I'm doing this right now: using my Roth IRA investments for a down payment on a house. So, I've been looking into details and here are the relevant ones for RE investing:
1. You can withdraw any and all contributions tax- and penalty-free, you just have to report it on your next tax return (Form 1099R);
2. You can withdraw up to $10K in earnings tax- and penalty-free, as long as you're using the money as a "first-time home buyer", which just means you haven't paid interest on a mortgage loan for the past 24 months (i.e., you can have owned a home in the past, just not have paid interest on a mortgage loan within the past 24 months);
3. The Roth IRA "ordering rules for distributions" specify that you must first take out your contributions, then any rollovers, and then earnings (i.e., you can't take out earnings first and then contributions);
4. To withdraw earnings, you must have been paying into the Roth IRA for at least five years.