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HGT — the height invariant

There are two HGT codes. Both say that written values contradict each other.

CodeSeverityWhat it says
HGT01errorCeiling height plus the slab above exceeds the storey height — it eats into the floor above
HGT02errorA ceiling height that pierces a storey is declared under a partial void

This is the sectional counterpart of overlapping in plan. check calls two regions overlapping on one level a contradiction (GEO02); on exactly the same footing it calls a lower ceiling punching through the floor above a contradiction. Both are about written data failing to hold together, not about whether the building is any good. That is why HGT stays on the check side.

The invariant​

For every space:

effective ceiling height + the slab of the level above ≤ storey height
  • The effective ceiling height is the space's own h: if it has one, otherwise the h: of the level it sits on. A space's h beats its level's.
  • The slab above is the next level's slab: (floor-construction thickness — structural slab, plenum and finish).
  • The storey height is the difference between the next level's z and this level's z. Storey height is never written; it is derived from the levels' z.
  • The tolerance is 0.5mm, so rounding never trips this.

koyu levels shows the stack-up as a section in text.

koyu levels examples/house/main.muro
R	z:5800	slab:500
L2	z:2900	h:2400	slab:500
  ↑ storey height 2900 = ceiling 2400 + slab 500
L1	z:0	h:2400	slab:400
  ↑ storey height 2900 = ceiling 2400 + slab 500

Who gets checked​

The invariant is asked only of spaces that satisfy all of the following.

  • There is a level above. The top storey has no floor above it to collide with.
  • Something above overlaps it. A space with storeys above it but nothing directly over its own footprint is not asked. A level above that carries no spaces at all counts as covering everything.
  • It is not semi-outdoor. A space meeting the exterior across type:open or air:1 — a balcony, a terrace, an external stair — has no ceiling.
  • It carries no vertical circulation declaration. The ceiling over a space with stair:, ramp:, escalator: or lift: follows the flight above it and cannot be stated as one number. This is a declarative exemption.
  • Both the slab above and the effective ceiling height are determined. Without either, the inequality cannot be formed. That state is missing information, not a broken invariant, so SUF01 and SUF03 say it instead and HGT stays quiet.

That last point bites. Forget slab: on the level above and the storey below goes unchecked and stays green. Run --strict so SUF03 is caught.

HGT01 — it collides with the storey above​

error

grid X 0 3600
grid Y 0 4000
level L1 0 h:2800 slab:400
level L2 3000 h:2400 slab:400
space /L1/a room X1..X2 Y1..Y2
space /L2/a room X1..X2 Y1..Y2
/L1/a collides into the floor above: ceiling height 2800 + L2's slab 400 = 3200 > storey height 3000

Cause — a ceiling height of 2800 plus a floor construction of 400 exceeds the storey height of 3000. The message prints all three numbers, so which one to move is settled on the spot.

Fix — move one of the three.

  • Lower the ceiling height — level L1 0 h:2400 slab:400
  • Thin the floor construction — level L2 3000 h:2400 slab:200
  • Raise the storey — level L2 3400 h:2400 slab:400

To lower the ceiling of one room only, write it on the space rather than on the level.

grid X 0 3600
grid Y 0 4000
level L1 0 h:2800 slab:400
level L2 3000 h:2400 slab:400
space /L1/a room X1..X2 Y1..Y2 h:2400
space /L2/a room X1..X2 Y1..Y2

If you meant to pierce the storey, that is a void — see HGT02 below.

HGT02 — insufficient coverage under a partial void​

error

grid X 0 3600 7200
grid Y 0 4000
level L1 0 h:5400 slab:400
level L2 3000 h:2400 slab:400
space /L1/a room X1..X3 Y1..Y2
space /L2/v void X1..X2 Y1..Y2
space /L2/b room X2..X3 Y1..Y2
boundary /L1/a /L2/v type:void
/L1/a collides into the floor above: ceiling height 5400 + L2's slab 400 = 5800 > storey height 3000. The void covers only 50.0% — under a partial void keep the ceiling height within the storey height (the height of the void part is derived)

Cause — a type:void boundary is a declarative exemption from the height invariant, but the exemption reaches only as far as the void covers the lower storey's plan. The example voids half the lower storey yet declares a ceiling height of 5400, piercing the storey. Over the other half there is a floor, and that part cannot be 5400.

The exemption is total only at a coverage ratio of 99% or more. The message prints the coverage to one decimal — a grain that cannot collide with the threshold.

Fix — bring the lower storey's ceiling height inside the storey.

grid X 0 3600 7200
grid Y 0 4000
level L1 0 h:2400 slab:400
level L2 3000 h:2400 slab:400
space /L1/a room X1..X3 Y1..Y2
space /L2/v void X1..X2 Y1..Y2
space /L2/b room X2..X3 Y1..Y2
boundary /L1/a /L2/v type:void

The height of the void part is never declared. It is derived from the void relation. The lower storey's ceiling height is the ceiling height where there is a floor above, not the height of the void.

To make the whole thing a void, give the void space the same region as the storey below. Coverage is then 100% and the storey-piercing ceiling height passes.

grid X 0 3600
grid Y 0 4000
level L1 0 h:5400 slab:400
level L2 3000 h:2400 slab:400
space /L1/a room X1..X2 Y1..Y2
space /L2/v void X1..X2 Y1..Y2
boundary /L1/a /L2/v type:void

What HGT does not say​

There is not one architectural judgement about height anywhere in koyu. Eaves height, maximum height, road setback lines, north-side setback, shadow studies, minimum ceiling heights — none of these exist in check or in koyu validate. About height, koyu says only whether the written numbers add up.

The two height-adjacent judgements koyu validate does carry are the proportion of a derived stair's tread and riser (stair.proportion) and a derived ramp's slope (run.slope). Both are cautions about a derived shape, not statutory height limits.