Fitch/Flitch beam for purlins

[AnthonyL]

I could do with some help on asking the right questions before I get someone to fix my sagging roof.

Due to some modifications some years back the roof has a slight sag due to an unsupported section of purlin, some 5.6m, and the removal of some roof support to open the attic.

The roof has been like this for at least 11yrs but I would like to get it corrected and it has been suggested in a report that I get Fitch (sic) beams installed by adding 8m steel to either side of the purlin as well as supporting the purlin at the mid-point of this span.

The purlin (185mm x 50mm - 6"x2") however is in two shorter parts bolted with a 700mm overlap at around the mid-point (ie around the

2.5m mark). I'm not sure how a builder will be expected to cope with this as clearly just running a length of steel either side will mean the run will not be straight.

I'm not too sure how a 5.6m length of steel is going to get into the house and attic.

The report refers to "Fitch" throughout - google seems to prefer "Flitch".

[Roger Mills]

How much space is there directly under the purlin? Assuming you can somehow get it in[1], I would be inclined to install a 7 x 4 I-beam directly beneath the purlin, and support it from that.

[1] You'd presumably need to feed it through a hole in an end gable

[AnthonyL]

Thanks - but I need firstly to address the issues as raised in the report.

[John Rumm]

Flitch beams normally work better with a layer of steel between two wood beams...

Having said that, it sounds more like your difficulty here was the removal of the support mid span rather than the strength of the beam. If you restored the support, then you could probably jack the beam back up again.

By taking a few tiles off near the eves and making a hole in the roof normally.

I would normally go with the latter spelling.

[Roger Mills]

Which issues are you referring to? I wouldn't worry about whether it's fitch or flitch!

In order to support the beam with steel, you need a single piece of steel the whole length of the purloin. If this is a flitch, it needs to be bolted tight to the side of the purlin. If the purlin is in two overlapping bits, you can't possibly do that. But you *could* put an I-beam underneath in a way which supports the whole purlin.

As John R says, you may be better off just re-supporting the middle. Except that there was presumably a good reason for removing the centre support, and it may not be that easy to put back.

[AnthonyL]

I believe there was a timber frame for the water tanks at the mid span which took some rafter load off the purlins. Its removal has left a nice clean space in the attic.

Gable front and back so maybe they would want to make a hole through that. Lot of trouble.

A closer look at the report shows both used - I don't like inconsistency in a professional report.

[AnthonyL]

Yes there is an outside wall undereath the purlins and the existing supports use that. It should be straightforward to put a block on top of that and one property down the road has done exactly (and only) that. I was hoping that would have been a sufficient solution but the report says otherwise.

Oops, that should by 8mm steel. The purlins are 185x50 and the proposed steel is 175x8 both sides for the full 5.6m unsupported span.

Makes sense but disappointing not mentioned in the report as that alone would presumably add a lot of strength to the purlin.

[AnthonyL]

I believe the centre support was removed in ignorance to create space in the loft and maybe as part of re-plumbing when a new heating system was installed.

[AnthonyL]

Just to add to my own post - if steel is to go either side of the purlin then maybe the overlap itself becomes superfluous and can just be cut out leaving 6x2 all the way?

[Martin Bonner]

That seems plausible to me (I was about to suggest it). The only problem I can see is the point that you have sawn the purlin in half, and not joined it back together with a flitch beam.

It sounds to me like you got a professional survey done - I'd ring the surveyor and ask him what he thinks.

(Flitch / fitch - I wonder if he dictated the report? If the audio typist isn't familiar with the terms yet, that could explain it. Still sloppy though.)

[sm_jamieson]

Its not very professional is it ? I've also seen "purlings" and "noggings" for what I would call purlins and noggins, but that might be a regional thing. Simon.

[AnthonyL]

Yes I paid for a structural report and as you gather have some reservations

I want to be in a position of better understanding before I do - as you might gather this is not an area in which I have expertise.

Certainly is the case with recent legal documents I have seen. No-one seems to care - or they assume that no-one else cares. The only correct (assuming 'flitch') spelling is in the computer calculation section which presumably is inbuilt to the software they use.

[Roger Mills]

Because it's in two pieces, it will simply add a lot of weight without contributing much to the bending stiffness.

[Martin Bonner]

Ah! In that case you might be seeing a regional difference. The surveyor uses "fitch" consistently, but can't get his software to spell it properly.

[Tim Watts]

I used to confusedly call it a filch beam... Better than a filtch beam I suppose, though I could see how a beam might be useful in such circumstances!

[tabbypurr]

Why don't you calculate the expected sag if you do just that, then you'll have a good idea for yourself as to whether it's a satisfactory solution. You didn't mention when these houses were built.

NT

[AnthonyL]

How do you propose I do that?

1968

[tabbypurr]

ory solution.

On 2nd thoughts is there a need to? If you have identical houses nearby, an d all they've needed is that extra support point to avoid sag, you already know an effective solution. I'd expect houses from 68 to be perfectly capab le, but not to current BR. I wonder if your professional may be looking at what current BR would require.

NT

[AnthonyL]

I've looked more closely at the "professional's" report. He has calculated based on an 8m span with an added support half-way. This is after having his surveyor come out and taken measurements (with a laser ruler!). My 5m tape covers the length of the currently unsupported span, so 2.5m with a mid-support. I suspect when he re-does his homework a flitch/fitch beam will be overkill.

It may be that I could rely on other properties - I have only been into the roof of one other so it is hardly a definitive sample. I don't know many folk yet - certainly not well enough to invite myself up into their loft.

[Vir Campestris]

Google for the Sagulator.

Andy