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Thread: Mastic, is it needed?

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    Mastic, is it needed?

    Our decking will be poured soon (framing is going in). A discussion came up about the mastic that'll go around the coping (only ~1/3 of the pool/spa perimeter)... and if it is needed. Our cement guy says he pours to the coping and it looks cleaner... everyone else has just sort of gone along with mastic but not 'explained' WHY you need it... so, is it needed? What does it do for you? We live in Southern CA and rarely ever (a couple nights a year) get frost on the grass... rain is... well, it can rain, but it's sporadic... so, what does the mastic do for us vs. just pouring the cement to the coping (quartzite flagstone).

    Thanks! (if this belongs in another forum, lemme know)
    Pool-15x31', Spa-7x9', depth 3.5-6', HP Pentair Whisper Flow pump/motor, Intelliflo 8-160, 4-to-8 speed, Pentair 320 Clean and Clear capacity filter (upgraded to 400,000BTU stackless heater). Stacked stone- Autumn Rose Classic, tile- Ridgeline Blue Slate, coping- Turning Leaf Quartzite. Start date 7/12/16! Progress thread

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    Re: Mastic, is it needed?

    By mastic, do you mean Caulk?
    Dave S.
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    Re: Mastic, is it needed?

    I guess it is a caulking, it's rubbery (I believe). Everyone we've spoken to here calls it mastic. It's this stuff here-

    http://www.ultimatepoolseal.com/wp-content/uploads/2011/06/Frisco02.jpg

    https://i.ytimg.com/vi/X6C4n6DS_eQ/maxresdefault.jpg
    Pool-15x31', Spa-7x9', depth 3.5-6', HP Pentair Whisper Flow pump/motor, Intelliflo 8-160, 4-to-8 speed, Pentair 320 Clean and Clear capacity filter (upgraded to 400,000BTU stackless heater). Stacked stone- Autumn Rose Classic, tile- Ridgeline Blue Slate, coping- Turning Leaf Quartzite. Start date 7/12/16! Progress thread

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    Re: Mastic, is it needed?

    Also, when I say a couple nights of frost, I mean just dew on the grass has crystallized and is gone as soon as the sun rises... we don't freeze here...
    Pool-15x31', Spa-7x9', depth 3.5-6', HP Pentair Whisper Flow pump/motor, Intelliflo 8-160, 4-to-8 speed, Pentair 320 Clean and Clear capacity filter (upgraded to 400,000BTU stackless heater). Stacked stone- Autumn Rose Classic, tile- Ridgeline Blue Slate, coping- Turning Leaf Quartzite. Start date 7/12/16! Progress thread

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    Re: Mastic, is it needed?

    Concrete expands and contracts some, whether it freezes or not. The "mastic" acts like an expansion joint.

    Maybe not needed in your area, but I've never seen a pool without it. If me, I'd ask your concrete guy to show you some pools where he poured to the coping that are a couple of years old and then decide if it is for you.

    Jim R.
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    Re: Mastic, is it needed?

    It is likely that your pool deck and and pool will move at different rates. An expansion joint is always a good idea between two hard surfaces that move at different rates. Sikkaflex self leveling polyurethane caulk/sealer is what goes in the joint.
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    Re: Mastic, is it needed?

    You need polyurethane caulk. The pool wall and coping move in a different direction than the decking so you need the flexible joint to keep water out and not crack.

    I often think PB's leave that gap too big (I would like 1/4") and I have not had good luck personally with self-leveling but those are choices you can make.

    Go with the caulk.....you will not regret it.
    Dave S.
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    Re: Mastic, is it needed?

    Poured coping?? Aka cantilever deck?
    Am i missing something?

    To duraleigh : in Connecticut I have seen 1/4" joints compress to barely visible. A half inch is preferred in New England. Where are you swimming?
    Ray



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    Re: Mastic, is it needed?

    Where are you swimming?
    middle of Florida. I have never seen those joints move that much but I have never been in that cold climate either. I could certainly see that much deflection that cold. This OP is in non-freezing weather, however, so I think 1/4 will work.
    Dave S.
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    Re: Mastic, is it needed?

    One thing that got me going on this question is that, we were/are planning on mastic on ~20ft of coping where the patio butts up to it... and then nowhere else on the pool (per the people who say we put in mastic)... if it's so important, why wouldn't it be everywhere the cement meets the pool structure (butts up to stacked stone on the side of the pool/spa everywhere other than the ~20ft of coping).

    Moisture wise, keep in mind, the Florida state average rainfall is 59.2'', 58.5'' for Connecticut... here it's 15'' (haven't gotten that in quite a while to be honest). We do not put our hand into the ground here and encounter moisture, it's near bone dry. I just fail to see how moisture would be an issue.

    Temperature wise, the average January temp in Connecticut is 17F, here it's 43F... it's entirely different worlds. Once the cement is cured, will it expand more than a standard joint in the cement?

    I'm thinking things are quite different with this when it comes to the regionality of the U.S... I guess the real question is, is there anyone from SoCal on here or the Southwest that has some experience with mastic vs. none?

    I will speak with my cement guy about pools from 5+ years ago and how they're doing. I have some friends with a pool, I'll ask if they have mastic and how it is doing (it's a pretty darn old pool). Lastly, I'll call the city inspector tomorrow to ask #1) if there's a code requiring it and if he has the time #2) what his thoughts are on it.
    Pool-15x31', Spa-7x9', depth 3.5-6', HP Pentair Whisper Flow pump/motor, Intelliflo 8-160, 4-to-8 speed, Pentair 320 Clean and Clear capacity filter (upgraded to 400,000BTU stackless heater). Stacked stone- Autumn Rose Classic, tile- Ridgeline Blue Slate, coping- Turning Leaf Quartzite. Start date 7/12/16! Progress thread

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    Re: Mastic, is it needed?

