How to install a subfloor

[akist]

We have an area we want to deck out in the loft space. The joists below are all 250x50. We also want to construct partition walls, made of 4x2.

There are many options and I would like your help to discuss ideas. Here's how I understand it so far and sorry for this long message.

The engineer has asked me to use 18mm plywood screwed into the joists below. Never to glue the boards as I will not be able to remove boards for repairs in the future.

Modern builders use T&G chipboard glued and possibly also screwed into joists below. Special boards exist that are covered by a film to protect from wear and tear while the builders are in.

There is also the option to use OSB, it's slightly cheaper than plywood.

I understand that if we use OSB/plywood, without T&G, we would then have to insert noggins where the boards meet to avoid movement and squeaks. But it may be hard to persuade the builders to go to such lengths.

T&G plywood does exist but is very expensive.

Constructing the partition walls.

Builders prefer to install the subfloor before the partition walls upstairs. However if the partition walls rest on the boards, it would be much harder to lift up any boards for repairs if needed, so we sort of lose the advantage of not gluing the boards onto the joists.

If we construct the partition walls before the subfloor, it will then require more cuts to be fitted and will be more awkward.

 

[Deleted member 174758]

The engineer has asked me to use 18mm plywood screwed into the joists below. Never to glue the boards as I will not be able to remove boards for repairs in the future.

A question. Just what sort if repairs do you imagine will be needed? The only place I can see it being necessary to ever lift a floor in a loft is above junction boxes, and a removeable access hatch can be cut in a sheet material floor to deal with such things. There is/was even a router jig called a Trend Routabout designed to cut flush, circular service access hatches with a router (not sure if they are still made - as a joiner I make my own neat hatches, thank you). But if your finished floor is going to be tile or laminate, what then? And what about walls or ceilings? If you need to get at services buried inside a wall or beneath a tiled floor as I did a few weeks back (friend with two leaks from a shower unit and its' waste pipe) you are left with cutting out material on the path of least resistance - the plasterboard on the othetvside of the wall or ceiling - then making good afterwards. Never heard anyone complain about having to do that and insisting that the PB are screwed on, but not scrimmed or skimmed. Sorry, but I don't think much of your engineer's approach (can you tell? )

A word of caution - if you do glue a floor down then prior to doing so all electrical and plumbing services beneath the floor MUST be thoroughly tested and the positions of everything checked and rechecked before committing. BTW some plumbers and sparkies seem utterley incapable of doing this, whilst others are on the ball

250 x 50 is a big joist size. That implies a span of 18 to 20 feet. If that is the case ensure that your joiners install at least one row of 200 plus x 50mm (i.e at least 80% of the depth of the existing joists) solid strutting between all the joists before they floor out. This will stiffen the sub floor and reduce any tendency for the floor to be bouncy or the plaster board beneath to crack in the future

Modern builders use T&G chipboard glued and possibly also screwed into joists below. Special boards exist that are covered by a film to protect from wear and tear while the builders are in.

Installed correctly T&G boards, be they chipboard or plywood, should always be glued and screwed to the joists and glued at the tongues with a D4 adhesive - as recommended by every chipboard flooring manufacturer. If you don't glue them you might get creaking in the future, but the same goes for 18mm plywood sheets with square edges which is just screwed down. Whatever you do put down, if you build walls on top of the boards, they won't be liftable, either. What did your engineer have to say about that?

There is also the option to use OSB, it's slightly cheaper than plywood.

I understand that if we use OSB/plywood, without T&G, we would then have to insert noggins where the boards meet to avoid movement and squeaks. But it may be hard to persuade the builders to go to such lengths.

If your joist spacing has been done to work with metric plasterboard (normally 2400 x 1200mm) then the joists will be on 400mm centres. This is not great for 8 x 4ft (or 2440 x 1220mm) sheets of plywood, OSB or square edge chipboards because you end up having to trim every sheet do that all your sheets meet end to end on a joist. T&G chipboard, especially the thicker 22mm stuff, can often sail under/over joists by 100mm or so and be joined there, providing that the sheets are properly glued and screwed down (NOT just screwed)

T&G plywood does exist but is very expensive

About the only type we see is spruce T&G plywood. It comes in 2440 x 1200 sheets (I think, haven't put any in for about 3 years so that might be slightly wrong). Only ever used that in multiple layers to build- up 36 or 54mm thick floors (offices, etc)

You can "manufacture" your own T&G plywood by routing a groove around all the edges and glueing loose 6mm plywood rips in (roughly 30 to 40mm wide) as tongues during the install. This was once a lot more common in shop and restaurant fitting

But, and it is a very big but, one of the biggest problems in using 8 x 4ft sheets of flooring is how to actually get it up into the attic. An full sheet of 18mm sheet stock is not only big and awkardly shaped, but weigjs 40 to 50kg as well whereas 2400 x 600mm sheets of 22mm P5 T&G chipboard are a lot easier to load up into an attic than any full size sheet will ever be

Builders prefer to install the subfloor before the partition walls upstairs. However if the partition walls rest on the boards, it would be much harder to lift up any boards for repairs if needed, so we sort of lose the advantage of not gluing the boards onto the joists.

