I plan to have the original, but badly damaged, 1 inch thick (26mm approx) floorboards ripped out and replaced in certain rooms of a Victorian property. The new floorboards will be 20mm thick solid oak floorboards. These days it seems impossible to get floorboards thicker than 21mm, and because the original boards are so thick, a simple swap will mean the new floor will be about 6mm lower than the original. This is a problem because it will create unsightly gaps between the bottom of the skirting (and door architraves) and will not align neatly with rooms where the original 1 inch thick boards are remaining.

My thoughts were to lay a 6mm thick sub-floor on top of the joists (in ply say) in order to raise the height ready for the new 20mm boards. However I understand that this is not a suitable thickness for a true sub-floor where the recommended minimum thickness is much greater. However, given that the boards themselves are capable of carrying the load, is it acceptable to lay a thinner sub-floor simply as a means of achieving the desired height?

I would appreciate opinions on this and other suggestions regarding best way to solve this problem?

Are you saying the original 1 inch flooring was directly on the joists?

I'd say yes if you can snap lines and nail into the floor joists like the original floor was done. Plus you'd want to use quartersawn wood for less movement. Thats the way most older homes near mid town here were done 80 years ago.

Yes they are. Which brings me to another question but perhaps for a separate thread - though I'll pose it anyway now: I've been told that new solid oak boards will invariably start to 'cup' if laid directly onto joists because the moisture level is greater on the bottom of the floorboard than on the top where it is exposed to the room. Can any of the solutions to the height problem also solve this one a the same time?

It's pretty simple to protect your floor from moisture. As I mentioned before, thousands of homes out here have had and still have hardwood nailed directly to the floor joists. But then I am in a very arid zone. http://www.fpl.fs.fed.us/documnts/techl ... lation.pdf

BTW, if this is going to be your finished floor theres a neat tool for placing larger lumber tight.

http://www.cepcotool.com/quikjack/

Also why not just go up to a 3/4 ply, thats only a half inch height difference.

I live in the United Kingdom where we can have four seasons in one day and moisture levels vary considerably from summer to winter - so it is something that requires careful consideration. Since posting this question, I've been told it would not be advisable to lay new hardwood boards without a suitable sub-floor and moisture barrier. The recommendation for achieving a suitable sub floor without raising or lowering the floor level would roughly be as follows (bearing in mind the original problem was a drop in height):

1. Take up old boards.
2. Cut strips of ply - equal to the width of the joists - and fix to the top of the joists to achieve desired level.
3. Fix wooden battens to the insides of the joists in order to support sub floor inserts i.e. plywood cut into strips that slot neatly between the joists.
3. With the sub floor now down, sand the whole area to ensure it is level (as the joists might be slightly proud of the sub-floor inserts in some parts).
4. Assuming the fixing of the sub-floor inserts and the sanding has been done correctly, there should now be a level and stable sub-floor.
5. Now lay tar or bitumen-centred 'building paper', to function as a vapour barrier. Whilst nails puncture polythene, the bitumen centre in this material clings to the nails. Apparently, it is now accepted that floors laid on joists that are vented should be laid on building paper.
6. Lay the wood floor - happy days!