Black edges on a bathroom mirror? What causes them and how to slow the spread

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Black spots creeping in from the edges of a bathroom mirror look like dirt at first. They are not. The dark patches are the mirror's reflective backing breaking down, and once it starts, no cleaning product will bring the silver back. What you can do is understand why it started and stop it from spreading fast. What the black edge actually is A mirror is glass with a thin layer of silver on the back, protected by paint. Where moisture, cleaning chemicals, or salty air reach that backing, the silver oxidises and turns black. Because the backing is behind the glass, the damage shows through as dark spots or a dark border you cannot wipe away from the front. People call it desilvering or mirror rot. Why bathrooms speed it up Three things attack the backing. Steam and condensation keep the mirror damp for hours, and water wicks into the edges where the backing is most exposed. Ammonia-based glass cleaners, sprayed directly on the mirror, run down and pool at the bottom edge, eatin...

Mosquito screen not fitting? Measure the frame and hardware before buying mesh

A mosquito screen can look simple until the replacement leaves a gap, catches the handle or refuses to sit in its track. Before buying mesh or ordering a frame, identify what is actually failing: the mesh, the frame or the fit. This guide helps you prepare a measurement brief, not a universal cutting size.

Quick answer: inspect first, order second

A sound existing screen is a useful starting point. Record its outside frame width and height, frame thickness and profile, retention hardware and installation side. If it is bent or missing, measure the receiving track and ask the supplier to confirm the finished size. Do not apply one standard deduction to every window.

Safety before opening the window

Insect screens are not fall protection. Andersen warns that they are designed for ventilation and keeping insects out, not preventing a child from falling. Keep children away from the work area and never lean out to reach an exterior screen. Do not obstruct a designated escape window. Use professional help for unsafe access, damaged glass or an unstable frame.

Why a screen leaves gaps or feels loose

Width and height are only part of compatibility. Frame depth, rail profile, corners, clips, pins and pull tabs can differ between systems. A bowed frame may no longer show the original size. A screen with the right overall dimensions can still fail to engage its hardware. Measuring only the mesh opening misses the surrounding frame.

What to gather without buying a kit

Use a metal tape measure, paper or a phone note, and clear photographs of the corners, track and retaining hardware. Label every dimension and unit. A caliper can help with frame thickness if you already know how to use one, but do not guess from a photograph or nominal catalogue size. Obtain the specific window or screen supplier instructions before dismantling anything.

A practical measurement sequence

  1. Identify the window and screen system. Photograph its installed position and how it is retained. Check rental permission before making holes or permanent changes.
  2. If the screen is designed to be removed and access is safe, follow its removal instructions. Do not force a stuck frame, pry against glass or assume all screens flex out.
  3. Lay a removable frame on a clean, flat surface. Check for bowing, separated corners and distortion. Tell the supplier about damage before treating its measurements as a template.
  4. Measure outside frame width and height, not just the mesh. Repeat at more than one point; label width, height and units.
  5. Record frame thickness, profile, corner style, hardware type and hardware positions. Include the side from which it installs and removes.
  6. If there is no usable original, identify the actual receiving track or mounting surface. Record widths at top, middle and bottom, and heights at left, centre and right. Send the raw readings and photos to the supplier.
  7. Ask for written confirmation of the finished frame size, required clearance and compatible hardware before ordering or cutting. Keep raw opening measurements separate from the supplier-approved order size.

Why there is no universal deduction

Roll Away publishes different rules for inside and surface mounting. Andersen has a separate measuring guide for particular 400 Series windows. Those are system instructions, not interchangeable rules. Window Hardware Direct also cautions that a standard allowance may not suit every track or spring-loaded fitting. Follow the instructions for the product you are actually buying.

A screen brief you can send to a supplier

Keep raw measurements separate from the final order
CheckRecord before deciding
Window identityBrand/model if known; room and opening style
Existing frameOutside width/height, units, damage and photos
Fit detailsThickness/profile, corners and installation side
RetentionClips, pins, tabs and their positions
No originalRaw track or opening readings at several points
Order decisionSupplier-approved finished size and clearance

Choose the repair that matches the problem

If only the mesh is damaged, ask whether the existing frame and spline system can be reused. If the frame is distorted or corners are failing, replacing mesh alone will not correct the fit. If the hardware is missing, identify a compatible part before improvising adhesive or screws. Mesh choice and appearance can be discussed after fit is settled; a stronger-looking mesh does not turn an insect screen into a safety barrier.

For ordinary dirt, use our safe glass-cleaning routine. If water appears on the glass, our kitchen ventilation check deals with a different problem. Replacing an insect screen does not diagnose moisture or a damaged window.

Common questions

Can I measure just the visible mesh?

No. A framed replacement needs the outside frame dimensions and fit details. The mesh area alone does not specify the frame.

Should I always subtract the same amount?

No. Mounting style, track and hardware affect clearance. Keep your raw readings and have the supplier confirm the finished size for that system.

Can pet-resistant mesh prevent a fall?

Do not rely on an insect screen as fall protection, whatever its mesh label. Use appropriate window-safety measures and supervision.

Sources and limits

Reviewed 8 October 2026. General planning information, not a cutting specification or on-site safety assessment. Manufacturer instructions and safe access take priority.



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