IFC import: plain profile-extruded IfcSlab renders offset with wrong thickness

[thebossnow]

What happened

Imported a minimal hand-authored IFC4 file (IfcProject → IfcSite → IfcBuilding → IfcBuildingStorey, one IfcSlab, four IfcWalls forming its perimeter) through editor.pascal.app/ifc.

• The 4 walls round-tripped perfectly: reported Length 10.00ft, Height 9.84ft, Thickness 0.656ft — exact matches for the 3.048m / 3m / 0.2m in the source file.
• The slab (IfcArbitraryClosedProfileDef + IfcExtrudedAreaSolid, no material layer set, PredefinedType = .BASESLAB.) came in as "Footprint 1 slab" but rendered offset to the side of the wall footprint instead of underneath it, and reports Thickness 0.16ft (≈0.049m) instead of the file's 0.15m.

Expected: the slab occupies the same footprint as the walls (same rectangle, verified below) and reports 0.15m / ~0.492ft thickness.

What I ruled out

• The file is internally consistent: the slab's profile points (#42–#46, (1.524,1.524)→(4.572,3.9624)) are the exact same absolute coordinates as the wall corner placements (#86, #108, #130, #152). The stored IfcExtrudedAreaSolid depth is 0.14999999999999999 (0.15m), not ~0.049m.
• Suspected a missing ObjectPlacement on the spatial tree/slab (my exporter originally left it null). Added an explicit identity IfcLocalPlacement on IfcSite/IfcBuilding/IfcBuildingStorey and the slab, re-exported, re-tested. Identical result — same offset, same 0.16ft thickness. So it isn't a null-placement artifact; the attached file already has the explicit placements.

A hypothesis, unconfirmed: the slab's extrusion is authored with the local placement's Z-axis flipped ((0,0,-1)) and a local extrusion direction of (0,0,1), so it extrudes downward in world space with a positive depth value (top surface at the placement origin, base below it) — a valid but slightly unusual IFC authoring pattern. The walls in the same file don't do this (local Z stays (0,0,1), and the wall's placement carries the position instead of baking it into the profile). If your depth/extent-recovery code assumes a non-flipped local frame, that combination might be what's tripping it up specifically for the slab. I haven't looked at packages/ifc-converter's source to confirm this — just flagging it as the one structural difference I can see between the wall (works) and slab (doesn't) authoring in this file.

Repro file (IFC4, ~5.5KB, walls + slab as described above):

