Double up joists instead of subfloor?

[erol_flow]

Hi, the corner of this room is rotten from the airbricks being cemented in, plus there's no French drain yet, and the floor was covered in underlay and thick glue backed synthetic carpets since it seems at least the 90's.

So I'm going to open all that up but want to put down this engineered oak floor I got for free. It's 18mm thick so needs a subfloor, I've also got the option of some actual oak (not engineered) however it's just 9mm thick...

I'm thinking what if I just reduce the 400mm between the joists by doubling them? That way there would be only a 140mm gap and will be far easier and quicker and frugal than buying a new floor...

Hopefully this is a good idea?

I want to create a little inspection area in this corner so I can see how it's all coming along to, otherwise it'll always bother me.

Many thanks.

 

[Cubist]

Hi Erol,
I'm puzzled, but nothing unusual about that.

You don't mention what lengths of the engineered boards you may be using so I'm not sure why you would want to reduce the width between the joists. Sistering is the technique commonly used to strengthen a floors joist structure and overcome problems caused by rot and decay that may have weakened any joints or connections between the timbers. This technique can also be used, as you propose, to overcome problems arising when the end of a floorboard does not sit near the centre line of a joist - or worse still misses it altogether.

Also; given the current 400mm between joists (circa 18 inch centres which is the crude standard) an 18mm engineered board will not flex sufficiently to be a serious problem to merit/demand the use of a sub-floor. Particularly if the new boards are tongue and groove and laid in a suitable pattern as this too servers to further reduce individual board flex.

On the other hand a 9mm thick floor-board would certainly need a subfloor as even short runs between joists may exhibit an unacceptable degree of flex or bounce. With these I would use either a 10mm marine-ply sheet(s) or backer boards to create the subfloor.

An inspection or access hatch is a wise move. I've installed such in all the first floor rooms here to allow access to electrics and plumbing.

Hope this helps,
Cubist

 

[erol_flow]

Hi Erol,
I'm puzzled, but nothing unusual about that.

You don't mention what lengths of the engineered boards you may be using so I'm not sure why you would want to reduce the width between the joists. Sistering is the technique commonly used to strengthen a floors joist structure and overcome problems caused by rot and decay that may have weakened any joints or connections between the timbers. This technique can also be used, as you propose, to overcome problems arising when the end of a floorboard does not sit near the centre line of a joist - or worse still misses it altogether.

Also; given the current 400mm between joists (circa 18 inch centres which is the crude standard) an 18mm engineered board will not flex sufficiently to be a serious problem to merit/demand the use of a sub-floor. Particularly if the new boards are tongue and groove and laid in a suitable pattern as this too servers to further reduce individual board flex.

On the other hand a 9mm thick floor-board would certainly need a subfloor as even short runs between joists may exhibit an unacceptable degree of flex or bounce. With these I would use either a 10mm marine-ply sheet(s) or backer boards to create the subfloor.

An inspection or access hatch is a wise move. I've installed such in all the first floor rooms here to allow access to electrics and plumbing.

Hope this helps,
Cubist

Ah yes, they are random sizes from over a meter to way under. The guy I got them from said they can't go on joists, Google search confirmed, something about the flex between the joists causing the laminate on the engineered planks to split. And I got it wrong they are 15mm and its like a thin peice of oak with softwood underneath, they are tongue and groove.

Also read on here about how a ply subfloor won't let moisture escape in the same way as floor boards, I'd like to do all I can to prevent further issues with dampness, I will be unblocking the air vents and cutting a drain along the edge of the cottage (otherside to this rotting corner).

Considering the other mitigations implemented, Do you think the difference in breathability between 10mm marine ply subfloor with 9mm oak planks, vs straight floor boards is worth much?

I'd really like to put these straight 9mm thin oak planks down, but was leaning towards the engineered stuff along side the doubling up of joists idea... but if the difference in breathability is not that much then the marine ply sounds great.

 

[a twig]

The main thing to worry about is horizontal airflow beneath the joists via air bricks etc, I wouldn’t worry too much about movement up and down in the gaps between floorboards

Normally you’d have thicker ply than 10mm as the subfloor though, id be worried about flex

 

[Cubist]

Before going any further I think you need to carefully consider how you would go about making sure the final surface level of the floor will be made uniform if you intend using both 15mm and 9mm thick boards. You may be lucky enough to find ply sheets, or similar, of 6mm to balance things up but typically you will find these are 5mm. Nevertheless, some kind of sub-floor would be needed to properly support the 15mm boards and frankly I think this should be somewhat heavier than I first suggested and in the range 18-22 mm thick. Take a look at some P5 TGV4 Tongue and Groove boards, B&Q and others supply them, these may be a much better option for you than plywood sheets.

