Had one home develop significant leaks (this highlighted the failure of our previous contractor to coat the roof adequately). 

It's basically walls and a roof and nothing more.

Need to replace roof, no biggy. 

They will help and will not charge you a single penny.They will help you, but some places charge through the roof.

If you put more down, you'll see your Cash on Cash ROI improve and your Total Return compress.In terms of your variable operating expenses, I use the following:Vacancy:  8%Property Management:  8%Repairs:  $50 per unit, per monthCapEx:  $150 per roof, per monthI think it makes more sense to use a flat amount per unit for repairs since repair costs aren't necessarily correlated with rent collected.I use similar thinking for CapEx, except it's a flat amount per roof rather than per unit.Are you planning to self manage to some degree?

You'll keep all expenses under 1 roof, a vacancy will NOT completely wipe out your rental income and you'll gain experience as a landlord (assuming you purchase locally and self-manage).

Net after all expenses and accounting for reserves of roof, etc. 

I know the current tenant in quite upset cause the house has roof problems.

But be sure to include all costs associated with the property: vacancies, rehabs, and long term issues like HVAC, roofs, etc.

The roof will not get an accelerated schedule, however having a cost and square footage broken out will extremely useful in the case of damage.