How to Order Functional Louvers If you are ordering a functional louver, you should order a louver that is approximately 2" larger than your rough opening on all sides. This will allow you to attach the louver to the substrate through the brickmould edge. Functional louvers are slatted for air flow with a screen on the back. Note: Slats are not operable. Louver Accessories Keystones and trim are available attached to standard products as fabricated items. Keystones and trim can also be ordered as unattached accessories by listing the matching part number as a separate line item on your order. How to Use This Section The products in this section of the catalog are organized by louver style followed by matching trim. Where applicable, matching keystones are referenced for each louver and trim piece. Note: The number of slats and size vary based on louver style and height. Louvers & Gable Vents Note: Always consult the local building code authority to determine suitability of products to meet specific project requirements.