Sistering timber joists effort similar to cost of steel?

[parsman]

Hi All,
We are looking to finally build the water tight shell of our Mansard extension. We had the structural plans drawn up 2 years ago.
I had a local builder which I often use look at the structural plan, where steel is minimal, i.e. only proposed for the central section of the floor span . The remaining structure relays on multiple use of 4 / 3 x sistered timber joists such as 4 x 47 x 195 C24. Now my builder has suggested that most of these joists should be replaced by steel rsj instead, as the time and effort to make and fit these joists would work out the same person metre to steel, and timber is frequently notorious for not being straight.

Could a posi joist be used instead for ease of install and consistency of straightness, and as a cheaper alternative to steel?
What would the equivalent posi joist be for the above 4x example?

Perhaps the builder prefers less hassle using steel instead of the timber design?

 

[blup]

You see massive steels being used now, that are specced by architects/SE's rather than builders, so its interesting yours sees steels as an easier option.. Its probably easier to get timber into place than steels, which are incredibly heavy, and not much extra work to clamp and bolt them together (many hands etc). Maybe its the head scratching of lining up all the timbers that he's concerned about, but that should bread and butter to a time served builder. Has he done bigger jobs like this before?

 

[parsman]

He has been doing smaller projects but has taken on more bigger projects recently, a couple of loft extensions, extensions and new basement and I would say his core skill is working with timber! He mentioned that typically timber delivered were never straight and joining 4x would require a lot of effort/ hands. I think he his recent work has made him more comfortable using steels then sistered up joists. I was thinking of suggesting posi joists or similar for these sistered joists as an alternative to steel.
I have also made queries to S engineers for the cost of coming up with a steel design and it is coming in at around £2k, which I'm not happy to fork out again for some new design being on a tight budget. Original S engineer is ignoring me at the moment.

 

[blup]

There's hassle in lifting and placing heavy steels. Joining timber with bolts, toothed washers, plates and nuts requires clamps and a long armed socket spanner, but is doable with a bit of grunt, but building is all about occasional bits of grunt. Maybe he is reckoning on craning the steels into place and thinks that will be easier, has he said anything about that cost?

 

[parsman]

Yes. He did mention they use a company that provides a crane service. And I suspect the higher cost of this method falls on me and less grunt work for them

 

[parsman]

I have got a quote of 1k + vat for calculating the replacement steels for a SE. I think it will be quiet costly materials wise this route. Current steel cost is about 1.5k but new Steel would probably quadruple or more that amount plus SE cost

 

[robinbanks]

Show us the plans you're talking about?

 

[parsman]

That was what I was thinking...will do shortly, btw second 20 yrs experienced chippie has the same opinion of swapping over to steel for the ridge beam etc

 

[stevie888]

I will often design multiple timbers rather than a steel where it's appropriate. You're not really sistering, merely putting several bits side by side to work together. It's very convenient for supporting dormer walls for instance: just a couple more joists to go in.

On one job some years ago, after 2 builders had refused to price it, I was asked to redesign a loft that had 2.5 tonnes of steel in it, one single piece weighing in at 750kg. My design had one single piece of about 40kg.

Back in the day you'd often see 12x12 timbers used structurally and we could reproduce that with 6 2x12s if we so desired (by a quirk of timber design codes such a beam would actually be rated for a higher loading than a single piece)

In such an application you wouldn't need spiky washers (which are a PITA) if the loading is applied to all timbers. A few bolts and plate washers are hardly brain surgery. Timber not straight change your timber yard!

If you use steel you frequently have to bolt timbers into the web to hang joists or rafters.

It sounds as though you had a sensible guy do the design first time: I would build what he's designed

 

[parsman]

Interesting to see if I get any builders willing to take on such a design....Anyway these are the plans with example photo of what the extension should look like. Can anyone explain why there needs to be 4x timber joined for 2T14 instead of say 2x? would be interested on your thoughts of the structural design. thanks

 

[parsman]

Any thoughts guys? Perhaps the use of flitch beams would make it less labour intensive (cheaper option then steel redesign) for those 4/3 X floor beams, so it would just be 2X timber and reduced risk of warping?

 

[stevie888]

The drawings won't open for some reason.

It will be a lot more work to form a flitch beam than a simple multiple timber beam and a flitch plate will cost more than the timbers it replaces

 

[parsman]

They opened for me. I will share again

structural - NorthLondon - Plans-1-7-6-7 (1).pdf

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