Roof Trusses and Why You Never Cut a Web

Why this matters

In a truss attic, the 2x4 that is in the way of the new plenum looks like the least important piece of wood in the house. It is a thin diagonal between two other members, nothing sits on it, and cutting it would take thirty seconds. It is also part of a structure in which every piece was sized for the load the others hand it, and removing one changes what all the rest carry. Trusses rarely fail the day a web is cut. They fail under the next heavy snow, or they sag slowly until the ceiling cracks along the line of the cut truss, and the one trade that was in the attic since the house was built is the one whose name is on the job.

This card is about how a truss works, why any cut or removal needs an engineered repair, the loads a truss attic was and was not designed for, and the attic platform question every HVAC and storage conversation eventually reaches.

How a truss carries load

A rafter roof carries load by bending: each rafter is a sloped beam. A truss carries load by triangles. The roof load presses down on the top chords, the triangle shape turns that into straight pushes and pulls along each member, and the metal connector plates at every joint pass those forces from one member to the next.

• Top chords are mostly squeezed (in compression) and also bend between joints.
• The bottom chord is mostly stretched (in tension). It ties the two heels together so the top chords cannot spread the walls apart, and it also carries the ceiling.
• Webs are the diagonals and verticals between chords. Some are in compression and some in tension, and which is which depends on the truss pattern and the load. You cannot tell from looking which one a given web is.

Every member is sized for its own force, and only that. There is no spare member and usually little spare capacity, because the truss was designed by software against the loads on its design drawing and built to that design. Cut a web and its force has nowhere to go but the members and plates around it, which were never sized to take it. A compression web that loses its job lets the top chord bend over a span twice as long as designed. A tension web that loses its job lets two joints move apart that were held together.

Bracing is part of the structure too. Long compression webs often carry permanent lateral bracing, a continuous 2x4 running across several trusses at the same web, sometimes with a diagonal to tie it off. It looks like leftover lumber nailed across the attic. It is what keeps those webs from buckling sideways. Removing it to clear a path is removing structure.

What the code and the industry say

The IRC (R802.10, in its subsection on alterations to trusses, 2021 edition, as adopted by your jurisdiction) forbids cutting, notching, drilling, splicing or otherwise altering a truss member without the approval of a registered design professional, and forbids adding load beyond the truss design without verification that the truss can carry it. Wood trusses are designed under ANSI/TPI 1, the national design standard the IRC adopts by reference.

BCSI (Building Component Safety Information), published jointly by the Structural Building Components Association and the Truss Plate Institute, is the industry's guidance on handling, installing, bracing and repairing trusses. It is consensus guidance, not law, though the IRC points to it as accepted industry practice for bracing where the drawings are silent. Its position on field damage and modification is the same as the code's: a damaged or altered truss gets a repair designed by the truss manufacturer's engineer or another design professional, and nobody improvises one.

The repair is a document. A truss repair comes as a detail sheet: which member, what added lumber or plywood gusset, which fasteners in what pattern. It is specific to that truss and that damage. A 2x4 scabbed across a cut web by eye is not a repair, and it may make the truss worse by stiffening one area and changing where the forces go.

The loads a truss attic was designed for

Most truss attics were designed to hold almost nothing. The IRC's minimum live loads for attics (Table R301.5, as adopted) are 10 pounds per square foot for an uninhabitable attic without storage and 20 pounds per square foot for an uninhabitable attic with limited storage. The table's footnotes define which is which by geometry: an attic counts as without storage where the clear height is 30 in or less, or where there are not two or more adjacent trusses with web configurations that can fit a rectangle 42 in high by 24 in wide. A common W-pattern truss with webs crowding the space under the roof usually falls in the no-storage category.

The bottom chord live load actually used is on the truss design drawing, along with any concentrated loads the designer included. The drawings are delivered to the job when the trusses are and are often on file with the building department. Without them, assume the minimum for that attic's geometry.

Equipment is a concentrated load. An air handler or a furnace sits on, or hangs from, a few feet of two or three bottom chords. Whether the design included it is on the drawing. Where it was not included, the truss manufacturer or an engineer can evaluate it; a common, sensible field practice is to spread the load across several trusses on a frame or strut rather than putting it all on one chord, but spreading it is not the same as verifying it. See related: Hanging Equipment From Framing.

