Fixing wooden joists on top of metal beams?

[Al Reynolds]

Evening,

I haven't been here for a while as I bought a house which didn't need much work so apart from a big pergola and some shelves I've done no DIY for two years. It's good to see a few familiar faces...

Anyhew, my mind has turned to the notion of converting our loft into two bedrooms and a WC. I'm still very much at the feasibility study stage so far. I have an *excellent* text by J Coutts on the subject which I would recommend to anyone who wants to learn all about loft conversions - it is on Amazon.

Most of the time, when a new floor is being constructed, the format is two long metal beams (I-profile) with joists between them. In most cases, the joists either sit in the web (the sticky-in bit on the side of the I beam) or are slung below the beams using joist hangers attached to timber in the sides and on top of the beam.

What I want to know is, can you fix the end of a wooden joist (say 75mm x 147mm) on *top* of one of these metal beams? I have good reasons to want to do this, most of which relate to the ideal position for the new staircase. If it is possible, how does it work? Do you bolt packing timber to the top of the metal beam and then fix the joist to that?

A further query: if you can do the above, could you have joists where one end is slung below one metal beam from joist hangers and the other end is supported on top of a (lower) metal beam?

Thanks in advance, Al Reynolds

[Bruce]

Surely this isn't within the scope of DIY.

You should consult a structural engineer, a loft conversion specialist or, if all you want is some broad guidance as to what is and isn't possible, your local building control officer.

[John Rumm]

Can't see why not. The harder bit might be "fixing" since you can't just skew nail them to the beam. A row of noggins however should hold the free ends sat on top of the beam in place.

I would just sit them on the top of the beam much as you would if it were a wooden joist/wall plate.

Yup, no requirement to have the same support mechanism at both ends.

I had one case where I had one end supported on a mid span wall, and the other end hung from hangers of a flitch that needed to be positioned above some existing floor joists:

[Owain]

You can nail into structural steelwork using effing big nailguns.

Owain

[mail]

Dear Al Yes is the answer. It is well within the scope of a DIY who is competent. For the top of the beam you place a sole plate on the top and bolt it onto the beam with, say, 12 mm (1/2") bolts at appropriate centres - probably alternate flanges at about 600 to 800 mm making each set 1200 to 1600 apart. Mark out where the joists are to go and avoid those locations. Use a 2" square or round thick plate washer on the timber and a normal nut and washer on the steel. Drill half way across one side of the flange (ie 1/4 span of the whole steel). It is also possible to use a Hilti gun and shot fire attachments but you would have to hire that and they are a bit of a fierce tool! Rather than tosh nail the joists to the plate I would use a Concept 2 screw driver but that is up to you. Unquestionably, you would need noggings between the joists for lateral stability of that end. I would also do this in mid span or alternatively use herring bone strutting. At the other end do it just as you suggested Chris George

[Al Reynolds]

Deciding what you want to have done and what is feasible is certainly DIY - the structural engineer and the loft-conversion specialists would be later ports of call, but there's no harm in thinking about what's possible in advance. You don't need an architect to decide what you want.

BCO would just tell me to ask a structural engineer. It's more than his jobsworth to give me engineering advice.

Al Reynolds

[Al Reynolds]

That's good to hear. Are joists normally fixed down to the wall plate?

cheaper to fabricate/install than metal beams. I suppose one advantage is that you can fabricate them on site?

Al Reynolds

[Al Reynolds]

All sounds eminently do-able...

That's sounds a bit dangerous! I always feel that I know where I am with bolts.

Yes the noggins at the end where the joists rest on the beam would seem pretty much essential in this application.

Thanks for all the help people, Al

[John Rumm]

Quite often they are just skew / tosh nailed to stop the end being kicked about - there is no real structural need for it.

They may or may not be cheaper depending on what you are comparing with. Larger ones are easier to handle on site, and you easily fix joist hangers to them.

[John Rumm]

The structural stuff is also DIYable with a bit of research.

Ours was quite happy to suggest solutions as well to be fair. One example: The architect had originally drawn our plans with a flitch beam over the bay window at the front of the house - each end resting on the front wall - this was because the strength of the lintel over the bay was unknown, and it seemed safer to use a beam here as a stringer to pick up the ends of the floor joists that would have otherwise been sitting on the lintel. However in reality there was no way of actually positioning it where he had drawn it, since you would have to cut through the existing loft floor joists, and that would leave the bonnet roof over they bay unsupported since it was held by the joists in cantilever. The BCO suggested putting the beam above the existing joists and using long hangers to suspend the joists below its level. This also necessitated attaching one of the other flitch beams to to its end - which represented a significant change in the load at the end of the beam. I modelled the change in the demo version of Superbeam, told the BCO it was still in spec, and he was happy to proceed on that basis.

[Al Reynolds]

My house is 8.5m gable-to-gable by 5.5m eaves-to-eaves.

The current roof timbers therefore run at right-angles to the 8.5m length. To get new wooden joists to run in the same direction as the current timbers (headroom is at a premium) I would need two steel beams running along the long length of the house between the gables so that I could suspend the joists between them.

Could anyone familiar with the regs tell me if the "span" of the floor would be 8.5m because that's how far apart the gables are? This would appear to make hanging the floor off the end-gables a no-go if that is the case.

I know this is going to be a lot of steel, but if I put in two steel beams parallel to and close to each gable (and possibly more in the middle), supported by the side walls, then used these to support two further beams running along the length of the property, would that mean the floor span is now only 5.5m?

Ascii-art (Times New Roman for correct scaling):

----------------------------------------------------------- | I I I I | | I HHHHHHHHH I HHHHHHHH I HHHHHH I | | I I I I | | I I I I | | I I I I | | I I I I | | I HHHHHHHHH I HHHHHHHH I HHHHHH I | | I I I I |

----------------------------------------------------------- I = eaves-to-eaves beams H = beams in gable-to-gable direction

To be honest, by the time I'd done all that, I'd probably be better off just going through the hassle of lowering the ceilings or raising the roof!

Al

[Bruce]

When I think of the careful, thorough and detailed training that my company gave operatives, supervisors and managers alike in the use of Hilti guns, I shudder at the thought of Joe Public being able to hire and use one with absolutely no safety training.

[somebody]

In message , Al Reynolds writes

Hi Al,

I don't know if it helps but for our loft conversion, the new floor joists sit in between the existing joists directly onto the existing wall plate. No steel required whatsoever. I appreciate from your later posts this may not be do-able, but something for you to think about.

It was a royal PIA to get them all in in this manner, but worth it. We have saved valuable headroom and a shi*load of money on steel.

Whilst there is no steel in the floor, the engineer decided the existing purlins should be replaced with steel. The new steel purlins have a timber plate bolted to the top (this means there is somewhere to fix dwarf walls with normal hammer and nail rather than messing about with a Hilti).

The various flanges, bolt holes etc were all part of the steel spec/drawings sent to the fabricator and they arrived on site as such. There was no working the steel so to speak, myself.

Hth Someone

[Martin Bonner]

Google "Austrian roofer" for plenty of incentive to be careful

[Tony Bryer]

The span of the beam is measured between centres of bearings, so if there is 8.5m between the faces of the walls and you have 100mm bearings, the span will be 8.6m. Spanning beams across this distance generally doesn't make sense.

Without knowing the details, it might be possible to put a front/rear span breaker beam at first floor ceiling level, tucked into a corner where a partition meets the ceiling: when boxed in it would be relatively inconspicuous.

[Bruce]

Here's one from today's London Evening Standard: