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.
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.
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.
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.
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.
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.
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.
Good valley trim is invisible from the parking lot and waterproof in a storm. That combination comes from geometry first, coil second.