pascal_test2.ifc

ISO-10303-21;
HEADER;
FILE_DESCRIPTION(('ViewDefinition [CoordinationView]'),'2;1');
FILE_NAME('','2026-08-10T14:52:59',(''),(''),'IfcOpenShell 0.8.5-1c5b825','IfcOpenShell 0.8.5-1c5b825','');
FILE_SCHEMA(('IFC4'));
ENDSEC;
DATA;
#1=IFCPROJECT('0X2rkhFXDCOAqVNZAthnn4',$,'untitled',$,$,$,$,(#10),#5);
#2=IFCSIUNIT(*,.LENGTHUNIT.,$,.METRE.);
#3=IFCSIUNIT(*,.AREAUNIT.,$,.SQUARE_METRE.);
#4=IFCSIUNIT(*,.VOLUMEUNIT.,$,.CUBIC_METRE.);
#5=IFCUNITASSIGNMENT((#4,#2,#3));
#6=IFCCARTESIANPOINT((0.,0.,0.));
#7=IFCDIRECTION((0.,0.,1.));
#8=IFCDIRECTION((1.,0.,0.));
#9=IFCAXIS2PLACEMENT3D(#6,#7,#8);
#10=IFCGEOMETRICREPRESENTATIONCONTEXT($,'Model',3,1.0000000000000001E-05,#9,$);
#11=IFCGEOMETRICREPRESENTATIONSUBCONTEXT('Body','Model',*,*,*,*,#10,$,.MODEL_VIEW.,$);
#12=IFCSITE('0q2RAHQ9zFNOG5yVK015mH',$,'Site',$,$,#22,#41,$,$,$,$,$,$,$);
#13=IFCBUILDING('2ivlxUwjjAHAPN_qSWj3OP',$,'Building',$,$,#27,$,$,$,$,$,$);
#14=IFCBUILDINGSTOREY('03qMmw5mX1fhw_uEBhpvQo',$,'Ground Floor',$,$,#32,$,$,$,$);
#15=IFCRELAGGREGATES('1Fex0KvXr9k8qi5ztnhGvk',$,$,$,#1,(#12));
#16=IFCRELAGGREGATES('21lLDdwILDU8sBR_j_qwlQ',$,$,$,#12,(#13));
#17=IFCRELAGGREGATES('3yquw45zzFgwkWPEJr70jf',$,$,$,#13,(#14));
#18=IFCCARTESIANPOINT((0.,0.,0.));
#19=IFCDIRECTION((0.,0.,1.));
#20=IFCDIRECTION((1.,0.,0.));
#21=IFCAXIS2PLACEMENT3D(#18,#19,#20);
#22=IFCLOCALPLACEMENT($,#21);
#23=IFCCARTESIANPOINT((0.,0.,0.));
#24=IFCDIRECTION((0.,0.,1.));
#25=IFCDIRECTION((1.,0.,0.));
#26=IFCAXIS2PLACEMENT3D(#23,#24,#25);
#27=IFCLOCALPLACEMENT(#22,#26);
#28=IFCCARTESIANPOINT((0.,0.,0.));
#29=IFCDIRECTION((0.,0.,1.));
#30=IFCDIRECTION((1.,0.,0.));
#31=IFCAXIS2PLACEMENT3D(#28,#29,#30);
#32=IFCLOCALPLACEMENT(#27,#31);
#33=IFCCARTESIANPOINT((0.,0.));
#34=IFCCARTESIANPOINT((9.1440000000000001,0.));
#35=IFCCARTESIANPOINT((9.1440000000000001,15.24));
#36=IFCCARTESIANPOINT((0.,15.24));
#37=IFCCARTESIANPOINT((0.,0.));
#38=IFCPOLYLINE((#33,#34,#35,#36,#37));
#39=IFCGEOMETRICCURVESET((#38));
#40=IFCSHAPEREPRESENTATION(#11,'Body','GeometricCurveSet',(#39));
#41=IFCPRODUCTDEFINITIONSHAPE($,$,(#40));
#42=IFCCARTESIANPOINT((1.524,1.524));
#43=IFCCARTESIANPOINT((4.5720000000000001,1.524));
#44=IFCCARTESIANPOINT((4.5720000000000001,3.9624000000000001));
#45=IFCCARTESIANPOINT((1.524,3.9624000000000001));
#46=IFCCARTESIANPOINT((1.524,1.524));
#47=IFCPOLYLINE((#42,#43,#44,#45,#46));
#48=IFCARBITRARYCLOSEDPROFILEDEF(.AREA.,$,#47);
#49=IFCSLAB('1a_QZryoL4RA$jFb_MWudY',$,'Footprint 1 slab',$,$,#68,#57,$,.BASESLAB.);
#50=IFCCARTESIANPOINT((0.,0.,0.));
#51=IFCDIRECTION((0.,0.,-1.));
#52=IFCDIRECTION((1.,0.,0.));
#53=IFCAXIS2PLACEMENT3D(#50,#51,#52);
#54=IFCDIRECTION((0.,0.,1.));
#55=IFCEXTRUDEDAREASOLID(#48,#53,#54,0.14999999999999999);
#56=IFCSHAPEREPRESENTATION(#11,'Body','SweptSolid',(#55));
#57=IFCPRODUCTDEFINITIONSHAPE($,$,(#56));
#63=IFCRELCONTAINEDINSPATIALSTRUCTURE('2onKV5145BKe963NFyNzps',$,$,$,(#91,#135,#113,#49,#69),#14);
#64=IFCCARTESIANPOINT((0.,0.,0.));
#65=IFCDIRECTION((0.,0.,1.));
#66=IFCDIRECTION((1.,0.,0.));
#67=IFCAXIS2PLACEMENT3D(#64,#65,#66);
#68=IFCLOCALPLACEMENT(#32,#67);
#69=IFCWALL('2LU6EYJkH6yPANWtd$EcwV',$,'Footprint 1 wall 1',$,$,#90,#85,$,$);
#70=IFCCARTESIANPOINTLIST2D(((0.,0.),(0.,0.20000000000000001),(3.048,0.20000000000000001),(3.048,0.),(0.,0.)));
#71=IFCINDEXEDPOLYCURVE(#70,$,.F.);
