A while back I accidentally cracked one of the wooden slats in my box spring. it’s a two piece king sized box spring under a king sized bed but the crack is only about 5 inches from the side of the bed and the wooden slat is mostly still level. Will this affect the mattress in any way?
Sorry to hear about the damage to the slat on your box spring. This is certainly not something to ignore- the slats provide support for your mattress and also protect the internal structure of the box spring itself. How long is the crack? A few inch crack in a king bed that does not produce any sagging with the mattress and the people laying above, should not present any problems as long as the it does not “catch” or damage the mattress You may want to run this one by the mattress manufacturer/retailer (in writing though) as damage to the slats may void any warranty.
From what you say it sounds like a non-critical crack at this point, since you say the slat is only 5” to the side and ‘still level’ but with additional pressure being put on it from both the mattress and sleepers consistently, the crack may get worse. So additionally, I’d keep a close eye on it and perhaps replace the slat or reinforce it if possible.
Many of the Trusted Members of the site are experienced with box spring manufacture and could advise you: such as US Box Spring or Jeff at Mattress To Go are great at helping consumers with all sorts of mattress related issues, they have a dedicated forum page here
As Phoenix would say…"a crack is a crack no matter how small” so keep an eye on it– the support under your bed is an important part of your sleep system.