roof-math

# Roof Math

Buy is not automatic. Stay length, cash left over, and a payment you can still live with.

Inspired by kusandriadi/finance property-kpr.md (MIT). Original Grok remake. Country-neutral. Complements cash-plan (budget first) and cover-map (home/required-car cover) and invest-map (do not treat a house as the whole portfolio).

Read `references/rails.md` before you write numbers. Fill the three templates.

## When to use
- Rent vs buy, mortgage basics, how much house, down payment
- Whether to keep renting, house-poor check, payment vs take-home
- Hidden housing costs, term vs interest, rate type, refinance education (not a product sale)

## When NOT to use
- Apply for a loan, scrape a bank, or quote a lender
- Collect SSNs or account numbers
- Household budget or high-interest debt as the main ask (use cash-plan first)
- Insurance stack for the home or required car (use cover-map)
- Treat the house as the whole portfolio (use invest-map after cash and cover)
- Legal, tax, or licensed-planner work they asked a pro for

## Inputs
- Country / currency context (keep products generic if unknown)
- Likely years in this place and mobility (job, family, school)
- Rent they pay today
- Price range and a down payment that does not drain all savings
- Take-home (monthly) and other debts if known
- Emergency cash (months of needs) and high-interest debt status
- Stability or family reasons they want named

## Procedure
1. Read `references/rails.md`. Refuse lender quotes, bank/broker APIs, applications, and storing SSNs or account numbers.
2. Gate with cash-plan: high-interest debt plan plus emergency cash still exist. No buy worksheet if those are missing. Roof-math does not invent a budget.
3. Stay check: mobility, years in place, emergency cash intact, high-interest debt, rent today. Buy lean: 5-7+ years, comfortable payment, emergency cash AND down payment both exist. Rent lean: high mobility, prices expensive vs rent, or (down payment + payment - rent) could be invested.
4. Cost stack: price, down payment (often 10-30% as education, not a local mandate), term, rate type (check the later floating rate, not only a teaser), monthly, extra costs about 5-10% on top of price, payment / take-home % (housing plus other debts usually <= about 30-35%). Flag house-poor if the payment strangles cashflow.
5. Name hidden costs: tax, maintenance, repairs, furniture/repair buffer. Property is illiquid. Family value is real; name it.
6. Term: longer means a smaller monthly and much larger total interest. Compare both. Extra principal early cuts total interest. Check prepayment penalties as a look-for.
7. Fill rent vs buy paths. Point leftover cash to invest-map (no tickers). A place to live is not an investment: modest gross rent yield, vacancy, repairs. REITs as an education alternative without physical hassle.
8. Refinance education only: compare move costs vs interest saved. Not a product sale. Do not FOMO on rising prices. No guaranteed appreciation.
9. Fill `templates/stay-check.md`, `templates/cost-stack.md`, and `templates/rent-buy.md`. Confirm locally.

## Output format
- Stay check (mobility, years, cash intact, debt, rent today)
- Cost stack (price, DP, term, rate type, monthly, extra costs, payment/take-home, house-poor flag)
- Rent vs buy paths, illiquidity note, one educational next step
- One line: educational only, not personalized financial, legal, or tax advice; confirm locally

## Safety
- Follow `references/rails.md` with no exceptions.
- No lender quotes. No bank or broker APIs. No SSNs or account numbers.
- No guaranteed appreciation. No Indonesia-only or US-only forms as mandatory.
- No legal or tax advice. A worksheet is not a loan offer.

## Stop conditions
- Three templates delivered, or refused for loan application / bank scrape / cash-plan gate / wrong scope