A shim is a thin strip of material, often wedged in a space to raise up or stabilize a unit. In the world of windows and doors, shims are used during the installation process to ensure a window or door is plumb and level. 10 essential steps for a proper window installation FAQ Friday is a weekly feature where we answer Frequently Asked Questions about our products, service or industry. To see other FAQs we’ve answered, visit our FAQ page. If you have a question you don’t see an answer for, just ask in a comment or contact us.