Subscribe or Renew Membership Subscribe Renew

Caulking Wide Gaps

SWJames's picture

I recently remodeled my bathroom and after I had already installed the ceramic tile onto the backerboard, I realized that the gap between the tile and tub is between 3/8" and 1/2". I was afraid to try putting conventional silicone caulk in such a wide gap because I figured I couldn't get it done neatly enough. So, I put a pre-manufactured tape-like product on and my wife and I don't like the looks of it. I want to re-do the caulking. Does this gap seem too big to seal with silicone caulk dispensed from a caulking gun?

(post #91486, reply #1 of 13)

3/8 to 1/2 inch is a little large but since that is what you have, you must deal with it, unless you are going to tareout and retile the whole tub surround. You can caulk a large joint like that, but do not try to fill it entirely with caulk.

Puchase"backer rod", a styrofoam rope, and place it in the joint first. Buy both sizes, 1/2 and 3/8 inch, and force fit it into the joint to within 1/4 inch of the surface. Apply the caulk over the filled joint and strike smooth. For pure silicone caulks, I  dip my finger in acetone or lacqure thinner and strike the joint. For siloconized latex caulk use water and a sponge applicator.  Keep a lot of clean wipe rags nearby and do not carry an excessive amount on your finger before cleaning it off.

Good luck,


(post #91486, reply #4 of 13)

Amen Brother to backer rod!!  About 99% of the caulk joints I see in this country (USA) don't have backer rod AND THAT'S WHY THEY FAIL.  Manufacturers of most sinks, toilets, tubs etc. don't leave room to tuck backer rod in where it is needed.  For example, base of a tub, base of a toilet.  It's stupid!!  There should be a 1/4" to 3/8" wide notch, with depth a bit more than the width -- this would be perfect for caulking with backer.

The important thing is to have the caulk relatively thick where it is supposed to hang onto the two substrates (tub & tile in this case) and quite thin in-between, so it can deform when the substrates move together or apart.  So if your tile is 1/4" thick, say, then the caulk should thin to 1/8" or less in the middle and then thicken out to 1/4" at the tub.   Backer rod is flexible as hell, and since the rod is round, tooling the caulk naturally forms it into a thin, flexible web in the middle.

Your 3/8" to 1/2" wide gap is good, not excessive.  However, you may have a problem with normal backer rod.  Your gap is probably not deep enough for it.   No problem.  Use adhesive foam tape (like weatherstripping) instead, as shown in attached sketch.  Note that the tape is narrower and thinner than the gap and is centered in it.  I show the tape a bit arched, like a half-circle, although that may be somewhat difficult to make.  Main thing: caulk thick at substrates, thin in middle.

Oh, also get the joint REALLY clean - scrub it out with acetone or brake cleaner -- remove the grease, oils, soap residue, etc.  (These cleaners are nice because they evaporate quickly.)

The words from the old song are a good motto for caulk:  Got to!

Good Luck!

Don't accept in your family, friends, or coworkers that which you would not tolerate in politicians, business leaders, or clergy.


(post #91486, reply #2 of 13)

Have you considered using an edge-tile treatment to cover the gap ? They sell small tiles that function like molding (cove, 1/4 round, ogee, ...).


Phill Giles

The Unionville Woodwright

Unionville, Ontario

<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" /> 

Phill Giles

The Unionville Woodwright

(post #91486, reply #3 of 13)

The two responses above sum it up quite well. I would choose the 1/4" ar 3/8" tile bead/ trim piece route as long as the backer board extends below the existing tile. You will need a substrate for the trim to be adhered to. I also think it will look as though you intended to do it from the start.

You can then caulk between it and the tub as customary. I would recommend using 100% silicone. This requires solvent to clean/ smooth it rather than water, but I find it's worth the effort because I get a better, longer lasting result.

I am still wondering how you got to this point. Wasn't the first course (lowest course) of tile laid first? Did you replace the tub after the tiling was complete? Please tell.

Flay your Suffolk bought-this-morning sole with organic hand-cracked pepper and blasted salt. Thrill each side for four minutes at torchmark haut. Interrogate a lemon. Embarrass any tough roots from the samphire. Then bamboozle till it's al dente with that certain je ne sais quoi.

