My limited experience from back in the construction days: Most charities are willing to accept labor donations, but hesitant to actually perform recommendations. Make sure you are working for a 501©3 http://en.wikipedia.org/wiki/501© not for profit, if you ask them if they are, they will know what you mean. As far as write off, Be clear with them about no fee/paperwork before you start, sometimes the manager and bookkeeper work on opposite ends of the building, and the bookkeeper may automatically pay an invoice that lands on their desk. Simply invoice them as usual, and ask for a receipt. They are familiar with the process. (or should be) Write off the receipt, and save it in case you or they get audited.
Post Script: Thank you for your willingness to give back.