Looks like these stairs were always meant to have carpet on them... hence the short cuts with the original carpenter. If you are going to use a quarter round, it may stick out too much and people may trip on the steps, as this shortens the depth of the steps. Ask a carpenter to come out and give you a quote to redo the steps.... and get a quote for more carpet as well.

agree with comments above; these were never meant to be exposed. You might talk to a stair shop about providing new finished hardwood treads that would go back all the way, and could use a very thin 1/4" veneer on risers. You don't want to add on top of treads because it would change the rise in the stairway and put it out of code, since the top step would be shorter than all the rest. Or look hard for a carpet you really like. Good luck!

That is framing lumber---not nearly tough enough for foot traffic on stairs. Even with sanding and painting, the treads will chip/splinter and the paint will wear off.

So, do it right---replace the treads and cover/replace the risers with hardwood treads and 1/4" hardwood plywood riser covers/replace the risers.

That "framing lumber" will finish beautifully and is plenty strong enough for foot traffic.

I’d use a veneer in the right length and glue it to each riser.

I really wish I could afford to have hardwoods installed but I just can't. It's not the materials but the labor costs. I'm being quoted like $170 per stair not including materials and I have 13 steps. I understand it's labor intensive but I just don't have the money. I do not want to spend money on new carpet since I absolutely hate carpet. I would rather save up for real wood. But now I'm stuck with ripped up floors. :-( I just saw videos of people diying with existing stairs and didn't know I'd run into this gap issue. So looking for something I can do myself to fix this in the interim. Perhaps I shall try the risers.

As mentioned above a thin piece glued to each riser to close the gap and the bonus is it will be finished , I would do white for the risers then paint the treads what ever color you want. White malamine would work for the risers just be really picky about measureing and never assume all of them will be exactly the same .You will not even have to paint the risers.

@Loreca to remove sub treads and risers down to stringer and re tread w real 1-1/6" hard treads and say a paint grade wood riser is around $200 per step all day. You also need to speak to your inspectors about run rise and grandfathering a new retread

Welcome to Home Renovation 101 where the first lesson is: It's never as easy as it looks on TV; and the second lesson is: If there's a possible shortcut that the building contractors could take and not get caught, then expect them to have taken it.

Do you have access behind the stairs? If not, are you comfortable cutting out a section of drywall to get access? If you can, my next question is: Are you capable and do you have the patience and tools for accurately cutting thin strips of 2x and 1x pine? If so, then my suggestion is to add width to the treads and risers from underneath and behind with strips, nails and glue. You'll need to be accurate and you'll probably need a few specialized adaptions to existing layout tools to get into the tight spot at the bottom treads, but if the goal is to paint the stairs, then the time might be a good investment and the payoff for body contortions at the bottom and top treads might be worth it to you. There'll likely be a visible seam on the faces and depending upon how fastidious you are, this could be minimized/eliminated with sanding and application of an epoxy and more sanding. If you can cut a shallow rabbet in the tread extension, then the need for accuracy on the riser extension is reduced. Once you get the first one worked out, the remainder should progress routinely. I can't tell from the photo, but you may only need to add width to the tread or the riser. That would be nice if the case and better than having to modify both. If you have a contractor do the work, it might be easier and cheaper to rebuild the staircase, but if you can do it yourself, then the result should be relatively inexpensive, but it's a fair bit of work. If you have concerns about your own accuracy and tools, then rebuilding the staircase might be your best option.

We often reface ugly risers as was mentioned above using this nice, smooth (but inexpensive) birch plywood from Home Depot. The thickness rarely interferes with the dimensions of the treads in a significant way. Another option that looks cool depending on your style is to put decorative tiles on the risers, but it's a lot more work.

https://www.homedepot.com/p/Birch-Plywood-Common-1-4-in-x-2-ft-x-4-ft-Actual-0-195-in-x-23-75-in-x-47-75-in-1503008/202088745

I agree that a paint grade birch 1/4" plywood is the least expensive way to bring the riser down to meet the tread. It can be primed and painted. You can also sand, fill staple holes and paint the existing treads. Minimum tread depth is, I believe, 10". Measure your tread depth now and subtract 1/4" to see if you can remain within code. You'll want to measure the riser height on each end of the tread for each step to be sure you don't end up with gaps again if the stairs aren't level. Smaller gaps can be caulked before being painted.

I just finished stairs with shims and joint compound

ack!