A published shell envelope: the main building's wall-skin planes, the horizontal reach of its eave and rake overhang, the lowest underside of that overhang at any point along the wall, and the plan clearance an addition must keep from each wall end to miss the corner trim.
Also called: lean-to through the fascia · attach height · shell clearance · overhang collision
- 1maxAttachHeight() samples the overhang underside across the addition's whole footprint, because on an end wall the rake underside climbs with the gable.
- 2A porch also carries a ridge cap standing proud of its steel, so its peak is accounted for too.
- 3The default attach height is as high as the shell allows, less a nominal attach drop for a lean-to (the flashing needs wall to lap up), and never above the main eave line.
- 4An over-wide porch cannot fit under the overhang, so the sanitizer shrinks its span about its midpoint until the attachment clears real headroom, rather than squashing it.
- 5Everything inboard stops at the skin mid-plane — buried, never past it.
- 6The same envelope is read by the audit scripts, so the geometry and the audit cannot drift apart.
A lean-to attached at the main building's eave height ran its own roof straight through the main fascia board. The real problem was that the finished shell's planes — the outside of the steel, the underside of the overhang — were not available to anything outside the main frame, so additions started at the wall line with nothing to stop against. Those planes are now published once and every addition attaches to them, which is why an attachment lands clear of the fascia instead of through it.
- Additions attached at a height that intersects the main roof assembly.
- Geometry rules duplicated between the builder and the audit.
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