Cut a Fink truss for loft hatch

[mrkjd]

Existing hatch to access loft is approx 40x70cm running between 2 Fink trusses (W design?) at 43cm centres in a 70's bungalow. The truss bottom chord doesn't actually span that far from the proposed cut as there is a support wall approx 50cm from it (though bottom chord total length from bungalow outer wall plates is approx 11m).

I would like to cut a truss to make an opening of 80x70 so the loft space can be better accessed for storage and I can get bulky (but light) items like suitcases up in there. The cut would remove approx 70cm from the centre of the bottom chord of one truss and this would include the metal join plate.

I understand that there are risks etc associated with cutting a truss but there must be a way that loft hatch installers etc do it. The common practice seems to be to temporarily brace between 4 noggins (2 either side) of the truss to be cut front & back, then cut out the truss, and double up on noggins joining the cut truss bottom chord to the truss bottom chords either side. The "temp" brace can then be removed.

I'm not confident about this structurally. Am I right in thinking that any load movements in the cut truss will be outwards towards the ends of the span? If this is the case, is it not best to double up the bottom chord inner sides of the trusses either side so forces on the doubled up noggins abutt these added timbers to help distribute any forces along the neighbouring joist bottom chords? I will also be leaving the "temp" braces as they will become joists for floor boarding which screwed to these joists should also help distribute forces from the cut chord to the neighbouring trusses and those beyond.

What is the "normal" practice for cutting in such a scenario? I can't think of a better structural way of doing it but am open to ideas / experience.

 

[Deleted member 174758]

As a joiner I have always been told that I should never under any circumstances cut a Fink truss unless a structural engineer has approved the alteration and issued detailed instructions and plans. The problem is that Fink trusses are built with an upward camber so that once loaded they deflect about 6 mm or so over time to become straight. The bottom chord is often made up of multiple pieces of timber held together with gang nail truss connectors and are relatively weak. They are designed with a heavy (critical) dependency on the bottom chords, which are in tension. Removal will leave them extremely vulnerable to collapse due to spreading, especially under snow loads, but also under just the load of the roof itself.

So cut away at your own peril because if you get it wrong...

...your insurers most certainly will not pay to put it all right

 

[KenGMac]

Totally agree with "JobAndKnock" above.

Indeed, do not make any alteration to any timber on any "modern" truss constructed with gang nails cut nothing without instructions and agreement from either a Structural engineer or, the designer of the truss.

Ken.

 

[mrkjd]

Thanks for the explanation. I didn’t know about the camber deflection. I can see how the forces come down from the ridge and are spread by the “W” to create an outwards force to the wall plates. Seems that central nail plate on the ceiling chord takes all that outbound tension. Having said that, I thought it was relatively commonplace for these lofthatch installers to cut and brace trusses somehow and self-certify so thought there must be a method out there.

 

[Deleted member 174758]

They might have a set of general arrangements and calculations done by a structural engineer - in fact if they don't have those I can't see how they could possibly be fully insured.

 

[KenGMac]

I have seen only [thankfully] two occasions where "someone" has cut such a truss, distortion ensues, ceilings, and other such seemingly odd manifestations of a truss having moved and [in effect ] failed.

The context of my involvement was that an insurance claim had been made for [of all things] possible Subsidence. do not ask??

Upshot, claim was declined, reason faulty design or poor workmanship, claimant left with a large cost.

Anyone "messing about" to the extent of chopping a bottom chord on such trusses in my opinion would have great difficulty in getting any sort of "Insurance"

Ken.

 

[mrkjd]

So there’s no way to do this job?

 

[KenGMac]

So there’s no way to do this job?

Yes there is, find out who manufactured the trusses, somewhere on a truss there "Should" be a manufacturers label with all the contact details you need.

Another way of locating the truss maker is that the trusses may have a name printed on the truss itself

Check the information you have for the property there may be information in there.

Ken.

 

[mrkjd]

I don’t think you will be very surprised to learn that there is no information visible on the trusses.

Thanks for the help Ken.

 

[mrkjd]

So what reinforcement options would a SE likely recommend? I'm sure many builders/ DIYers would just go for an "overkill" approach and rely on that. I'd like to know what those options are so I can figure out if they are a route worth going down. I wouldn't want to be removing roof to double trusses but I would be up for doubling trimmers/noggins, tying between trusses at strategic points etc.

 

[Deleted member 174758]

Having said that, I thought it was relatively commonplace for these lofthatch installers to cut and brace trusses somehow and self-certify so thought there must be a method out there.

At lunchtime today I was chatting with my boss about Fink trusses. He has about 30 years experience of timber framed houses, many of which incorporate Fink trusses. He told me that it is common to set the trusses at 600/610mm (24in) centres and that anyone installing an aftermarket loft hatch would probably opt for positioning their hatches between existing trusses as "cutting them is a fool's game unless an s/e has designed a solution for you".

Rather than (in effect) asking the same question again and again in the hope that you'll get an answer that you want to hear, why don't you Google "cutting Fink trusses" and read some of the horror stories yourself. Then maybe talk to a structural engineer or a truss manufacturer to confirm that the two of us really are giving you the "skinny" on not cutting them.

Doubling up, sistering, trimmers, noggins, etc won't work unless they are part of a design where a structural engineer has done the calcs and issued a layout drawing

 

[mrkjd]

I am asking the same question as nobody has really answered it. You are (in effect) just outlining the risks and legal requirements again and again. Which I didn't actually ask for.

But I understand your answers pointing out those risks are in good faith and you are not just being "one of those people" we get on this forum who respond to every DIYer's query with "consult a professional" pedantry. So thanks for your contributions.

I still think that it is not particularly uncommon for DIYers/Builders to cut a truss ceiling chord, reinforce minimally, and suffer no issues and the "horror stories" are the exception rather than the rule. There "should" therefore be a relatively simple, DIY-able practical solution for "most" scenarios.

But bottom line is nobody has volunteered what was done in a similar situation (perhaps as instructed by a SE) and my Googling / Forum surfing has been "mixed". I am not confident to move on with the job so I will shelve the idea as it is clearly something that it is best to err on the side of caution about given the potential consequences.

 

[Deleted member 174758]

So what you are saying is that what the truss manufacturers, structural engineers and God forbid even lowly joiners say is utter tosh. There are all sorts of people put there who will happily "cowboy" a job without giving a tinkers cuss about the consequences. I suggest you find one of those.

 

[mrkjd]

Nope I do not believe that is what I said.

You seem to be upset about something and turning troll?

I’ve thanked you for your previous contributions even if they didn’t actually answer the question but now you need to wind yer neck in and don’t bother contributing further to this thread if you can’t keep things civil.