Replacing floor boards, then sanding and varnishing etc

Question

We've recently bought our first house (yay!) and are looking to have exposed floorboards throughout. However I realise it's a big job and due to work etc I'll probably have to tackle this one room at a time.

I'm going to start in the upstairs spare room (3m x 3m) and I was initially just going to replace the floorboards that needed replacing - i.e. the broken/cracked ones but I've rang 3 local timber merchants to discuss replacement boards and each one has told me that the existing 18mm boards are too thin to be exposed and I should look to getting at least 20mm. One also said that the ones at the moment will be 18mm softwood(?) which is rubbish quality, never designed to be exposed and therefore won't come up that great if I do sand & varnish them?

So basically I have a few questions....

• If I'm going down the route of replacing all floorboards is it worth treating both sides of the boards with something before I actually fix them? They come untreated from the timber merchants and while I realise I won't need to varnish both sides is it worth putting some sort of treatment on the underside too?
• Do you think I'll still need an industrial sander if all the boards are new and therefore not paint-stained or anything else? Or will I get away with just giving them a quick going over with an electric hand sander?
• How do you know how tight to clamp the boards together? It's summer here at the moment so the boards will obviously be at their largest - Because of this I was thinking I should probably do them quite tight because they'll shrink over the winter which then means gaps could start to show. Or am I just overthinking the expansion/shrinking situation (I have no idea how much to expect see!)
• Is it worth/can I put some sort of underlay between the joists? It's a first floor room and so I'm thinking of noise below it might be worth putting something inbetween the floorboards and the ceiling below? Or am I just talking rubbish here?!
• Any other hints / tips before I start would be massively appreciated.

Answer 1

If you're staring with raw planks... PLEASE apply the same finish to all surfaces (underside as well as topside). Otherwise, the surface with the lesser finish will tend to absorb/lose moisture much more quickly than the surface with the heavier finish, and the planks will "cup", "bow", and warp with seasonal humidity changes.

Better than starting with raw planks would be starting with tongue-and-groove planks. The tongue/groove joints helps lock each pair of adjacent planks together so they commonly bear the weight of people & furniture. The T&G joint also helps prevent dirt from dropping down between dried/shrunken planks, preventing the joints from closing again in humid weather & causing buckling of the floor. Last, it helps stop breezes from blowing up through the floor from the space below.

It's possible to install the new flooring as a "floating" (not nailed down) floor over foam insulation (mostly for thermal insulation, but there IS a little sound insulation value)... but that's a whole different type of floor and really begs for a completely different approach. If you try to install any resilient insulation under nailed planks, you'll just get "nail pop", where the nails pop up through the surface of the floor and trip you, possibly tearing bare feet. There are completely different materials available (google "pergo") for floating-floor applications.

If it were my floor... I'd leave all the original floor planks in place regardless of condition, and cover them with all new flooring, diagonal or perpendicular to the original planks' direction.

EDIT: ALWAYS allow new flooring to "season" in its destination room for several weeks prior to installation so it's in humidity equilibrium with the room, preferentially always install during the period of highest humidity and make all joints TIGHT, and ALWAYS leave at least 3/4" (18mm) gap around all edges. You'll probably need to remove any existing mopboards/baseboards & trim before flooring, then cover the gap with them after flooring.

Answer 2

I haven't installed, just refinished, so I can't answer most of the questions.

For new boards, the only reasons I'd sand them before finishing would be to help ensure that the junctions between boards are level or to remove old finish. Fresh from the factory, neither should be an issue. DURING finishing it may be desirable to sand between coats, depending on what kind of finish you're using and how many layers you put down (I'm not sure you can "hot coat" more than every other layer). Of course if the new flooring is prefinished -- or if you apply finish before installing -- you may still not need that sander.

TIP: Factory-finished flooring can use varnishes you wouldn't really want to deal with yourself, which can be longer-lasting before they first need refinishing. That makes prefinished worth considering.

TIP: There are now random-orbital floor sanders, which are a lot easier for a novice to use than the old belt-type sanders; they sand more evenly, so there's much less chance of digging in farther than you intended. If you're going to rent a floor sander, I think these are worth the extra cost.

There are softwood floors out there which look fine (websearch "heart pine floor", for example), so I wouldn't necessarily replace just for that reason. (To get an idea of what wood will look like under a clear finish, sand it clean then give it a quick swipe with mineral spirits. Determining what it would look under an amber finish, or after staining, is more difficult.) However, there's also a point at which floorboards start getting too thin to be worth refinishing.

Websearch for "flooring underlayment" says these are certainly marketed for wood flooring, and finds several articles on the topic. Not having done it, I have no opinion.

If the boards are composite (fancy name for plywood), expansion and shrinking due to humidity isn't an issue. If they're solid wood, they will move along their width (across the grain) but not along their length; if you know the wood, there are tables which can estimate this. Quartersawn wood moves less than plainsawn, but costs more. I'm honestly not sure how one factors that into flooring, though I tend to agree with you that installing during the highest-humidity season should pretty much solve the problem.