Modelling pitched roof as a flat roof

[Bayan khayat]

Hi,
I am modelling a semi-detached UK house with a pitched roof that has loft insulation on ceiling level only. Can I model the roof as a flat cold roof? If yes, what considerations should I take into account to get similar results to the actual pitched roof (i.e. when applying the equivalent Sd-value, ACH, U-value, and other related parameters)?
Thank you,
Bayan