The backer board should overlap the tub flange so the tile has a stable backing to attach to. This requires shimming the backer board out from the studs so the wall is plumb. There should be a 1/4" gap between the tub ledge and the backer board.
The flange is supposed to be behind the cement board so you are not taping to the flange itself only to adjacent boards. If your cement board was kept high and there is a gap below the board and the flange that's not the best thing. The cement board should be almost tight to the the top of the tub (1/8 to 3/16 above).
Leave a 1/8-in. space between the tub and the tile to allow the installation of a flexible bead of caulk. This allows you to wrap tile around tub corners, as shown in the photo below. Don't start the first row of tile by resting it against the tub or shower.
Fill the gap with silicone caulk. First, make sure the surfaces are clean and dry. Next, fill the tub with water. Then caulk the gap. Leave the water in the tub for the recommended curing time, then drain the tub.
ANSWER - The transition joint between the top of the bathtub flange and the wall tile is not supposed to be filled with a cementitious grout. It should be filled with a resilient ASTM C920 sealant that is either a 100% silicone sealant or a polyurethane sealant.
In general, sealant is the prefered sealing solution for bathtubs and showers, as it is more water resistant than traditional caulk. Silicone sealant forms a watertight seal that is resistant to heavy temperature changes.
BEST SEALANTS FOR CAULKING BATHTUBS
Loctite Polyseamseal Tub & Tile Adhesive Caulk has been specifically formulated to create a long-lasting, flexible seal that is resistant to mold and mildew. It works on both fiberglass and ceramic surfaces as a two-in-one product, making it highly versatile.
How Close to a Wall Can a Freestanding Bathtub Be? It is typically recommended to place your freestanding bathtub at least 5 inches from the wall when you are using a freestanding faucet between the tub and the wall. However, if your faucet is coming out of the wall, the tub may need to be closer to the wall.
If the approach is parallel to the bathtub, a 30 inches (760 mm) minimum width by 60 inches (1525 mm) minimum length clear space is required alongside the bathtub. If the approach is perpendicular to the bathtub, a 48 inches (1220 mm) minimum width by 60 inches (1525 mm) minimum length clear space is required.
Leave a 1/4-inch gap between the backer board and the wall. Leave a 1/8-inch gap between each backer board sheet. The corners and edges of the sheets should not touch one another.
Use radius trim on the perimeters where you have installed backerboard over drywall.
But there's one massive exception: it's no match for especially moisture-rich environments. That makes finishing drywall around a shower flange or tub enclosure tricky, because using drywall as a backer for tile in these areas is likely going to lead to ruining both.
There should be a 1/4" gap between the tub ledge and the backer board. Technically, after the tile is installed over the backer board that gap between it and the tub ledge should be filled with an ASTM C920 sealant (caulking e.g. silicone).
Can I Tile Directly Onto Cement Board? Yes, cement board is the perfect base for tiling, as it's waterproof and provides a smooth, even surface that won't be disturbed by moisture.
For most interior applications, Durock® Brand Cement Board Panels are not affected by water and do not need to be waterproofed. Durock® is water durable but not waterproof. If you're looking for added protection to water sensitive material behind the Durock®, you may consider adding a waterproofing membrane.
For gaps even smaller than 1/8”, regular caulk will do the job just fine, but if your gap is larger than 1/8“, you may want to consider using sanded caulk. You can start by squeezing a bit of caulk into the space. To smooth it out when you are done, simply wet your finger and go by touch.
You want at least 6 inches of extra space from the wall to your tub. This prevents you from being too cramped but also prevents moisture from being trapped in your walls or tub.
Rough-In Dimensions for Baths & Showers
Shower supply line (vertical): 80 inches from floor. Tub supply line (vertical): Faucet valves should be 20-22 inches above floor and 4 inches to left and right of centerline, respectively.
Shower/Bathtub
Clearance: At least 24" of space is needed in front of a shower or tub entry. Leave at least 15" of space between the shower and toilet or other obstacle.
Generally, silicone is most effective for small to medium-sized gaps, typically ranging from ⅛ inches to ½ inches. Attempting to seal large gaps with a single application may result in collapse or shrinkage. When facing wider gaps, epoxy-based sealants or specialized gap-filling products may be a better choice.
Typically, you should use caulk in bathrooms due to its flexible, waterproof, and non-porous composition. Caulk is also easier to remove and replace when necessary, which will minimize damage to your fixtures. Keep in mind that sometimes a project will require both grout and caulking.
Tape the edges of the bath
Before you start sealing, apply some masking tape above and below where you're going to seal, leaving a gap in the middle for the sealant. This will help keep your sealing accurate and prevent any getting onto your tiles.
A silicone-based caulk can be applied to the open gap to provide a seal. This method of repair is advised for gaps that are ½-inch or smaller. Silicone-based sealants are more durable than standard caulk. The flexibility of the silicone allows it to last longer in areas with higher moisture and it tends not to crack.
Tubs will naturally sink and flex from the weight of the water, putting stress on the sealant joint. If you fill the tub before applying the sealant, you can account for this stress and ensure the finished seal will properly expand and contract when you fill and empty the tub in the future.