Hi I am in the process of building an extension with no ceiling . I want a pitched roof but with no ceiling rafters at plate level to give maximum ceiling height. I believe I can use a warm roof design can anyone advise. Thank you

As long as the span isn't too big, you can use rafters and collar ties. You can also use scissor trusses that are better for load bearing and typically have a 7-8 outside pitch and a 5-6 inside pitch. I just built a 1400 sq. ft. log home with scissor trusses spanning 24 ft. 7 on top and 5 inside, 13 ft. to center. Looks great. You can also insulate and ventilate better and it's easier to run HVAC and wiring.

Warm roof design?Can you explain this please?

My only concern would be the limited pitch and what you plan on using for roofing materials.Shingles are designed for 8/12 pitch or greater,less than that and you need to lap less than the standard 5",also using ice and water shield is recommended.

Cold lap and modified bitumen can also be used for lesser pitches(and you can insulate heavily under those 2 as well,if that is your need/purpose.)

From a point higher than the ceiling,you can run the cross ties to the building making a ceiling higher than the header.But this would depend on the length of said rafters..

Once you raise the ceiling joists from the top of the bearing walls, you must de-rate the size of the rafters according to the methods prescibed by your building code.

If you do not wish to use any ceiling joists, cross-laterals, or collar ties... the ridge must be designed as a beam or girder and must be designed by an engineer.