The builder is definitely not the person designing the roof structure. From the drafting shop, the file is sent to the truss fabricator for design of the individual truss profiles, and of course pricing. In my experience, it is really easy to identify a costly roof from a basic low cost roof based on the appearance and spans needed, lower slope and bigger spans being more expensive. The truss factory is among the most efficient producers of material we have in the industry though. So even a costly roof design is still viable to specify for a client. The trusses are prefab done off site in a mechanized factory in a lower cost rural area and trucked in to the site. This is fast and efficient. Asking farmers to make the trusses by hand would be a far inferior means of building a house.
Typical of the construction business, to do a better spec’d truss package will not cost 10-20% more than a basic package. You can spend double or more than double on the custom roof. Such is the case of the upcoming project garage I am working on now. A typical four car garage truss pack can be easily purchased for under $2000. Yet a custom two car garage roof somehow came out to be $3000. That is almost double the cost for half the space. In my world that is a 4x upcharge. Is it worth it? I think so because this will be a really interesting and useful garage with a completely vaulted ceiling inside. Essentially the garage could easily contain a lift system allowing someone to work on their own car or store an additional care. Having a costly truss package is a lot cheaper than building a third stall.
This garage also merits a more costly opener, such as side lift rather than typical ceiling mount, and likely a fancy door too with better insulation and windows. So there are further implications of going with the more costly product, that is the future selections made necessary to specify later will also be more. A smart builder will be able to estimate the increased cost of custom work and also the collateral cost in the future to make it all function properly. This is also why it is a slippery slope to attempt to mix lower cost builder grade material with higher cost custom material, they don’t necessarily cooperate with each other. If you go the higher cost route from the beginning of a project, then likely everything else will need to be upgraded too. By the time the house is built you can experience a massive divergence from what you’d intended to spend to what you must spend to include the products you want.
This illustrates what I mean by builder grade truss profile being very inexpensive, yet custom profile being very costly. Both serve the same purpose, but one garage will have a higher ceiling and a much bigger price tag. On a million dollar house these details are worth pursuing, on a basic house, not so much.