Engineered Oak fixed over old boards *not* at 90 degrees; how bad can it be?

[Safety First]

Greetings!
In short, I have failed to get the full technicolor detail of the horrors of this flooring crime, so am asking what it is that will happen?
I read it everywhere- 'Must overlay new boards at 90 degrees to old'

I found on Google a mention that it can make the new boards 'go wavy'- if you lay in line with old one's, but I do not understand this warning.

If the original boards are (very) fixed, and the new ones also, how does it go wrong, exactly?

Someone mentioned that if you do fix new engineered boards over old boards at the 90 degrees to the old (i.e. parallel to the joists) then you are utterly screwed* if you ever need to get to the sub floor.Just imagining this* scenario got me considering the main question above...

Any advice/info much appreciated. Cheers,
M

 

[big-all]

may work fine may come unstuck
but at 90% all boards are supported on several boards every metre and providing they are all within say 2mm off level over the metre all new planks will be level with the next one across the new boards with no chance off pivoting on the joint and breaking the board apart unlike when parallel to the old boards with a say 2mm step at the board joints

 

[Safety First]

Thank you big-all, for your post. Seems I have made one of those rather readable but non-answerable questions; thanks for taking the time!

May I ask what you mean? I'm not contesting what you are saying by any means. I am trying to work out whether we have the means to mitigate the problems associated with parallel boarding, or whether it is simply a disaster in the making. For example, someone could ask if floor boards really do need to be perp' to the joists? Obviously they really do!

About the 2mm figure; Do you mean, say, if the two old boards, along their join, were 2mm diff in height? As far as I can tell our boards are the same thickness, though the closest to a cause of something like this would be the mild cupping. Maybe we could sand the boards along their joins? My wife did suggest I plane the raised edges of the boards,.. along all the joins- almost 200 lin' m!

About the breaking boards apart... do you mean with the boards parallel the top boards might ( in being pivoted over a pronounced join) actually snap an old (beneath) board along it's length? We have hope (if we do this) that the 8.5 inch new board will no be to prone to this happening when laid over the 6 inch old boards.. And then there's also the matter of screwing the old boards down very tight first?

So, really, really not arguing with the advice! Just trying to see how wingable it all might be?

Cheers, again.
M

may work fine may come unstuck
but at 90% all boards are supported on several boards every metre and providing they are all within say 2mm off level over the metre all new planks will be level with the next one across the new boards with no chance off pivoting on the joint and breaking the board apart unlike when parallel to the old boards with a say 2mm step at the board joints

 

[big-all]

what underlay did you use

ok try a different tack
you have a plouged field with furrows 125mm apart and 10mm deep
your planks are say 161mm wide about every third one will pivot in the center and the joints are unsupported so will act like a seesaw
now you may be fine so if laid leave as is till problems like rocking/clicking/groaning get your attention and come back to us it may never happens

 

[Safety First]

Hi Big all,
Thanks again for taking the time- again.
Underlay- don't get it. We have not done it yet...
We have pine boards that are a little cupped, over joists at varying centres. I can beef up their fixing to the joists. We (SWMBO) wants an engineered oak floor, and this laid along same line as old boards- going with the main light source/largest window.

It is clearer now- thank you- that the issue is to do with the frequency of height differences, if there are any. I.e, at 400 centres you have only a couple -ish of ups/downs per metre that a floor board must cope with when laid onto joists of slightly differing heights, at 90 deg' to the joists. Eng' boards at same direction as old boards will wander up and down 5-7 times per meter, along their tongue and grooved connections.

Yes? If so, then if we must have it as SWMBO wants it, we need to get the floor as flat as poss? Cue the 200m plane-off..!

Tempted to remove old boards in 8x4' areas laying 22mm ply over as I go. This is something I'd like to pick your brains on...

I once removed all the boards in a room to find the joists just fell about the place; a real nightmare- there were no diagonals (herringbone struts?) at all.The boards held the joists in place, etc, etc. To avoid disturbing the delicate arrangements of the joists (on their brick piers with slate packers etc) I can do as above, removing and replacing 8x4 sized areas at a time....

I'm no fan of the t+g chipboard stuff and dont trust flying joints, so will have to add noggins to pick up ply board where it spans between joists.

I think?

 

[dazlight]

Sand down the cupped boards or overlay with 9mm plywood. Then lay the Wood anyway you want