges, and screws substituted for nails all reduce it.
• Roof framing to wall. Where the tabulated uplift on a rafter or truss connection is small (200 pounds or less in the 2021 edition's R802.11), toe-nails are permitted; above that, a connector rated for the uplift is required. In a high-wind zone, assume every rafter or truss has a connector and that it was sized for that location.
• Trusses themselves. The IRC is explicit that truss members shall not be cut, notched, drilled, spliced or otherwise altered without the approval of a registered design professional (R802.10.4 in the 2021 edition). A cut web or chord is never a field decision.
• Top plate. The double top plate ties the wall together and receives the roof connectors. When a bore or notch for pipe or duct removes more than 50 percent of the plate's width, the IRC requires a galvanized steel tie at least 0.054 inch thick (16 gauge) and 1-1/2 inches wide fastened across the opening with at least eight 10d nails each side, or equivalent (R602.6.1 in the 2021 edition).
• Studs. In exterior and bearing walls, a notch may take no more than 25 percent of the stud's width and a bored hole no more than 60 percent, with doubling required for holes over 40 percent, and holes kept at least 5/8 inch from the edge (R602.6).
• Wall to foundation. Sills are anchored with 1/2 inch anchor bolts at no more than 6 feet on center, within 12 inches of each end of each plate section, embedded at least 7 inches (R403.1.6), and high-wind and seismic regions require closer spacing or engineered hold-downs.

The trade acts that break a link

The last row deserves its own sentence. A connector's rated capacity applies only with the nails or connector screws the manufacturer lists for it, in every hole the listing requires. Drywall screws are brittle and snap in shear; short nails pull out. A full set of the wrong fasteners can leave a connector carrying a fraction of its rating while looking complete.

Worked example: one wall line, two trades

A single-story house in a 140 mph design wind zone, trusses at 24 inches on center bearing on an exterior wall with a 2x4 double top plate, which is 3-1/2 inches wide. A connector clip ties each truss to the plate, as shown on the original plans.

The plumber. For a new bathroom vent, the plumber notches the double top plate 2 inches deep across its width to pass a 1-1/2 inch vent. 2 divided by 3-1/2 is 57 percent of the plate width removed. That exceeds 50 percent, so R602.6.1 applies: a 16 gauge, 1-1/2 inch wide steel tie across the notch with eight 10d nails into the plate on each side. The plumber leaves the notch open.

The HVAC installer, a week later. The new bath fan's 4 inch duct runs to a soffit vent along the same wall. The clip at the truss directly over the notch is in the way, and the installer cuts it off to pass the duct. That truss is now held to the plate by its original toe-nails, if any, and the plate under it is notched more than halfway through.

What the house lost. At that truss, two links in a row are broken: the roof-to-wall connection and the top plate that was supposed to carry it into the wall. Neighboring trusses still connect, so the house will not fail on that one truss alone in ordinary weather, but uplift and racking loads concentrate where the chain is weak, and that truss sits at the end of the eave where suction is strongest.

The repair in kind. The steel tie goes across the notch: 16 gauge, 1-1/2 inches wide, eight 10d nails each side, so 16 nails in total. The duct is rerouted between trusses so it does not cross the connector. A new connector of the same model goes on the truss with every fastener its listing requires (the count comes from the connector manufacturer's catalog for that model, not from the old clip's leftover holes). Both repairs are photographed, and the ticket names the connector model and the nail count.

Where it would stop. Had the installer cut the truss's bottom chord instead of the clip to fit the duct, the repair is no longer in kind: it goes to a registered design professional or the truss designer for a repair detail, and nobody closes the ceiling until one exists.

How to look for a broken link

On any attic or wall job in a high-wind area, before you leave:

• Every truss or rafter on the run you worked along still has its connector, with nails in every required hole.
• Any plate you bored or notched over half its width has a steel tie across it.
• Nothing in the attic sits on, through or hangs from a cut truss member.
• In a crawlspace or basement, anchor bolt nuts are present and snug where you worked.

Photograph the connectors on either side of your work. If something was already broken before you arrived, the photo is the record that the shop did not do it, and the right next step is to tell the owner, not to quietly fix someone else's structural cut without the proper repair detail.

References

• International Residential Code, R602.6 and R602.6.1 (stud and top plate drilling and notching), R802.10.4 (truss alterations), R802.11 (roof uplift resistance) and R403.1.6 (foundation anchorage), 2021 edition, as adopted by your jurisdiction
• ASCE 7, as adopted through the building code, for design wind pressures
• Connector manufacturers' catalogs and evaluation reports for rated capacities and fastener schedules
• See related: Design Events: What the Code Assumes Will Happen to the House
• See related: How Service Work Quietly Removes a House's Hazard Resistance