How to Block Light Coming Through Curtains

Light bleeds through curtains from all angles: the fabric itself, the sides, the top, and the gap where the pole ends before the wall does.

Most curtains aren’t made to block light by default, they’re made to look good.

Fixing the problem rarely means buying new curtains. It usually means addressing where the light is actually getting in.

What You’ll Need

Tools

• Tape measure
• Drill and wall plugs
• Screwdriver
• Scissors or fabric scissors (if trimming lining)
• Step ladder

Materials / Options

• Blackout curtain lining (clip-on or sew-on)
• Return brackets or wrap-around curtain track
• Blackout blind (roller or Roman) for layering behind curtains
• Curtain pole extension or wider replacement pole
• Weighted hem tape (to help curtains sit flush to the floor)

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How to Fix It: Step-by-Step

Step 1: Work Out Where the Light Is Coming From

Before you buy anything, stand in the room when it’s bright outside and watch where the light enters. It’s almost never just through the fabric.

• Check the sides: gap between curtain edge and wall
• Check the top: light above the pole or track
• Check the bottom: curtain not reaching the floor
• Check the fabric itself: hold it up to a window. If you can see light through it, the weave isn’t dense enough

This tells you which fix to prioritise. Side gaps are the most common culprit.

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Step 2: Deal with the Fabric First

If light comes through the curtain itself, you’ve got two options: line it or replace it.

• Clip-on blackout linings attach directly to your existing curtain header tape — no sewing needed. They cost around £20-40 and work well on most pleat styles
• Sew-on linings give a neater finish but need a basic sewing machine or a willingness to hand-stitch
• If the curtains are thin polyester or sheer fabric, lining them will help but won’t fully block light. A separate blackout blind behind is a better long-term answer
• Check the product is rated as blackout, not just “thermal” or “lined” — those don’t block light the same way

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Step 3: Fix the Side and Top Gaps

This is where most people lose the battle. A curtain that doesn’t wrap around the window frame leaks light from both sides regardless of how thick the fabric is.

• Mount your pole or track so it extends at least 15-20cm beyond the window frame on each side
• Use return brackets at each end so the curtain wraps back to the wall, closing off the side gap entirely
• For the top, fit a pelmet or valance above the pole to block the gap between the curtain header and the ceiling
• If you’re using a track instead of a pole, a wrap-around track (also called a bay or return track) handles this automatically

If your current pole is too short, you can usually buy extension brackets rather than replacing the whole thing.

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Step 4: Handle the Floor Gap and Final Checks

A curtain that floats above the floor lets in a strip of light at the bottom — more obvious than it sounds in a dark room.

• Your curtains should sit on the floor or puddle slightly, not hover above it
• If the curtain is slightly short, weighted hem tape sewn or ironed into the bottom hem helps it hang straighter and flush
• Once everything is fitted, do a full blackout test: close the curtains in daylight and give your eyes 30 seconds to adjust. Any remaining light sources will become obvious
• For rooms where total blackout matters (shift workers, babies, home cinema setups), a double layer of curtain plus a fitted blackout blind is the most reliable combination