What you have to concerned about is the 'point load weights' that may be applied by someone walking across the floor. The deflection of the board can be significant and repeated application of such loads can lead to the failure of any fasteners, nails/screws, or even fracture the board completely. I certainly would not trust a 9mm board to bear my weight - try putting one of the 9mm boards over two house-bricks spaced about 400mm apart and step gently and gingerly on its middle.

Whatever material you finally choose for the sub-floor some careful planning of the layout of these and the cuts needed to get a good fit to the room will be essential.

No disrespect intended but how competent are you with power carpentry tools?

 

[erol_flow]

The main thing to worry about is horizontal airflow beneath the joists via air bricks etc, I wouldn’t worry too much about movement up and down in the gaps between floorboards

Normally you’d have thicker ply than 10mm as the subfloor though, id be worried about flex

Oh really thanks, that's great then, there's about 2ft gap between the actual ground and the suspended floor and with all the airbricks opened should be all good then to stick some thicker ply I suppose...

Problem with thick ply though is how much higher my floor becomes. At the moment floor boards are 20mm which is nice, the rooms flow into eachother at the same level.

 

[erol_flow]

Before going any further I think you need to carefully consider how you would go about making sure the final surface level of the floor will be made uniform if you intend using both 15mm and 9mm thick boards. You may be lucky enough to find ply sheets, or similar, of 6mm to balance things up but typically you will find these are 5mm. Nevertheless, some kind of sub-floor would be needed to properly support the 15mm boards and frankly I think this should be somewhat heavier than I first suggested and in the range 18-22 mm thick. Take a look at some P5 TGV4 Tongue and Groove boards, B&Q and others supply them, these may be a much better option for you than plywood sheets.

What you have to concerned about is the 'point load weights' that may be applied by someone walking across the floor. The deflection of the board can be significant and repeated application of such loads can lead to the failure of any fasteners, nails/screws, or even fracture the board completely. I certainly would not trust a 9mm board to bear my weight - try putting one of the 9mm boards over two house-bricks spaced about 400mm apart and step gently and gingerly on its middle.

Whatever material you finally choose for the sub-floor some careful planning of the layout of these and the cuts needed to get a good fit to the room will be essential.

No disrespect intended but how competent are you with power carpentry tools?

No disrespect felt I'm very good with power carpentry tools as I'm a hobby furniture maker. I want to keep the overall floor not much higher than 20mm to keep it level with the hallway and there's a nice victorian tiled hearth which currently sits flush with the 20mm floorboards that are down.

This is why perhaps doubling up my joists at the centre of the current gaps could be my best option...

*edit I won't be using both 9 and 15mm boards, just one or the other.

 

[jamest]

If it were me doing it, I'd cut 18 or 15 mm ply or other suitable board and lay it between the joists (with suitable batons fixed to the joists to sit the ply/board on). That way, you do not raise the floor level and you have a continuous solid subfloor to lay your boards onto

 

[erol_flow]

If it were me doing it, I'd cut 18 or 15 mm ply or other suitable board and lay it between the joists (with suitable batons fixed to the joists to sit the ply/board on). That way, you do not raise the floor level and you have a continuous solid subfloor to lay your boards onto

Thanks for this, makes alot of sense.

 

[Cubist]

It's a lot more work but I do like JamesTs idea of insetting a heavier (thicker) ply or particle board between the extant joists to provide the support needed for the surface boards. It'll be rather like a macrame exercise and I would consider the merits of using an adhesive rather than nails or screws to fix the surface boards and particularly the 9mm ones.

 

[Flyfisher]

Although there is a standard spacing for floor joists, I can't see any reason not to reduce that spacing by inserting additional joists.

Seems to me that the most important thing for this bit of work is not to increase the floor level, so I'd work backwards from that requirement. That makes inserting a suitably strong sub-floor more difficult so why bother if it's easy enough to insert another joist instead. In an extreme case even add two joists. Nothing wrong with a bit of over-engineering

As mentioned above, just ensure there is good airflow beneath the floor.