$\backslash$table$[[$Task$,$Length,Dependency$],[$a$.$ Write and agree on contracts,$3$ weeks,$],[$b$.$ Secure financing,$4$ weeks,a$],[$c$.$ Obtain permits,$7$ weeks,a$,$ b$],[$d$.$ Site work,$4$ weeks,a$,$ b$,$ c$],[$e$.$ Build foundation,$3$ weeks,b$,$ c$,$ d$],[$f$.$ Erect walls,$4$ weeks,d$,$ e$],[$g$.$ Build roof,$5$ weeks,f$],[$h$.$ Rough in plumbing,$2$ weeks,f$],[$i$.$ Rough in electrical,$2$ weeks,f$],[$j$.$ Rough in furnace and air conditioning,$1$ week,f$],[$k$.$ Finish exterior,$6$ weeks,f$,$ g$,$ i$],[$I$.$ Finish interior,$6$ weeks,f$,$ g$,$ h$,$ i$,$ j$],[$m$.$ Finish plumbing,$2$ weeks,h$,$ i$],[$n$.$ Finish electrical,$1$ week,i$,$ I$],[$o$.$ Finish furnace and air conditioning,$1$ day,j$,$ I$],[$p$.$ Wait for inspection,$2$ weeks,k$,$ l$,$ m$,$ n$,$ o$]]$

If the project proceeds on schedule, how much time should have elapsed before the roof is built? If appropriate, give a range of time.

Roof building should begin after

weeks and should end after

weeks.