it is a viable option however cost wise to do the whole roof i wouldnt like to hazard a guess.
however when it is time to get work done on your roof you may find this option's problem.
if and when your slats need replacing and the slates will need to be removed for this . You may find that the insulation you have now put on will become a problem as the foam will get broken and need removing from the back of the tiles and the roof joists so creating further work and obviously added cost later.
however even with that said it maybe a viable option for the time being. all depends if you can save to have the job done correctly or this cheeper option. with the problem of paying extra later.
" this type of insulation i understand is usualy spayed under the felt " inside the loft " and therefore only normaly comes in contact wit hthe felt and the wooden beans in the loft not the slates or slats.
before posting this i checked on a site that specialise in this and they usualy fix the issues with the roof before doing this type of work . i would suggest they have good reason to do this for reasons i have already highlighted above
http://sprayfoaminsul...cqxp58CFVaY2Aod-BkN1A .