I read one other thread on this topic and the consensus is they are good for running services (MVHR) and are light which is why I am specifying them. However, I don't have a lot of experience and the floor design (from manufacturer) stated a design deflection of 14mm which seems a lot. When I questioned the deflection they said it could be improved by reducing the spacing from 600mm to 400mm, subsequently the price went from £2200 to £3400. I have no doubt the deflection will be improved but is it worth the extra?

Max span of longest joist = 5.2m

I intend to glue 22mm T&G boards to the joists.

I have metal web joists, with 22mm T&G chipboard flooring glued and screwed, with a maximum unsupported span of around 6m. In that instance the flanges on the joists are much wider than on shorter spans. I haven't noticed any deflection.

2.15kn is, conveniently, 215kg per square metre. If your span is 5.2m and, let's say your other direction is 4m that's a room area of 20.8sqm, so that's about 4.5 tons for everything - the self weight of the floor, ceiling, insulation, floor deck, carpet, furniture etc

Put another way, if you nip down to the local rugby club and ask 20 players (100kg) and their wives (50kg) and ask them to stand paired up, one couple per square metre of your fully built, carpeted, insulated, decked etc floor.. Then your floor in the middle would have sagged by about the thickness of your index finger

... is 14mm 'design'? deflection.
That is an awful lot... it will feel like a spring....
400mm c/cs, and maybe deepen the section, or widen the flanges?

Good luck...

British Gypsum has standard designs for metal stud walls that allow more movement then this in a floor.

Also some of this deflection will happen as soon the the floor and ceiling is installed, before you build any walls on top of the floor.

Posted By: ward32reducing the spacing from 600mm to 400mm

We have metal web joists and I'm very pleased we chose them. One of the main reasons for using metal web joists is the ease of running services. But there is a downside. Unlike either traditional joists or I-beam joists, you can't cut through the metal web joists - neither the webs nor the flanges. So you have to run services where the openings are. That can be a bit tricky for big stuff like ventilation ducts and soil pipes, especially near the MVHR unit where there are lots of ducts. We've ended up with a soil pipe run across the floor of the plant room because the space between the joists is so full of ventilation ducts. I suspect that reducing the joist spacing from 600 mm to 400 mm would magnify this problem, so I'd be nervous about doing it. I'd certainly suggest getting full layout plans for all pipework coordinated with the joist plans before ordering to make sure it is possible to lay your services.

I'm slowly fitting all the services into our metal web joists. Yes, they limit in some areas (the shower tray outlet has to be exactly right against the joists underneath!), but in the main I've been able to make sufficient adjustments to the services layout to fit with the joist spacing - around 27 - 28 cm gaps between the joists.

It was also important to allow for boxing in for drainage behind the toilets, giving more flexibility, and having our airing cupboard between the two bathrooms so we can use part of it for drain runs and manifolds.

Yes, it would have been good to have the services layouts done before the frame design was agreed, but even for a retired architect it was a step beyond my expertise!!

You will not notice the deflection walking on it, even if you are obese. The main issue I have found when running ducting etc is when the design needs strongbacks (a timber section) running at right angles to the joists, or they are supported in places with engineered or solid timber joists.

If it were me, I would ask the designers to work to tighter deflection limits, or get them to spec the joists at 600mm centres and then actually fit them at 400mm or 300mm centres.

Engineered timber joists stand more of a chance of being 'bouncy' than solid timber joists. It is not really a function of deflection as such, more a function of springyness!

You might be perfectly fine, and a deflection limit of 14mm is totally normal and what is stated in the design codes etc, but sometimes you get unlucky and the floor ends up being springy.

Not sure I'd use Interesting to describe it!! I had to clear some insulation out of the posh lounge to make way for the IKEA kitchen delivery - although as they missed 25% I shouldn't have bothered!! I was expecting to have to go back to them - they are well known for missing stuff - but the amount was annoying!! It was slightly weird that the delivery note had lower numbers for some items, even though they actually delivered the right number!! So I got the 10 drawer fronts I wanted, even though the delivery note said they only gave us 2?

Stuffing the 75mm glass fibre in two layers wasn't too bad, it's the 140mm that I dread!! But, it's good to get some of a nasty job out of the way!! Back to wiring ....