    It sounds like you are convinced you don't need it.....you may not. That said, I think it will be hard to redo that area if cracking occurs.
    Dave S.
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    Mod Squad pooldv's Avatar
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    Re: Mastic, is it needed?

    Yes, I would put an expansion joint everywhere two different surfaces meet. Pool and deck, deck and spa wall, deck and patio, deck and house, etc. It is less about moisture and more about allowing movement. Although, keeping water out is also a good idea. When the deck moves and the pool doesn't it will move the coping which will crack the grout lines, possibly knock coping loose from the mortar bed, cause the grout line between the water line tile and the coping to crack and possibly knock off waterline tiles.
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    Re: Mastic, is it needed?

    I'm in socal and our builder used mastic. He also poured sand while still wet and gave it texture so it blended better.


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    Re: Mastic, is it needed?

    Out here in CA, there is more worry of deck movement from water getting in-between and undermining the deck base, than freezing or anything else. We, and this manufacturer likes to see a joint that is as wide as it is deep. So, that calls for, in some cases, a base of either sand, or foam inserted in to he joint prior to the pour.

    Look specifically at the poly-sulfide based joint sealant "deck-o-seal". It is a 2 part kit that you mix on site and must use all or toss the left over.

    They have recently come out with a mastic already mixed in a bag HS-1 SL. This stuff is great, it is ready to go, you can pour it right out of the bag if you want, or into an applicator bottle. And what you don't use, you can seal the bag back up, and it will stay in liquid form for the next job.

    "Sanding the joint" as its called looks nice, but i believe that it interferes with the flexibility of the mastic by making the joint "harder" or less flexible than it was designed to be. The whole idea of this joint is for it to be flexible so it can handle movement. The joint will harden through the years without the sand, and that will be when it begins to fail/separate/rip due to its age.
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    Re: Mastic, is it needed?

    Out here in CA, there is more worry of deck movement from water getting in-between and undermining the deck base, than freezing or anything else.
    My question then is that if that is a concern, why is the mastic not used everywhere? Against the house, against the wall, around the pool where the cement meets the stacked stone (check my build for what I mean). I understand the concept of water getting under the decking, but if that's the case, why mastic only where there's coping butting up to it? This is what our PB told us (only here and not anywhere else), so that's what got me thinking. Also, if THAT much water is going to get under the decking and cause problems, it seems like it's a problem with the pre-decking work/prep and that water would've gotten in there anyways and caused problems regardless.

    I'm not arguing against mastic etc... but I'm wanting to understand how it does anything (here in SoCal in particular) when it's just used along ~20ft of our pool/decking, and nowhere else. Seems to me that water will find its way under there if it wants to regardless of mastic (inevitable crack in a joint, elsewhere around the pool where there's no mastic, around a drain etc. perhaps?)... blame my scientific and inquiring mind seeking examples of why it's needed.

    As an FYI, I spoke with our city inspector, he said it's not required and he has seen both with and without that have looked fine. Our cement guy has said he has redone some things on projects 7 years old with pools with no mastic and they're fine. In the end, we're not using mastic.
    Pool-15x31', Spa-7x9', depth 3.5-6', HP Pentair Whisper Flow pump/motor, Intelliflo 8-160, 4-to-8 speed, Pentair 320 Clean and Clear capacity filter (upgraded to 400,000BTU stackless heater). Stacked stone- Autumn Rose Classic, tile- Ridgeline Blue Slate, coping- Turning Leaf Quartzite. Start date 7/12/16! Progress thread

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    Re: Mastic, is it needed?

    It probably wouldn't hurt to put it in all those places. I assume it is a cost thing. High end projects i see have it all over the place. You don't want lateral movement up against the pool because that force can push and loosen coping stones and/or knock tiles off. And you wouldn't want vertical movement because that could be a tripping hazard or just plain looks un-sightly, having two different elevations close to the waters edge. I didn't mention before that some soils expand when they get wet, while others expand when they dry out too much. This may cause the decking to heave or shift in either case. The mastic also helps seal the moisture in preventing the dry out. Granted, we are talking about the soil having to really dry out to expand, but after several years of CA drought, we are beginning to see these effects in some areas. I'm also seeing more underground leaks last year and this year than i ever have in my years.

    I have no other explanation for it, other than the drought.
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    Re: Mastic, is it needed?

    I would certainly use an expansion joint and put polyurethane caulk. You can get it in multiple colors. Our soil here in Texas has lots of expansive clay. Things move all over the place. Pool moves. Decking moves. Large changes in seasonal underground moisture levels. But no freezing.

    Our ur deck is cantilevered over the top of the pool. But it is isolated from the pool shell and tile. So far so good. 2 years now and not any problems. But if you have traditional coping you need some room for things to move around.

    By by the way. Put in extra steel in your pool. Make you feel a lot better about things and doesn't cost to much. I didn't know this was an option till too late. Pb never even discussed it. The decking is stamped concrete with a multicolor hardener and release colors. Looks pretty good and holding up well. I have an expansion joint back where the deck meets the house.
    Mark morejp4 College Station, TX 20k inground plaster pool, Hayward Omnilogic and Ecostar VS pumps, AA in floor cleaner, Hayward swimclear 525 cartridge filter, Venturi Skimmers, Spa connected to pool via rock overflow and river. Currently using liquid bleach for clorine at a target of 2-3 PPM, Borates at 50 PPM, and Muratic acid and baking soda for ph control (ph always drifts high due to water features running a lot)

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