Modeling a Hip Roof and Estimating Loads on Interior Walls: Lessons From Real‑World Inspection and 3D Structural Analysis

Recently, I had the opportunity to model an L‑shaped house with a hip roof and perform a detailed attic inspection to understand how loads were being transferred through the rafters, pony walls, and interior partitions. This experience brought together field inspection, practical engineering judgment, and structured 3D modeling — and reminded me of the teachers and mentors who shaped my engineering mindset.

This article summarizes the modeling workflow, key observations from the inspection, and how 3D analysis helps confirm which interior walls are bearing and which can be safely altered.

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Why a 3D Model Was Necessary

The house used:

• 2×4 rafters at 16″ spacing
• 2×6 ridge beam
• 2×6 joists acting as tie members
• Plywood collar ties
• Multiple pony walls (pony frames) supporting rafters

These pony frames created non‑uniform and non‑obvious load paths, making a 2D section inadequate. Only a 3D model could capture the true rafter‑to‑pony‑wall interactions.

Roof overhangs were excluded (except at the entry), because exterior-wall reactions were not part of the analysis.

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Planning the Structural Model

Before modeling, I always:

• Sketch the roof and wall layout
• Identify key nodes and elements
• Number nodes and elements logically
• Avoid chaotic node jumps
• Keep member numbering clean for debugging

These habits come from early training: organize your structure before touching the software.

I also kept member-end stiffness unreleased to streamline modeling time. The goal was not detailed moment distribution, but reaction estimates and load paths.

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Findings From the On‑Site Attic Inspection

During the attic inspection at Haddon Road SW, it became clear that this house did not follow a simple “rafters + ridge + joists” system. Instead, there were:

• Mixed rafter support conditions
• Pony walls bearing on mid‑house walls
• Some rafters transferring load to joists rather than walls
• Construction methods typical of the 1950s

Because pony-wall positions were irregular, estimating bearing conditions by inspection alone would be unreliable.

The 3D model confirmed:

• Several interior walls were bearing (with measurable reaction)
• Some walls carried light unintended loads
• Others were fully non‑bearing at the stud level, except for headers

This validated which walls could safely be removed and where structural reinforcement was needed.

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Engineering Decisions Based on the Model

Key structural recommendations included:

• New engineered headers:
  • 6′ span (living room)
  • 5′ span (bedroom)
  • 4′-8″ span (entry closet)
• Double floor joists under the new openings
• Keeping existing headers and king/jack studs for lightly loaded walls
• Blocking beneath jack studs was required

The model clarified the load path:
rafters → pony walls → joists → interior partitions.

This ensured accurate decisions about which walls could be partially or fully removed.

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Reflections and Acknowledgments

Every structural model I build brings me back to the teachers who shaped how I think.

My Father (Prof. Zaghloul) & Prof. Mahmoud Helmy

They taught me the discipline of node numbering, preparing clean data files, and the dos and don’ts of structural modeling.

Prof. Rasid Shehata

Introduced me to the logic behind matrix structural analysis — the foundation of modern engineering software.

Prof. David Lau — and Prof. Ed Wilson’s Influence

Prof. Lau taught matrix structural analysis at Carleton University, where I served as his TA for five years. He emphasized efficient stiffness matrices and reducing degrees of freedom to streamline computations.

It is important to mention that Prof. Ed Wilson — the pioneer behind CAL90 — was actually Prof. Lau’s professor.
Prof. Wilson and his students developed CAL90, the foundation for SAP80 and SAP90, forming the early lineage of SAP2000.

This connection made Prof. Lau’s teachings deeply meaningful, because the simplified methods he taught were rooted directly in the origins of the software we still use today.

Prof. Ahmed Trabiah

Taught SAP input fundamentals long before modern graphical interfaces.

My Student Hoze

Showed me alternative matrix‑solution paths, teaching me that engineering is not about “my way only” — there are always multiple correct approaches.

Mr. AbdelGadir Abbas (AMEC)

Taught me practical techniques such as cutting and isolating complex STAAD models.

Prof. Gehad Rashad

Showed me how to use shell elements in slab and concrete modeling, and how to interpret shell–frame interactions correctly.

These mentors shaped not only my technical skills but also my mindset:
Be precise. Be practical. Be open to different solutions.

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Final Thoughts

Modeling a hip roof with pony walls isn’t just about numbers or software — it’s about understanding how real houses behave. Homes evolve over decades, renovations occur, and construction methods differ from era to era. The load path is rarely obvious.

A reliable evaluation requires:

• On‑site inspection
• Thoughtful 3D modeling
• Engineering judgment grounded in fundamentals

This combination ensures safe wall removal, proper header sizing, and a clear understanding of load transfer through the structure.

And personally, every model is a reminder of the teachers and colleagues who shaped my engineering journey.

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