Re do it all. Sorry but this is the best way. The stringers look bad too You can buy pre* cut stringers and the rest is math if you have basic woodworking skills you should be able to do it!

Edit spelled pee instead of pre

The stringers are probably bad. Agree. Once you have the bottom riser off you'll see if it's rotted. Wood with direct ground contact is almost always the first to show signs of deterioration. Depending on how the treads and risers were done you may have to pull the first tread also.

The spindles you should be able to cut off with an oscillating tool and replace with standard 1x1 spindle (at least that's what it looks like). Usually you need to drive a structural screw through the top and bottom rails to be sure it's solid. It's pertinent to check the top and bottom rails for signs of weakness and rotting (if so replacing the whole thing may be best).

The handrail unfortunately looks like it got massacred. You can try to bang in more nails and fill it with something but replacement is the better route there as well.

By redo it all do you mean to re do the whole railing+stringers+treads? If so, I am assuming to leave the newel posts as they are if they are not rotting?

Thanks. Dumb question: how to get the bottom riser off? Pry it off, or remove the tread and then pry it off?

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Also, the treads and the deck flooring is of something composite material secured with square slotted screws. Is removing the tread just removing the screws and lifting the tread off? Sorry for this basic question, but I am really new to how deck stairs and railings are done.

You would want to check to see if it's either screwed in or nailed in. If it's nailed in prying it off is the answer, but if it's screwed then you have to remove the screws.

As far as the composite tread, in theory, it's that simple, yes. Just be careful if you do have to pry it, composite material is notoriously soft and marks easily. Also, does the 4x4 post extend through the tread?

Thanks for the explanations.

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Yes, newel extends through the tread. The treads are cut around the newel.

Ya anything that's not rotting you can salvage or reuse some how but the reality is when u demo it alot of stuff is gonna get damaged or you will be spending alot of time tediously removing it.. so sometimes it's better to just re do it. If the stairs are composite board those can be reused. Generally when u see lots of rot around like that chances are other areas are rotted just hidden behind paint.

Hoping your riser and skirt are protecting a separate set of stringers that you can't see but sometimes the skirt itself is the stringer. I can't tell from here

Take a screwdriver and lightly stab a few suspect areas or tap it with the butt of the screwdriver if u dont want to damage it. Good wood makes a nice solid knock whereas rotted wood doesn't make as much of a noise, a spongy noise or any at all

Good points. Thanks. One more Q: Once I remove the composite treads, do I reinstall them using screws at new locations? Then do I need to fill the holes where old screws were? (sorry, new to this stuff).

Nah. It would just be for aesthetics composite will likely outlive you. I personally would though if u can find a matching filler

Thanks. This was very helpful!

I believe the stringers are separate from these, at least that what I gathered when I got under the stairs for a look see. The stringers looked good from my first looks. However, now that you point this out, I will take another closer look tomorrow from under the stairs.

I will try that tomorrow. Assuming I need to replace one or more risers, what kind of lumber do I go buy?

Another Q: what kind of lumber do I need to replace the riser? If it matters, this is near Boston MA.

Just don't buy spf.(spruce pine fir) Cedar at the very least. Treated wood is best but it doesn't take to paint as well as the latter due to the coating and its usually damp. Check your local hardware store there's some special boards that they use for fascia that holds up well and usually comes primed

Just do the wood tapping. If the stringers are treated and sitting on shingles or something it should be ok

The square slotted screw is probably a Robertson drive type. They are popular for decks as they don't strip out as easily.

Thank you for the reply. I know zero about lumber and only now beginning to learn, so this was very helpful.

That is good to know. Should I need to put in new screws, I now know what to get.

Did that. See Photo 5 that I added in my post. The wood parts under the top painted fascia seem good. Looks like I just need to replace the top parts (?)

Link to the photo in the original post: https://preview.redd.it/v9vcztj3zp2a1.jpg?width=1000&format=pjpg&auto=webp&s=7f964b7cf02bed33322d36b7e69901215621a89b

Took some bad parts out. Photo 5 in the post now. Looks like the rot was starting from the bottom where it was touching the soil. Some roots of the plan were creeping behind the front white fascia!

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What you wrote about the wood selection still holds? Looks like I need something that can touch soil. Wonder if there is any thinig I can treat the wood with to make it last longer.

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Edit: typos.

I agree with some of the other commentors - would probably rebuild entirely, maybe save the treads for reuse if they look decent. Cutting stringers and building stair railings aren't the easiest things for a beginning carpenter, you will want to read up on it and watch a few youtube videos at a minimum if you are doing it yourself. Use treated for any framing (the stringers etc), and then for any trim boards and the railings you will want to use something rot resistant that holds paint well such as cedar, or Accoya. If rebuilding the whole thing is a feels a bit out of your wheelhouse, you can certainly replace some of the fully rotted boards and then use a product such as Abatron Epoxy on smallish chunks of rot. Epoxy is fairly easy to use as long as you can follow directions, and it's a very high quality fix for minor rot. The bottom of the balusters and where the toprail is toenailed to the newel could good candidates for epoxy. Whatever you do, disregard any advice to use bondo, spray foam, caulk, etc to fill any large voids (haven't seen any yet...)

looks like once you start you will find that there are way more issues than you can see right now.

I think you will be replacing the whole thing.

Pro tip: Be ever so careful when removing the carriages so as to preserve them to use to lay out the new ones ( unless you want a different step pattern).

Use PT for all of it you might be able to reuse those treads. Might.

Them Pee-cuts'll get ya every time

You definitely can. It's called cut and seal. It's basically copper azole which is what they use on treated lumber.
Creosote is the best but I think it's illegal now as it causes cancer.

That is great to know. Thank you for all this wonderful info.

Just to give you an answer on the miter saw if/when you go that route, a 10" saw should be more than sufficient for minor home repairs like this and small to medium woodworking projects. If the 12" was a viable option budget-wise, I'd say better to use that extra instead on a basic folding miter saw stand, a quality blade or two, that sort of thing.

I mean it's a tradeoff, right? And only you know what kind of work you're aiming to be able to handle in the near future. A 12" can handle wider and thicker stock, and when that's what you're working with (like if you're cutting 2x6's for some framing work or something) then it's great, and the extra weight is an afterthought for the tool that does what you need it to do. But from what you're describing, I'd say a better quality 10", a stand, and a good blade or two should serve you very well for awhile.

Great points. Exactly the kind of insight I was looking for. Thank you.

Thank you for this info. In the new photo I added, Photo 5, it appears that the rot is on the trim boards where they touch the soil. The risers behind the trim boards appear good.

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Basic question: I was looking on HD for boards to replace the trim board with. The only pretreated option I am getting is pine. Am I looking for pretreated Cedar or Accoya?

Cedar is naturally rot resistant-doesn’t require treatment. Accoya is probably not readily available at the big box stores.

If you’re not looking to upgrade to a miter saw quite yet you can probably get away with using a speed square as a guide for your circular saw for all of these repairs and many more.

I agree. I am looking into this and already planning for it. In the process I bought an angle finder as well. Thanks for the idea!