Before doing another thing you should get this looked at by a building professional with a view to establishing whether or not the roof and walls need to be supported as the dwelling may not be safe.
A year is a surprisingly short time for such problems to develop unless you used really poor quality timber that hadn't been properly dried out before being used in the construction.
Using Direct labour has downsides.
A contractor on a small job means a single point of contact when things go wrong.
An architect heading a small design team means a single point of contact when things have been designed incorrectly and/or improperly certified.
You are now going to have to wade in and sort this mess out yourself, because you had neither on the job and even in the current situation, most architects wouldn't want to take this on...
However, all may not be lost - you say you have an engineer and a "roofing carpenter", whatever that is.
Firstly you need to find out what is wrong with the roof.
To do that you'll need an engineer to check the structure and possibly an architect and/or a damp specialist to look at other issues- not all problems with roofs are structural, some are cased by condensation leading to dry rot or water ingress leading to wet rot.
In cases of under-design, the walls may not be resolving and carrying to ground the imposed load of the roof, particularly any unresolved vectors.
On traditional houses a concrete "ring beam" was often used at the top of the walls to tie the masonry structure together [although these can themselves give problems] and give a firm springing point for the wall plate onto which the roof timbers were fixed.
Above a certain span, the roof must be a collar/tie roof to stop the rafters "spreading" and it sounds like you installed a cut timber roof.
These used be very popular, but relied on a working knowledge of such construction by both carpenters and blocklayers - typically there would be a central loadbearing spine wall towards with spreaders would shed their load from struts supporting purlins from which would be hung "hangers" to help teh ceiling rafters span the gulf.
Interference, omission, or removal of any of the composite load spreading members - spine wall, spreaders, struts, purlin or hangers - could cause problems.
You need to check what the engineer actually certified - was it limited to the design of the structure only, was it limited to visual inspection only, and if so was there anything visibly wrong with the roof.
You can start by talking to your engineer or you may wish to employ another building professional to check the property over before going near him.
He will review the design and the built work and comment.
FWIW
ONQ