I hope some of the MUGsters can chime in with their own experiences and insights here. Every bit helps, especially when navigating something as nuanced as mattress support and foundations.
On the topic of box springs, there’s often a lot of confusion due to the terminology still being used in the marketplace. The reality is that true box springs those containing actual springs or a coil grid inside a wooden frame have largely disappeared from the mainstream mattress world since the 1990s. Today, they are rarely found outside of high-end mattresses lie Vi-Spring or among specialty retailers. The shift away from traditional box springs coincided with the rise of thicker mattresses, those taller than 8 inches, which made the cushioning function of a true box spring less essential. At the same time, platform beds became more popular, reducing the need for an additional base layer with spring suspension.
Historically, box springs played a key role when mattresses were thinner and needed extra give to remain comfortable. However, starting in the early 2000s, concerns around motion transfer became more common, particularly among couples, and that spelled the beginning of the end for real box springs. The added bounce of a true box spring tended to amplify motion across the bed, which is exactly what most people wanted to avoid.
Around that time, many manufacturers transitioned to semi-rigid foundations such as the Sigma-Flex, which were fairly common in the early 2000s. These foundations were especially prevalent before the widespread adoption of one-sided mattresses. As the industry moved toward one-sided (no-flip) mattresses, the dominant style of “box spring” became the Semi-Flex. Despite the name, these don’t actually contain springs. Instead, they use a flexible metal grid over a rigid wood frame essentially acting more like a slatted wood foundation.
In our experience, the semi-flex doesn’t provide the same level of support as a well-constructed wood slat foundation with slats spaced less than 3 inches apart. So if you’ve had a “box spring” from a major brand at any point in the last 15 to 20 years, odds are it didn’t contain springs at all, but rather one of these semi-rigid alternatives.
You mentioned: “We recently had a Zinus low-profile wood foundation (Amazon.com) on this same platform bed sitting on top of the slats, and we found once taking it off it actually reduced our back pain, but we still have some.” While it’s hard to know for certain without inspecting the setup, it’s possible that the Zinus foundation was either too supportive or not supportive enough (allowing for sag). Either extreme can affect alignment and contribute to back pain. It’s something worth experimenting with as you troubleshoot.
As we’ve said in other discussions, placing your mattress directly on the floor is a good way to assess the maximum level of support your mattress can receive. If that feels too firm or unyielding, and you’re craving a bit more “give,” then you might consider an alternative support system like the IKEA LONSET slats. These have a bit more flex and can mimic the softer suspension of an old-school box spring. That said, they’re not particularly heavy-duty, and in some cases can make a mattress feel too soft or unstable so it really depends on your personal preference and body needs. We generally don’t recommend the IKEA Luroy slats, as those are similar in flex and spacing to many existing platform beds and may not change much for your situation.
I know this overlaps with other conversations we’ve had, but we like to post publicly about these things because many people reference these threads later when facing similar challenges. It’s helpful to provide detailed information for future readers as much as for the current discussion.
To your point about reinforcement, adding supports running head-to-foot under the slats can make a meaningful difference. If you’re handy, you can build your own using 1x3 framing lumber for the horizontal rails and 2x4s for the vertical legs. Just be sure to measure everything carefully so the support bars sit at the right height.
That said, vertical slats above the existing slats that are not anchored or connected to horizontal supports may not do the job. The real issue lies in the middle section of the bed—between the center rail and the cleats on the side rails. If you simply place a 1x3 board in that section, it won’t prevent the underlying slats from flexing. That’s why we recommend our 1.5" foundation or a similar product. These types of foundations have internal braces running from head to foot, along both the sides and the center. This design avoids placing all the load solely on the existing slats, which often aren’t strong or stable enough on their own.
In fact, we generally suggest placing our 1.5" foundation directly over the slats you already have to add a layer of reinforcement. Even with this setup or any foundation, for that matter it’s still critical to have cross braces and a center support leg that touches the floor. That’s what helps prevent sagging in the middle and ensures long-term structural integrity.
We do sell a 1.5" foundation that’s designed to sit directly on top of your slats. It adds rigidity without adding much height, and can help extend the lifespan of your mattress. There are also products on the market marketed as “bunkie boards,” but a word of caution: not all are created equal. Many that we’ve seen don’t provide meaningful support and, in some cases, can actually make things worse. Some are just double-walled cardboard. Others are sold in interlocking panels that don’t span the full width of the bed, or are advertised as “wood slats” when the slats are spaced too far apart or are too thin and flexible to offer any real structural benefit.
We’re not just trying to sell you ours. Please feel free to send us any links to products you’re considering and we’re happy to take a look and offer an honest opinion on whether they’re suitable.
Another approach is to lay plywood across the slats. For best results, use ¾" MDF that’s smooth and sanded, so it doesn’t tear the fabric of your mattress. For mattresses with a quilted bottom like most innerspring or pocket coil design. This can work well and provides a solid base. But keep in mind that if your room is prone to dampness or lacks ventilation, this can become a breeding ground for mold or mildew. We strongly recommend avoiding solid surfaces if your mattress uses a cotton bottom fabric, or if it’s an all-foam design (like memory foam or latex), both of which need airflow to stay dry and clean over time.
Now, for those dealing with King-size mattresses dipping on the sides, a firm ridge in the middle, or uneven wear, welcome to the club. This is a common issue with virtually all King-size beds. The problem stems from the shape of the mattress. That “dead zone” in the middle of a King—where no one typically sleeps—stays firmer than the rest of the mattress over time.
Here’s a trick from the factory floor that can help. Instead of rotating your King-size mattress 180 degrees like most people do, try rotating it 90 degrees. Yes, that means the mattress will hang over the sides by about 2 inches on both ends but only temporarily. The goal is to redistribute wear by forcing your body into that middle section that otherwise gets ignored. This helps compress the center and even out the surface.
Let it hang for a week or two. Then repeat the process every few months. It may seem a bit unorthodox, but it works. Over time, the mattress will break in more evenly, and you’ll reduce the sense of unevenness that builds up over years of sleeping on the same zones.
Hope this helps someone out there, even if it’s a bit redundant. Thanks again for keeping these discussions going they’re helpful not just for us, but for everyone trying to make sense of all the mattress options out there.
