Engineered metal web joists

[garyo]

I'm starting to plan out the building regs on my new extension. I have two rooms that have 5250mm clear spans, and I'm already struggling for head height so don't want an excessive floor (joist) depth.

I'm almost outside of the standard span tables, but might just scrape a 72x220 at 400cc (which are good for 5362mm if less than .50 kN/m2) That would be 32 joists in total : £500.

Initially I thought I'd put a steel half way across the span with a timber bolted in to the webs to get the floor depth below 170mm, but then I thought I'd get a quote from one of the leading engineered joist suppliers to save myself all the bolting/fixing/faffing time. I was expecting it to be a bit more, but fell off my chair at £2000.

Is that the going rate, or should I keep plugging away for quotes from elsewhere? My mistake might have been that I specified metal webs rather than the OSB webbed ones, on the basis that I could also run a soil pipe through if required. I could probably adjust the design to avoid that if it would make a big difference.

 

[^woody^]

The span tables are conservative. Actual calculations can make joist sections work whereas span tables say not.

 

[garyo]

Thanks... I'll see what the SE can come up with. Thinking about it I could bolt a 50mm wall plate at either end too, which would reduce the span by 100mm.

I guess I need to decide how much I'm prepared to pay for the 40mm of head room in going to save.

I could do without putting a steel mid span. All that extra joinery is going to cost days.

 

[Doggit]

How much is the steel really going to cost, as it would let you drop back to 50mm joists which would work out cheaper, and give you less spring in the floor. And will the 40mm (plus the inevitable sods law leeway) cause real problems.

 

[garyo]

I think the steel would probably be the best solution in terms of achieving my floor depth goal, and like you say, it will might even work not much more expensive than the huuge 75 x 220 beams, even allowing a bit for an SE calculation. The main reason I looked in to the engineering beams was to see if I could arrive at something roughly the same price that would save me having to spend an extra day fitting joist hangers etc.

The other benefit of the steel and standard joists is that I won't start getting stressed if they're exposed to weather - it'll be a slow DIY build so the odds of this are high!

 

[tomfe]

I am surprised the quote for engineered joists came in so high, they are normally twice the price as standard joists, have you tired some different manufacturers. You'll find it much simpler to use joists rather than trying to get a steel in place etc.
You say you have a span of ~5m what's your width?

 

[garyo]

I am surprised the quote for engineered joists came in so high, they are normally twice the price as standard joists, have you tired some different manufacturers. You'll find it much simpler to use joists rather than trying to get a steel in place etc.
You say you have a span of ~5m what's your width?

Having looked at costs again, the 75 x 220 beams would cost me more like £900 I think.

It's a 12 metre length spread over two rooms, so if I was to divide the span using a steel in each room it'd look like the following, needing 20 beams at 600cc or 32 at 400cc.

 

[Doggit]

I'm not sure what's the on the side wall, but is it possible to run the steels across the room rather then down their lengths

 

[tomfe]

the 75 x 220 beams would cost me more like £900 I think.

I'd stop messing around and just get them 'oked' by BC and ordered.

 

[garyo]

I'm not sure what's the on the side wall, but is it possible to run the steels across the room rather then down their lengths

Yes I could do - I didn't think there'd be much in it. Fewer joists to bolt in though I guess?

I'd stop messing around and just get them 'oked' by BC and ordered.

Yeah I think you're probably right. I just need to convince myself the 50mm floor depth saving isn't worth the extra few days work (or extra £1000 odd)

 

[Doggit]

At some unfathomed level, it's obviously important to you, otherwise you wouldn't be querying it.

If you take the beam across the room, it may just work out smaller, and more stable, the joists will be less than 75mm wide, and the costs less than the OSB joists. Yes it'll take you longer, but I'd say it was the best compromise, and I think you'll forget the extra time, and be pleased that you achieved what you really want to.

 

[garyo]

At some unfathomed level, it's obviously important to you, otherwise you wouldn't be querying it

It's this kind of thing I have going on above, and with the PIR insulation I'm going to need in the roof and below the rafters, the floor depth may make a big difference to me (in the garage if not the actual bedrooms)

If you take the beam across the room, it may just work out smaller, and more stable, the joists will be less than 75mm wide, and the costs less than the OSB joists. Yes it'll take you longer, but I'd say it was the best compromise, and I think you'll forget the extra time, and be pleased that you achieved what you really want to.

Good point - we'll be in the house for a long time. Rotating the steels around may also give me less wastage out of a 4.8 or 6M timber. I note that at the moment to do the clear span of 5445mm in 225x75 I'm going to have to cut 600mm off the end of a lot of 6M timbers.

 

[Doggit]

Taking the steel across the room should allow you to use 200x50, so to me, it's a no brainer. I don't know whether you could get down to 175x75 at 400 on a 3.2m span from the central steel beam. Have you got an SE to do the calculations, or are your just working this out yourself.

 

[garyo]

I'll have an SE doing it all in one go, but need to give him a brief/design to work to. I have a couple of ensuites above so need to give some thoughts to the joists with regard to running services too, before I get the SE input.

 

[Doggit]

The services can be run outside, so don't let that put you off completely; best of luck with it, and let us know how you get on.