#72=IFCDIRECTION((0.,0.,1.));
#73=IFCARBITRARYCLOSEDPROFILEDEF(.AREA.,$,#71);
#74=IFCCARTESIANPOINT((0.,0.,0.));
#75=IFCDIRECTION((0.,0.,1.));
#76=IFCDIRECTION((1.,0.,0.));
#77=IFCAXIS2PLACEMENT3D(#74,#75,#76);
#78=IFCEXTRUDEDAREASOLID(#73,#77,#72,3.);
#79=IFCSHAPEREPRESENTATION(#11,'Body','SweptSolid',(#78));
#85=IFCPRODUCTDEFINITIONSHAPE($,$,(#79));
#86=IFCCARTESIANPOINT((1.524,1.524,0.));
#87=IFCDIRECTION((0.,0.,1.));
#88=IFCDIRECTION((1.,0.,0.));
#89=IFCAXIS2PLACEMENT3D(#86,#87,#88);
#90=IFCLOCALPLACEMENT(#32,#89);
#91=IFCWALL('1wt2kQo0b6qQ1SsGQnsZiL',$,'Footprint 1 wall 2',$,$,#112,#107,$,$);
#92=IFCCARTESIANPOINTLIST2D(((0.,0.),(0.,0.20000000000000001),(2.4384000000000001,0.20000000000000001),(2.4384000000000001,0.),(0.,0.)));
#93=IFCINDEXEDPOLYCURVE(#92,$,.F.);
#94=IFCDIRECTION((0.,0.,1.));
#95=IFCARBITRARYCLOSEDPROFILEDEF(.AREA.,$,#93);
#96=IFCCARTESIANPOINT((0.,0.,0.));
#97=IFCDIRECTION((0.,0.,1.));
#98=IFCDIRECTION((1.,0.,0.));
#99=IFCAXIS2PLACEMENT3D(#96,#97,#98);
#100=IFCEXTRUDEDAREASOLID(#95,#99,#94,3.);
#101=IFCSHAPEREPRESENTATION(#11,'Body','SweptSolid',(#100));
#107=IFCPRODUCTDEFINITIONSHAPE($,$,(#101));
#108=IFCCARTESIANPOINT((4.5720000000000001,1.524,0.));
#109=IFCDIRECTION((0.,0.,1.));
#110=IFCDIRECTION((0.,1.,0.));
#111=IFCAXIS2PLACEMENT3D(#108,#109,#110);
#112=IFCLOCALPLACEMENT(#32,#111);
#113=IFCWALL('09Cl9kPrPEWuLu8h9oLi4e',$,'Footprint 1 wall 3',$,$,#134,#129,$,$);
#114=IFCCARTESIANPOINTLIST2D(((0.,0.),(0.,0.20000000000000001),(3.048,0.20000000000000001),(3.048,0.),(0.,0.)));
#115=IFCINDEXEDPOLYCURVE(#114,$,.F.);
#116=IFCDIRECTION((0.,0.,1.));
#117=IFCARBITRARYCLOSEDPROFILEDEF(.AREA.,$,#115);
#118=IFCCARTESIANPOINT((0.,0.,0.));
#119=IFCDIRECTION((0.,0.,1.));
#120=IFCDIRECTION((1.,0.,0.));
#121=IFCAXIS2PLACEMENT3D(#118,#119,#120);
#122=IFCEXTRUDEDAREASOLID(#117,#121,#116,3.);
#123=IFCSHAPEREPRESENTATION(#11,'Body','SweptSolid',(#122));
#129=IFCPRODUCTDEFINITIONSHAPE($,$,(#123));
#130=IFCCARTESIANPOINT((4.5720000000000001,3.9624000000000001,0.));
#131=IFCDIRECTION((0.,0.,1.));
#132=IFCDIRECTION((-1.,0.,0.));
#133=IFCAXIS2PLACEMENT3D(#130,#131,#132);
#134=IFCLOCALPLACEMENT(#32,#133);
#135=IFCWALL('10Q_ZH6RP6MRUKqbM2I9_f',$,'Footprint 1 wall 4',$,$,#156,#151,$,$);
#136=IFCCARTESIANPOINTLIST2D(((0.,0.),(0.,0.20000000000000001),(2.4384000000000001,0.20000000000000001),(2.4384000000000001,0.),(0.,0.)));
#137=IFCINDEXEDPOLYCURVE(#136,$,.F.);
#138=IFCDIRECTION((0.,0.,1.));
#139=IFCARBITRARYCLOSEDPROFILEDEF(.AREA.,$,#137);
#140=IFCCARTESIANPOINT((0.,0.,0.));
#141=IFCDIRECTION((0.,0.,1.));
#142=IFCDIRECTION((1.,0.,0.));
#143=IFCAXIS2PLACEMENT3D(#140,#141,#142);
#144=IFCEXTRUDEDAREASOLID(#139,#143,#138,3.);
#145=IFCSHAPEREPRESENTATION(#11,'Body','SweptSolid',(#144));
#151=IFCPRODUCTDEFINITIONSHAPE($,$,(#145));
#152=IFCCARTESIANPOINT((1.524,3.9624000000000001,0.));
#153=IFCDIRECTION((0.,0.,1.));
#154=IFCDIRECTION((0.,-1.,0.));
#155=IFCAXIS2PLACEMENT3D(#152,#153,#154);
#156=IFCLOCALPLACEMENT(#32,#155);
ENDSEC;
END-ISO-10303-21;

Generated by aiblueprint-mcp's new (minimal) IFC exporter while validating it against your importer — noticed this while testing rather than going looking for it. Happy to provide more variants if useful (e.g. a slab with local Z not flipped, to test the hypothesis above).