I'm an European, too. I am quite familiar with both SI (by school) and USC (by my efforts to understand US engineering works). Mr. Delaforce I think you say that SI is a system that follows the physics and is an "absolute" system of units. It's true, no doubt. However in my earlier school I starting physics with m-kgf-s units. As unit, kilogram force was never SI, but it has been used in technics (probably via German books) and for some reasons it was a unit also in physics books. When they turned to full SI in physics books, I was totally confused when they "revealed" that 1 kilogram of mass weighs 9.81 Newtons rather than 1 kilogram force- for me it was a perfect non-sense. Fortunately, in school you cannot simply declare that you "just don't like it" and I went ahead.

Mayur, your original question was "If I know the weight of the roof plates, say = 4593.8 lb then can I assume the gravity force (f) to get LBF OR How to get LBF? ". You simply know the mass 4593.8 lb hence you know the weight 4593.8 lbf. It is about how your system was constructed- it is a relative (gravitational) system of units. 1 lbm weights (exerts) 1lbf in earth's gravitational field and there is no reason to convert lbm to lbf, or vice-versa; you may simply declare that w=m and skip the "third" letter in lbm and lbf. OK, if you are a scientist working on Earth you may need to consider the local gravity field intensity (g- what we call usually "gravity acceleration") and weight will be slightly different as w = m⋅g/gc≈ m.

Mr. Delaforce, just few historical details. The "metric" system was a child of French revolution and its history is a perfect example of mankind "harmony" that is still driving this Planet. The French Academy approached the British and the Americans in early 1790 with proposals of joint efforts to define a common standard of length based on the length of a pendulum. "The committee" agreed but the proposal brings up the choice of latitude for the pendulum- Thomas Jefferson wanted to be 38°N, Talleyrand for 45°N and Great Britain for London's latitude. French went ahead alone and did it based on other reference. Napoleon, because the French people were quite resistant to the decimal system (and for sure also for other political reasons), introduced in 1812 a new system of measurement "mesures usuelles" (customary measures). Louis Philippe reinstated the original metric system and this took effect in 1840, after a half of century of efforts.

Best regards.