Difference between "thinset" and "dryset" mortar...?

Discussion in 'Remodel Forum & Blog' started by Fistor, Jul 30, 2008.

  1. Fistor

    Fistor Geotechnical Engineer

    Messages:
    48
    Location:
    Vancouver, BC
    Can someone explain the difference between thinset and dryset mortar?

    Some stores seem to carry one or the other, but not both - I'm wondering if perhaps these are the same thing, but under different brands...?
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,003
    Location:
    New England
    Not quite...

    There are many types of thinset; primarily modified and unmodified. An unmodified is also called a dryset thinset (mortar).

    A thinset is typically spec'ed for a maximum of 1/4" thick application layer. If you need more than that in one layer, you'd use a medium bed mortar (often called granite or marble motar). Depending on the formulation, these can ususually be installed up to around 1/2" thick, and have more sand in them to support big tiles (not that thinset can't be used for big tiles - depends on how flat the floor and tile are and other considerations).

    With a modified thinset, it needs to both cure (chemical reaction with the cement and water) AND dry out. One of the primary things in a modified thinset is a laytex or similar compound. This needs to dry out.

    Mixing a thinset must be done to the manufacturer's specs - in essence, on a modified thinset you need to first wet all of the components, let it slake (rest), then mix again to then cover all of the mortar cement bits with laytex that was hydrated in the first action. If you don't do that, it won't have the proper consistency nor will it have the stated strength. This will have a HUGE difference when it comes time to use it.

    For help with tiling, suggest www.johnbridge.com.
  3. Fistor

    Fistor Geotechnical Engineer

    Messages:
    48
    Location:
    Vancouver, BC
    So if I understand correctly...

    dryset = unmodified thinset = subtype of thinset

    In other words, dryset is a type of thinset that requires (more) additives than just water?

    Reading instructions for both thinset and dryset (and looking at all the products on the shelves), it gets confusing, but it seems that both recommend the use of additives.

    Very confusing! :confused:
  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,003
    Location:
    New England
    A good dryset mortar can be made into a modified mortar if you mix it with an additive rather than water. You can do it either way. The additive makes it stickier and stronger that may not be necessary or even desirable...just depends on what you are trying to accomplish. For example, all CBU (cement backer unit or cement board) call for them to be bedded in thinset. The thinset actually is there to fill in voids to give 100% support - the screws or nails are what hold it in place; so, there's no need or even desire for a modified thinset here.

    Where the thinset can't dry ( installed on a waterproof membrane and a large, impervious porcelain tile), you'd probably not want to use a modified thinset. If you sandwich a modified thinset between two waterproof layers, it can take literally months for it to dry out and achieve max strength. On the other hand, a dryset mortar cures anaerobically - needs no air, so it would achieve design strength regardless of the conditions.

    Regardless, you get what you pay for within the same type - a cheap dryset will have a bigger sand to cement ratio, and may only meet the minimum specs for strength. A premium dryset can have more strength than a cheap modified thinset.

    Some people prefer to buy a good dryset, use it with water when needed, and essentially have a modified thinset if they use the addative...it lets them only have to buy one type of dry stuff.
  5. Fistor

    Fistor Geotechnical Engineer

    Messages:
    48
    Location:
    Vancouver, BC
    OK, thanks for the detailed explanation, that clears up a lot.

    My applications at present are for CBU for a tub enclosure, and for CBU for the bathroom floor, over the subfloor and beneath the tile.

    Thanks again,

    Carl
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