Which underlay for sheet vinyl going over tiles?

The green fibre boards is cheapest way and you will not have problems with grout lines

Latex self level

Instead of relying on soft underlay—which will indent over time and allow grout lines to show through—the correct professional method is to apply a self-levelling compound or subfloor prep.
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Soft underlay is not recommended under sheet vinyl because heavy furniture will press into it, causing grout lines to show through over time.
The correct professional method to get a completely smooth floor is:
1. Prime the Tiles: Apply a suitable acrylic or neat primer over the existing tiles to ensure proper adhesion.
2. Apply Self-Levelling Screed: Pour a flexible, self-levelling compound over the tiles (or fill the grout lines with a floor feathering compound like Arditex NA or Mapei Latexplan).
3. Smooth & Sand: Lightly sand any high spots once dry to ensure a flat, seamless finish.
4. Lay Vinyl: Lay your cushioned sheet vinyl directly onto the smooth, prepared surface.
If you prefer a dry solution without liquid compounds, install 3mm–6mm plywood sheets over the tiles before fitting the sheet vinyl.

Xtra floor flex pro

The best way is to put a coat of latex over the tiles

Latex screed

Thin ply wood would be better than. Underlay

Underlay wouldn't be suitable for vinyl flooring
The area needs self leveling which should be done couple of days before vinyl installation

Latex screed, I would recommend.

Can't put underlay under sheet vinyl you would have to feather finish the grout lines

Self leveling compound

Self levelling

1. Vinyl Underlay: Use a foam or cushioned vinyl underlay that is designed specifically for vinyl flooring. These types help smooth out imperfections like grout lines and provide a stable base.

2. Plywood Underlay: If your tiles are uneven, consider using a plywood underlay (6mm thickness). It will provide a solid, smooth surface for the vinyl to adhere to and prevent grout lines from showing. Ensure the plywood is well-secured and level.

3. Self-levelling Compound: For a smoother surface, you could also apply a self-levelling compound over the tile to fill in grout lines and create a flat base for the vinyl.

Make sure the underlay is compatible with your vinyl and the existing tile surface. Once the underlay is in place, you can lay your sheet vinyl for a seamless finish.

Thick vinyl underlay

To lay sheet vinyl over tile and prevent grout lines from showing through, consider these underlay options:

1. Plywood Underlayment (1/4-inch): Use 1/4-inch plywood to create a smooth, even base. Secure it over the tile and fill any seams and holes.
2. Vinyl-Specific Underlayment: Foam or cork underlayments designed for vinyl can help absorb imperfections and provide a flat surface.
3. Self-Leveling Compound: If the grout lines are deep, apply a self-leveling compound to fill in the gaps and create a flat base for the vinyl.

Plywood or self-leveling compound are the best choices for a smooth installation. Be sure to clean the tiles thoroughly before applying the underlay.

To install sheet vinyl over tiles without the grouting showing through, you'll want to choose the right underlay. Here are a few options:

1. Cushioned Vinyl Underlayment: This is specifically designed for vinyl flooring and can help absorb imperfections from the tile underneath. Look for a thicker option (around 1/4 inch) for better coverage of grout lines.

2. Foam Underlayment: A dense foam underlayment can provide cushioning and sound absorption, helping to smooth out the surface beneath the vinyl. Ensure it’s suitable for use under sheet vinyl.

3. Fiberglass Underlayment: This is another good choice that offers durability and can effectively smooth out the tile surface. It’s less compressible than foam, so it provides a stable base.

Before installation, make sure the tiles are clean and any loose tiles are secured. Additionally, you may want to consider using a self-leveling compound to fill in any significant grout lines or uneven areas for a smoother finish.

Plywood

For laying sheet vinyl over tile, a good option is a foam underlayment specifically designed for vinyl flooring. Look for one that provides cushioning and helps minimize imperfections. Here are a couple of suitable choices:

Cushion Underlayment: This type provides a soft surface that helps hide the grout lines while also offering sound absorption and insulation.
Vinyl-Specific Underlayment: Some products are specifically designed for vinyl flooring, providing a smooth surface that reduces the visibility of grout lines and offers moisture resistance.
Make sure to choose an underlayment with a thickness that balances cushioning and stability, typically around 1/8 inch to 1/4 inch. Also, check that it's compatible with the vinyl sheet you plan to use.

The best underlay to apply under sheet vinyl over tiles is plywood or hardboard underlay. This will provide a smooth, durable surface, preventing the tile grouting from showing through the vinyl.

Don’t use underlay! These tiles need screeding first, use a suitable primer first.

For most cases plywood or cement backer boards are used