Why PEMB hips and valleys fight back
Pre-engineered metal buildings rarely present a single tidy roof plane. Lean-tos, transitions, and unequal eave heights create valleys and hips where two slopes meet at awkward dihedral angles. The structural members can be modeled in the building software, but the trim — the bent plate that nests in the valley or caps the hip — still has to be brake-formed to an angle that matches the real geometry.
Guessing the break on the shop floor wastes coil and time. Calculating from pitch and plan run gets you a first-article bend that usually nests with only minor tweak. That first-article step is not optional when panel profiles, hems, and sealant laps shift the finished nesting angle away from the pure geometric dihedral.
Inputs that matter
- Pitch — commonly rise over 12 (enter 4 for a 4:12). Each side of a valley can have its own pitch.
- Plan run — horizontal distance associated with each roof side. Independent runs let you model main building vs lean-to geometry.
- Which side owns the rise — many calculators compute valley rise from the primary (Side 1) inputs when eaves step. Read the assumption; verify against elevations.
If Side 2’s theoretical rise does not match Side 1, you are looking at stepped eaves or a non-symmetric intersection. That is normal on PEMBs — just do not ignore it when detailing trim length, closure pieces, and wall flashing returns.
Break angle vs inside angle
Shop language gets sloppy here. Separate two ideas:
- Bend angle from flat — how many degrees the brake folds the sheet from a flat panel. Flat is zero bend (180° unfolded).
- Inside angle — the angle measured inside the bent trim after forming. A 60° bend from flat yields a 120° inside angle on a simple V.
Program the press with the bend-from-flat value your calculator reports, then confirm with a short sample. Coating thickness and inside radius shift the finished nesting angle slightly. Write both numbers on the fab ticket so the brake operator and the inspector are not arguing about which angle was intended.
Member cuts: level and plumb
Beyond trim, intersection geometry produces member level cuts and plumb cuts for jack rafters, purlins, or frame flares depending on the system. Use those angles for layout; still verify against the 3D model when the intersection is skewed or multi-plane.
On many PEMB packages the primary frames already include the correct end cuts from the manufacturer’s software. Field and secondary members — especially after a late lean-to add — are where hand calculations and StructLogic’s Hip & Valley tool earn their keep.
Regular vs irregular plan
A “regular” hip on a square plan often assumes a 45° plan angle, which makes hip run = common run × √2. PEMB valleys between unequal buildings are frequently irregular: different pitches, different plan runs, stepped eaves. Use a tool that accepts independent sides. A framing-square hip chart from residential carpentry will not save you on a 2:12 into 4:12 lean-to.
If someone hands you a single “hip length” from a residential app, ask for both pitches and both plan runs. Without those four numbers (or a clear stepped-eave assumption), you are estimating, not detailing.
Shop workflow
- Pull pitches and plan dimensions from the approval drawings or model.
- Compute valley/hip length, plan angle, and trim break.
- Brake a short sample; check nest on a scrap of matching panel profile if possible.
- Adjust for hem, sealant lap, and manufacturer trim standards.
- Only then cut production lengths.
Document the sample bend: coil lot, inside radius, measured nest, and any shim or open joint. That note saves the next building with the same panel and pitch pair.
Field fit and waterproofing
Geometry gets the trim close; waterproofing finishes the job. Valley trim must shed water without relying on sealant alone, and hip caps must shed without ponding at the ridge transition. If the nest is open at the panel ribs, fix the break or the panel engagement — do not bury the gap in caulk and hope the next storm is gentle.
Coordinate trim length with eave and rake closures so you are not splicing in the worst drainage location. Splices belong where the manufacturer details them, with the correct lap direction for the slope.
Common mistakes
- Using the same pitch on both sides when the lean-to differs.
- Confusing bend-from-flat with the angle the inspector measures inside the trim.
- Ignoring eave step — trim length and closure pieces will not match the wall.
- Trusting a 2D sketch for a three-plane turret intersection — model it.
- Skipping the sample bend because “the calculator was right last job” — coil and radius change.
Good valley trim is invisible from the parking lot and waterproof in a storm. That combination comes from geometry first, coil second, and a sample bend before production.