They generally need the floors in place to support the stud walls, so that is a standard approach. If any of the walls run parallel to the joists, but aren't directly above an existing joist, then the builders will need to install short trimmer pieces (solid strutting) between the two adjacent joists at 400mm centres so that the load of the wall is transmitted into the two joists (via the floor), not on an unsupported floor (which may well creak over time). If stud walls run directly above a joist, it should be doubled-up to strengthen/stiffen it. Where the wall runs across the joists that should be OK. In all cases care must be taken to ensure that sole plates are nailed into joists or strutting as this makes for a less squeaky floor as well

If we construct the partition walls before the subfloor, it will then require more cuts to be fitted and will be more awkward.

See above. That would be a lot more work and makes for a poorer solution, which I'm sure is why you very rarely see it done

 

[tell80]

Why not use, as you seemed to half suggest, T&G Chipboard flooring? Follow package instructions, & Simply screw it down - use noggins if you want, chipboard works, its good.

 

[Deleted member 174758]

Follow package instructions, & Simply screw it down - use noggins if you want, chipboard works, its good.

It comes in packages, with instructions? Wow! You must pay a fortune for your flooring.

All reputable manufacturers now state that T&G chipboard should be glued to the joists and glued at the tongues - this is to avoid unintended squeaking. So once it goes down it's a so and so to lift (hence a variety of methods for installing access hatches, etc). Simply screwing it down sans glue is a recipe for a squeaky floor - which is the most complained about fault with it

 

[tell80]

Happens I dont buy the packs but the packs we use have instructions on the wraps as I imagine that they do with the packs on offer at the sheds.

 

[Nige F]

Why not use, as you seemed to half suggest, T&G Chipboard flooring? Follow package instructions, & Simply screw it down - use noggins if you want, chipboard works, its good.

You seem to half suggest you know what you're talking about. Good.

 

[akist]

A question. Just what sort if repairs do you imagine will be needed? The only place I can see it being necessary to ever lift a floor in a loft

The loft is liveable space with 3 rooms and 2 baths. I am trying to convince plumber and electrician that I want it clear of any cables or pipes unless absolutely necessary. I was told that cables can run parallel and max. 150mm below ceiling due to regs, so am trying to convince them to run cables like that rather than through joists inside the ceilings. We will see. But yes, I do not expect any repairs (touches wood) - however the engineer was adamant not to glue anything.

Whatever you do put down, if you build walls on top of the boards, they won't be liftable, either. What did your engineer have to say about that?

That would be a lot more work and makes for a poorer solution, which I'm sure is why you very rarely see it done

My preference is partition walls before floor boards / sheets. If the boards go down first, then you could be building partition walls over stretches over voids and hard to tell afterwards.
But it is much, much easier putting down boards first, then build walls over them, I admit.

you end up having to trim every sheet do that all your sheets meet end to end on a joist.

The carpenter said that he will make sure the long ends will be cut to be over joists and the short ends don't care as the joists are 400mm and 350 mm apart. Ideally he ought to put a noggin at those ends too, if I can convince him.

one of the biggest problems in using 8 x 4ft sheets of flooring is how to actually get it up into the attic.

We will push them up from below. There are no ceilings yet. That is my idea anyway...

250 x 50 is a big joist size. That implies a span of 18 to 20 feet.

It is 225x50 - sorry. There are also steel beams everywhere. Largest span is 4.5m, most spans are 2.8m. I think we have one extra noggin at the longer spans (must go check again).

 

[Deleted member 174758]

My preference is partition walls before floor boards / sheets. If the boards go down first, then you could be building partition walls over stretches over voids and hard to tell afterwards.

Not if you measure up accurately and do yourself a drawing first. You don't need to be able to do CAD or SketchUp, either - a simple graph squared pad to make a scaled drawing is enough. In any case you may need to double-up beneath walls in order to stiffen the floor structure (depends on what your S/E has specified). A lot of times structural engineers seem to be happy if your sistered joists are within 100 to 150mm of the wall sole plate (at least from the GAs on several refurbs I've been on)

The carpenter said that he will make sure the long ends will be cut to be over joists and the short ends don't care as the joists are 400mm and 350 mm apart. Ideally he ought to put a noggin at those ends too, if I can convince him.

It's not a matter of convince if you hand him a written instruction...

We will push them up from below. There are no ceilings yet. That is my idea anyway...

You may be OK providing you have enough headroom in the attic to get an 8ft (2440mm) board slightly angled (so, say 2250mm height x 1350/1400mm width), but the slope of the roof can cream cracker that on some builds. Good luck pushing up 30 or 40 sheets at 40+ kg a pop. Not so long ago we did a diaphragm flooring job (basically two skins of 9mm ply nailed one over the other in an overlapping pattern to form a stressed skin) where we started every day by humping 50+ sheets of 9mm birch ply up to the attic level. That used to warm you up for the day, and they were only half the weight you are talking about. Good luck!

It is 225x50 - sorry. There are also steel beams everywhere. Largest span is 4.5m, most spans are 2.8m. I think we have one extra noggin at the longer spans (must go check again).

The "noggin" should be a row of solid strutting. Noggin is any old size - solid strutting has to be the same thickness as the existing joists and not less than 80% of the depth (so for a 225 x 50 that would mean 180 x 50 minimum). Sorry to be a pedant, but there is a difference. Herringbone strutting is a better way to do things (stiffer), but takes more time to install.