A worked example: one web and one platform

A tech is replacing an attic air handler in a single-story house with W-pattern trusses at 24 in on center. The new unit is taller than the old one, and the new supply plenum, as planned, runs straight into a diagonal web of the truss beside the unit. The tech's helper suggests cutting the web out and putting a 2x4 across the gap afterward. Both of them are working from boards laid across the bottom chords, never on the drywall between them, with the circuit to the old unit switched off and proved dead, and with the visit planned around the attic's afternoon heat.

The web stays. The tech does not know whether that web is in tension or compression, and the drawing is not on site. The options are to rotate the plenum takeoff 90 degrees so the plenum rises between trusses in the 22-1/2 in clear bay, or to use a lower-profile plenum with the takeoff on the side. The tech rotates the takeoff; the plenum clears the web by about 3 in, and no truss member is touched.

Then the platform question. The homeowner wants plywood laid around the new unit so she can store boxes up there, a 4 by 8 ft area. Here is how that conversation goes:

CUSTOMER: While you're up there, can you put plywood
  down around the unit? I want to store boxes.
TECH: I'll put down a walkway to the unit for service,
  that's part of the job. A storage deck is different.
  These trusses were likely designed for no storage.
CUSTOMER: It's just a few boxes.
TECH: A 4 by 8 platform is 32 square feet. If the
  trusses are rated for 10 pounds a square foot up
  here, that's about 320 pounds over the whole area,
  and the plywood itself takes some of that.
CUSTOMER: So I can't use it at all?
TECH: You might, once someone checks. The design sheet
  for these trusses says what the ceiling chords can
  carry. I'll note it and you can get it from the
  builder or the city. If they're rated higher, a
  deck is fine.

Checking the arithmetic the tech used. At 10 psf, 32 sq ft carries 32 times 10, or 320 lb. A 3/4 in plywood deck weighs a little over 2 lb per sq ft, about 70 lb across 32 sq ft, which is dead load the truss also carries. That leaves roughly 250 lb for boxes, people and anything else. At the 20 psf limited-storage rating, the same area carries 640 lb less the same 70, about 570 lb. The difference is the whole answer, and only the design drawing settles which number applies.

The record. The tech photographs the rotated plenum with the web intact, notes that no truss member was cut, notes the unit's weight and that it bears on a frame spanning three trusses with the design drawing not on site to confirm that concentrated load, and notes that a storage deck was declined pending the truss design drawing.

The attic platform question in general

Three different things get called a platform, and they have different answers.

The walkway is not a storage deck in waiting. A floored path to the furnace is the first place boxes get stacked. When you install one, say so to the customer in plain words, as the tech did above.

Storage trusses exist. Some houses have attic-storage or room-in-attic trusses designed with a floor load and an open center. They are identifiable on the drawing and usually obvious in the attic: an open rectangle through the middle of the trusses with no webs in it.

What changes the answer

A girder truss. A truss that carries other trusses, often doubled or tripled and bolted together, is a girder, and it carries far more than a common truss. Hanging anything from it or cutting near it is a question for the engineer before it is anything else.

Damage that is already there. If you find a cut web, a missing plate, a cracked chord or bracing removed by someone earlier, photograph it and tell the customer before you leave. It is not your repair to design, but it is your finding to report, and an air handler on a damaged truss is your problem the moment you set it there.

Snow country and high wind. The design loads on the drawing include the roof's snow and wind. A cut that seems tolerable in a mild climate is a cut into the margin the next storm needs.

References

• International Residential Code, 2021 edition, R802.10 (wood trusses, including alterations and bracing) and Table R301.5 (attic live loads), as adopted by your jurisdiction
• ANSI/TPI 1, National Design Standard for Metal Plate Connected Wood Truss Construction, adopted by reference in the IRC
• BCSI, Building Component Safety Information, Structural Building Components Association and Truss Plate Institute
• See related: Hanging Equipment From Framing
• See related: Attic Access and Equipment Service Paths