Curious if anyone’s tried adding an extra insulation layer underneath to help with both sound and thermal movement?
Extra insulation does help with noise, especially with lighter synthetics. I’ve seen folks use a high-density foam board underlayment—cuts down rain noise a lot. As for brittleness, it can help a bit, but if the material’s prone to curling from temp swings, insulation only goes so far. Fastener placement and making sure the deck’s flat are still huge factors.
I’ve managed a few buildings with synthetic slate, and I’ll say the lighter weight is a big plus for older structures—less stress on the framing, especially if you’re retrofitting over an existing deck. As for insulation underneath, I’ve seen mixed results. High-density foam board does help with noise, but it’s not a cure-all. In one of our properties, we tried adding a 1/2” foam layer under the synthetic tiles. It definitely softened the sound of rain and hail, but didn’t do much for thermal movement. The tiles still expanded and contracted with temperature swings, and we had some minor curling at the edges after a couple of seasons.
Honestly, I think the bigger issue is making sure your substrate is dead flat and your fasteners are spaced right. We had one section where the deck was just a little uneven—barely noticeable to the eye—and that’s where most of our problems showed up. Insulation can mask some noise, but it won’t fix movement if the base isn’t solid.
One thing I’d watch out for: adding too much insulation can sometimes mess with how the tiles sit or even void warranties, depending on the manufacturer. Always worth double-checking that before you go layering stuff on.
If you’re in a spot with big temp swings (we get -10°F to 100°F here), synthetic slate will always move a bit more than real stone. That’s just the nature of the material. But if you’re mainly after noise reduction, a thin foam underlayment is probably your best bet. Just don’t expect miracles on the thermal side.
Curious if anyone’s tried those newer “acoustic” underlayments? I’ve only seen them in catalogs so far...
I get what you’re saying about the foam not really helping with thermal movement, but I actually had a different experience with noise. We put down a recycled rubber underlayment (not foam) under our synthetic slate last year, and it made a bigger difference than I expected—especially during hailstorms. It was a bit pricier, but honestly, I’d do it again just for the peace and quiet. Didn’t notice any issues with curling or movement, but maybe our climate’s milder (we rarely get below 20°F). I do agree about the flatness of the deck though—learned that lesson the hard way on an old shed roof.
That’s interesting about the rubber underlayment—honestly, I hadn’t thought much about noise until I started working on a couple of reroofs last fall. One customer mentioned how loud rain was on their synthetic slate before adding any underlayment, so your experience lines up. The price tag on the recycled rubber stuff made me hesitate, but if it really cuts down hail noise, that’s probably worth it for some folks.
I totally get what you mean about deck flatness too. First time I tried to lay synthetic slate over an old, uneven shed roof, it looked fine from the ground but up close... not so much. Ended up having to pull a bunch off and shim the deck. Lesson learned: spend the extra time up front checking for dips and humps.
As for curling or movement, maybe you’re right about climate playing a part. Where I am, we get some wild freeze-thaw cycles and I’ve seen a few edges lift over time, especially where the deck wasn’t perfect. But if you’re not dropping below 20°F much, that might save you some headaches.
Thanks for sharing your take—it’s cool seeing how different materials and climates change the whole game.
I’m kind of obsessing over noise now, honestly—didn’t even realize rain could be that loud on synthetic slate until I started researching. If you had to do it again, would you go with the pricier underlayment right away, or try something else to deal with the sound? Also, for anyone who’s dealt with deck shimming, how do you actually spot those dips before you’re halfway through the install? I’ve only done minor repairs so far, but I’d rather not find out the hard way next time.