What is the correct spacing for 1x3 slats on a DIY platform I’m building. The mattress is a Tuft and Needle king size mattress. Thanks!
Any foam mattress with a polyfoam support core will generally do best with a firm, flat, and evenly supportive support surface underneath it that has minimal to no flex under the mattress and for larger sizes with at least one center support beam that has good support to the floor to prevent any sagging in the middle of the mattress. The components (either a bedframe and foundation or a platform bed) need to be strong and durable enough to support the weight of the mattress and the people sleeping on it without some of the parts bending, sagging, shifting, or breaking with extended use. The support surface under the mattress (which may be a solid surface, slats or a steel or wire grid) should have enough surface area to prevent the mattress from sagging through any gaps or spaces in the support surface over time but still allow some airflow under the mattress.
If a foundation has a slatted surface, then I would suggest that the gaps between any slats are no more than about 5" (with 1 x 3 slats) although I would strongly suggest 4" or less which would be better yet. If you ever planning on going to a latex mattress, then I would suggest 3" or less for the slat spacing. And remember to provide proper center reinforcement so the slats don’t sag.