Arabella Weir as Minty Marchmont - Posh Nosh

(post #91486, reply #8 of 13)

The directions with the backerboard said to leave a 1/4" gap between the backerboard and the tub, which I did. I figured that if I should also leave that much gap between the tub and any point on the tile, which I did. The tile is 8" by 12" and is thicker than smaller tiles. To leave a 1/4" gap between the tub and the edge of tile nearest backerboard meant that the front face, the finished face of the tile, protruded far enough away from the wall that the gap between the front edge and the tub was 3/8" to 1/2". The reason is that the edge nearest the backerboard was right above the rounded-upward lip at the edge of the tub. By moving away from the wall, the gap increased when the lip disappeared.

(post #91486, reply #9 of 13)

"I figured that if I should also leave that much gap between the tub and any point on the tile, which I did. "

There is your error. The backer board should be held shy of the tub. The tiles the should extend BEYOND the edge of the backer board and almost touch the tub. By keeping the back board high, there is LESS (not zero) oppertunity for the water to wick up behind the tiles.

Now, you have a real TLC job. Tile fill-in is no longer an option since there is nothing there for the tile trim to be secured to. The foam backer rod seems to be the only option. However you must be careful of how far IN you tuck the foam rod. You should be trying for a compression fit between the tub/ lip upper edge and the under/ bottom edge of the backerboard. This is assuming the backer board is indeed above and not behind the tub perimeter. Is it? Or is it on a different (further back) plane than the tub edge/ side?

Flay your Suffolk bought-this-morning sole with organic hand-cracked pepper and blasted salt. Thrill each side for four minutes at torchmark haut. Interrogate a lemon. Embarrass any tough roots from the samphire. Then bamboozle till it's al dente with that certain je ne sais quoi.

Arabella Weir as Minty Marchmont - Posh Nosh

1/4 inch tile bead/trim piece (post #91486, reply #11 of 13)

Frankie, a decade ago, in reply to someone asking how to caulk a wide gap, you (and another poster) mentioned small "tile bead/trim pieces" that could be used instead of caulking or grout.  I have a gap of about 1/4" between my new quartz backsplash and ceramic tiles on the wall.  I'm terrified of caulking, especially since the space varies from nothing to 1/4 inch, so this seems like an attractive option.  But I've been unable to find any source for such small pieces.  Can you direct me to a website?  Thanks!


Picture attached.

2013-04-20_09.27.02.jpg2.19 MB

caulk (post #91486, reply #12 of 13)

1/4 is not a wide gap.  Use tape on the tile and tub to limit the mess.  Whatever caulk you use, tool and them remove the tape.

A tile shop would have the (post #91486, reply #13 of 13)

A tile shop would have the "tile bead", but the thinnest you'll find is 1/2 to 3/4".

Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

(post #91486, reply #5 of 13)

Dave and Woody are right on: backer rod.

If you consider the thin tile approach to filling this extra space, just remember not to pack it in too tightly.   An expansion gap of sorts is needed at any location where a tile field contacts a dissimlar material, or a change in plane.  Specifically, always use caulk at the corners and between the tile and the tub.  Caulk is flexible ands allows some expansion to occur, whereas grout will crack and fail over time.



(post #91486, reply #6 of 13)

You should not try to use caulk like a piece of trim. No matter how good a job you do caulking that gap, it will never look quite right.

Look into small pieces of cove or qtr. rnd tiles. You will be much happier with the results.

(post #91486, reply #7 of 13)

That's not caulk...., it's carpenter in a tube!

(post #91486, reply #10 of 13)

When you are all ready to caulk this joint (closed cell filler in place, clean, etc) I suggest you tape the limits of the caulking seam you plan to make with blue tape. 

Next, just do, say one end, maybe 32 inches.  Place the caulk, and quickly strike it smooth, COMPLETELY down to the tape (NO CLIFF AT THE TAPE), and quickly remove the tape.  You may then have a little quick touch up, but this will save you trying to clean the smear-out on the surface, which usually contributes to messing up the seam in the process.  That silicone is really hard to work with if you don't handle it right.  Spend some time honing the end of the tube with sandpaper. 

Actually, I prefer some stuff called Polyseamseal All-Purpose Adhesive Caulk, water clean up, made by OSI.  It can be purchased from and the item code is 80515.  When applying this I use the same procedure except instead of having a roll of paper towels I have a damp sponge to clean my striking finger. 

I have seen silicone rubber (even with fungicide) get a black fungus that digs deep in the silicone.  Also, silicone has a offensive reflective surface that contrasts with the grout IMO.