Advice on using cement board instead of wood under countertops

[Tomhanks]

Hello everyone!

Is it a good idea to replace the wooden base of countertops with cement board if the wood is rotting due to water dripping from the edges? Thanks in advance for any advice!

[Downaski]

Hi there! Replacing the rotted wood with cement board can be a decent idea depending on how you're using the countertop. Cement board handles moisture much better than regular wood, so it makes sense if water exposure is a frequent problem.

Just keep in mind that cement board isn't structural. If the wood you're replacing is actually supporting the countertop, you'll still need something solid underneath like treated plywood or another moisture-resistant material. You can then put cement board on top if you're planning to tile over it.

Also, it might be worth checking why water is dripping from the edges in the first place. Sealing the edges better or adding a drip guard could help prevent future damage.

Hope this helps!

[Tomhanks]

Thanks! If cement board isn’t structural, would using marine-grade plywood underneath be a good choice for support?

[Downaski]

Absolutely, marine-grade plywood would be a great choice for that. It’s designed to withstand moisture much better than regular plywood, so it offers solid support while helping to prevent future rot. A common approach is to use the marine plywood as the base layer for strength, then add cement board on top if you're going to tile or want extra moisture resistance. Just make sure everything is sealed properly, especially around the edges and any cutouts for sinks or fixtures, since those spots are most vulnerable to water damage. That combination of marine plywood and cement board gives you both durability and protection.

[Tomhanks]

Thanks, that makes sense! Do you think using a waterproof membrane between the plywood and cement board is worth the extra step, or is it usually okay to just rely on sealing the edges well?

[Downaski]

You're welcome! And yes, using a waterproof membrane between the marine-grade plywood and cement board can definitely be worth the extra step, especially if water exposure is a recurring issue. It just adds another layer of protection and helps prevent moisture from seeping into the plywood over time.

That said, if you're confident the area doesn't get too wet and you’ve sealed all the edges and cutouts really well (like around the sink), you might be okay without it. But honestly, I’d lean toward adding the membrane just for peace of mind, especially in kitchens or bathrooms where water has a way of sneaking in.

Better to overdo it a bit now than deal with rot again later!