I'm starting a remodel project later this week. I haven't been by there to look for myself, but the homeowner tells me the subfloor is particle board. I suspect it may be MDF. A common method in this area for building subfloors years ago was to lay 1/2" plywood down on the joists, then felt paper and some type of 3/4" subfloor. Many of them were particle board, others MDF, more recently Advantech is used by itself without the 1/2". My question is, would any of you recommend going over the MDF (if that's what it is) with CBU and tile, or is it in the same category as particle board and needs to be torn out?

It might be OSB, since a lot of people mistakenly refer to OSB as chipboard, particle board, waferboard, etc... I've never heard of anyone using MDF for flooring since the boards would never meet OSB stress-test results & it wouldn't meet code.

OSB has flooring-grades (Advantech being one which is quite popular) which are resistant to moisture and can be used directly on the joists instead of plywood.

You may be right, although this particular homeowner may know what he's talking about. He's an insurance adjustor and does a lot of work around construction and such. Unfortunately, a lot of the homes in the more rural areas haven't been subject to any type of code enforcement until this year, so it was pretty much left up to the builder to put down whatever type of subfloor they wanted. Unfortunately, I think we'll end up having to tear this stuff out for a tile installation.

Im not a big fan of any material other than exterior plywood.On all my jobs if i find anything like particle board i rip it out.I feel that if moisture ever gets to it it will swell and can harm the installation. Most newer homes in my area have OSB and my preference is ripout or mud bed. My last house was 16 oc with 3/4 OSB glued down.I screwed down 5/8 exterior ply, mortared 1/4 cement, and set 16x16 travertine with tec full flex. In 3 Years i still had a couple of minute grout cracks. In my new house i will only do a mud bed over OSB.

Just Some Guy, you were correct. I checked it out yesterday and it's OSB. I don't know the thickness of it, or if there's any other type of subfloor underneath, but at least it's not particle board.

phew, that's a close one.. typically OSB flooring is 23/32" or 7/8"

I really like plywood, and for certain applications it is the winner hands down, but for uniformity OSB can't be beat.

Did you know that plywood is assembled by hand (the alternating sheets of veneer are typically stacked by hand & the knots you see are patched by hand as well? The grading is often visual, so there can be variance from one mill/grader to the next.
OSB on the other hand is a fully automated process & there are sophisticated density/thickness scanners at the press outfeed which ensure uniformity, and if you buy a #1 grade panel from one mill, it will meet the same criteria as #1 from another mill.

You learn something new every day. I just finished a job where the entire particle board subfloor had to be removed, and what a pile of garbage that stuff is. I can't imagine using it for anything. If fell apart as we removed it, and where it got wet, it swelled 2-3" thick. We replaced it with T&G plywood, and even the homeowner, who doesn't know jack about construction, remarked that the floor seemed so much more sturdy. The cost was significantly higher, but it did make a good subfloor.

Particle board has some uses, but structure isn't one of them lol.. cabinetry & shelving are the areas it should be used. It simply isn't rated to do the same jobs as plywood or OSB.

On another note, OSB is not the same product as waferboard, even though they are made with the exact same material. Waferboard flakes are randomly placed on the mat prior to pressing, while with OSB, the flakes are 'oriented' (thus the name) in alternating levels to increase the strength. Waferboard was created in the 1970s, and was touted as the greatest thing since sliced bread but it had mediochre results in stress tests. (this is the time period which formed people's poor opinion of the product) OSB followed, and test results proved that it was a viable building material in areas previously limited to plywood only.

How can you tell the difference between the two just by looking at them? Is there some characteristic you look for?

The surefire way is to check the grading stamp which will indicate the designation of the panel, but you may be able to tell by the appearance because the waferboard is actually rated as 'random waferboard'... so if you see no pattern to either face this is a possibility.

It might be better to check & familiarize yourself with the difference the next time you're in a lumberyard, because different manufacturers have different strand sizes as well.