Installing a wooden floor on joists

Andrew Baker asked us the following question (see here for his original comment):

It's a 200-year-old listed mid-terrace cottage. The majority of room still have the original boards, which are in a serviceable condition (which is a great testament to the use of oak in itself). The room I am looking to work in has a few original boards but the majority of it is new ply or pine boards, all covered with carpet.

The existing boards are laid directly on joists. Joists have centres of around 400 to 500mm. All boards nailed to the joists. The underfloor void is about 300 to 400mm high above bare ground. This void is not currently ventilated with airbricks, and while this isn't causing too many problems it is something we are looking to rectify. We are also looking at ways to enable ventilation between the room and the underfloor void.
Being in a listed property, when I replace my flooring it will have to be solid planks (no tongue) and on a suspended floor (ground floor).

So my question is, how does this affect preparation and installation? I'm thinking specifically of underlays (something to protect the wood, not cause condensation and reduce sound transfer, but remain breathable). Also of fixing methods - I would rather not glue (difficult to take up again), and secret nailing will be difficult - what are the alternatives?
Finally, I'm also interested in the choice of wood. I have read 18mm thick wood is not sufficient for a suspended floor?
I hope you may have an opportunity to address these questions, and am fairly sure the answers will interest a broad range of people.
Thanks and regards.
Andrew Baker

Answer (options, tips and issues to be aware off)
Installing directly onto joists above a void (which should indeed have airbricks to insure ventilation especially for removing excess humidity) asks for floorboards that are at least 18mm thick, 20 – 21 mm is preferred. Another issue to consider is the space between the joist: 300 - 350 mm is best otherwise the boards may ‘flex’ too much and could even break/snap.

Normally we would advice solid (or load-bearing Wood-Engineered floorboards) to be installed directly onto the joists by secret nailing in the tongue. If the void underneath is ventilated properly and doesn’t show any signs of moist no extra underlayment is needed.
Ventilation causes air movement = draft, but is important to keep your house climate healthy.
The ‘modern’ construction of the T&G boards allows movement (shrinkage especially) of the boards without feeling the effect of this draft. (Filling gaps between ‘old-fashion’ floorboards is just a very temporary solution, after a while because of seasonal movement the filling material will drop in the void and you can start all over again and again and again.)

Oak floorboards of 21mm thick without T&G can be face-nailed on top of the joists (but note maximum space between joists), as long as every board rests on at least three joists. Because the short sides of the boards don’t have connecting T&G’s for stability between the boards it’s recommended to end every board on a joist.

As mentioned above, non T&G boards can create draft, but we see another problem also: sourcing proper Oak floorboards suitable for installing as floor. These boards need to be dried to floor specifications, which is between 9 – 11% moist. Most kiln and dried timber contains 15% moist.

Can we suggest two other options to consider?

Option 1
Install plywood on top of the joists first, creating a stable and sound subfloor, minimising draft. Using Elastilon’s self-adhesive underlayment you can then install Oak overlay strips (solid wood of 6 or 10mm thick) tightly, which creates a ‘floating’ floor that can be ‘lifted-up’ easier than glueing boards or strips directly onto the plywood.

Option 2
Select ‘standard’ Oak 21mm thick T&G floorboards with square edges (non-bevelled) and with secret nailing install the floor directly onto the joists.
The unbevelled boards will create an ‘old-fashioned’ appearance but the T&G’s will avoid draft and are easier installed.

We hope this answers your questions, we welcome any further comments on this post if you need further advice or have more questions.

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