I overengineered and built a catwalk in my attic.

The Problem & Context

I have an attic that showcases:

• Fink Roof Trusses
• Blown In Insulation

I wanted to add some flexible duct work and verify that the air sealing of the drywall was complete. It was difficult without a catwalk as a "work area".

Research Initial - Plywood only

Initial consultations with other homeowners came up with the solution to 

• clear the blown-in insulation from a section and put down a strip of 1/2" plywood. The side effects of this are:
• every time I enter or leave, I need to excavate then re-cover the plywood with blown-in insulation or fiberglass batts.

I don't like that initial "solution" because most times I enter or leave the attic, I am hot, itchy, and tired, so I expect that future me will leave a giant hole in the thermal envelope by forgetting to "re-cover" my home if I choose to apply that advice.

Research Additional - 2x12s and Plywood

Additional research online showed people hoisting 2x12s into their attic and putting them at 24" OC perpendicular to the bottom chords of the roof trusses. The comments to those who suggested and applied this is:

• "you just made any additional work in your attic 10x harder because they have to drill through those beasts." 
• You could make the ceiling sag because the load is being directed into the bottom chords of the Roof Truss

As the poor fool who will have to work in the attic is probably me or paid for by me, I wanted something that could hold weight and allow flex ducts to pass through, so I chose to reject this option. 

Research Final - Flat Truss and Plywood

The only structural member that satisfied my requirements is a "truss".

Most trusses are designed by engineers and constructed before installation (a hydraulic machine presses metal gussets onto them) then they are delivered to a jobsite to be lifted by crane and secured in place. As the entrance into the attic is via a small hole with no ladder, I would need a construction plan that could handle the small entrance.

As the existing Fink roof trusses suggested that the snow load was being transferred to the exterior walls, I needed to consider how static loads (storage thrown up into the attic as a temporary(permanent) solution) would be directed. While considering this, I determined that the low level of weight expected on this catwalk could be directed into 

• any fink bottom chords that are directly above a wallplate (aka interior wall).
• any exterior wall.

This seemed conservative enough for me. I didn't expect any appliances to be stored, but I did expect someone to put a stack of books or cans of food onto it even after I left warnings that this is not for storage.

Research - Design Considerations

I reviewed the types of "flat" trusses and determined a few types that could be applied :

• Platt
• K
• Warren
• Howe

Per my review, the best cost to benefit would be the "Platt" truss.

The Platt truss consists of a series of equilateral right triangles. Truss nodes with support have an inverted "V" on top and the middle of the unsupported span has a "V" with parallel diagonal chords to this. If you had only 2 supported nodes, it would look something like this: "/\\\\\V/////\". Also every single diagonal has a vertical that makes an equilateral right triangle, and the end of the truss often does not bother having a complete "V", so it looks more similar to this: "/\\\\V////\\\\V////\". That example shows that the ending supports are "half a V", and there is one support in the middle.

Any "short" nodes due to insufficient space for a perfectly equilateral triangle should be kept as close to a support node as possible because the truss shape in the middle of an unsupported span should not be messed with (or it will greatly decrease the strength here).

Research - Design Dimensions

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the project is not done, but if i wait to have it done to post anything it will never be posted.