What size RSJ?

[martyc]

Hey all,
On a set of plans i have a 175 * 75 wood purlin supporting a roof mid rafter.
Gable to gable is 24 foot.
Im thinking of using an RSJ instead and using it as a "floating wallplate" supported by the end walls as this will be tidier and less work(ill have someone lift the RSJ!)
Anyone knoe what size RSJ id need just so i can get an idea on price...i assume it would not have to be as big as 175 *75.
Cheers,
Marty.

 

[Robert too]

so you want to span nearly 7 m . you cant prop it anywhere mid span from a loadbearing internal wall ?

I think you will be in for a surprise here .


you will have to provide the spans of the rafters and also the tiles you have on the roof , to give somebody a chance , if they dared have a go at it .

 

[^woody^]

You won't get the answer by guesswork on a forum.

It needs to be designed and calculated - or one night, you may be looking at the stars when you are in bed

 

[Bowness72]

If you can tell me how much roof it will be supporting i can give you a size which will give you an idea of the price but i doubt if it will be smaller than the timber because that sounds undersized to me anyway. Also, building control will probably want to see a structural calculation to prove the beam is adequate, whichever you use.

 

[martyc]

Hi,
Yeah this is exactly my point..i have done a lot of DIY over the years and i cant help but think that 175*75 is extremely light for this purpose and i am certain it will sag or at the very least be bouncy in the middle!..But this is whats in my plans!

Anyway.
Its a lean to roof and the span is 10'6"(ridge to soffit)..RSJ will span 24'
Regular slate with 2 velux windows each 58*95cm..slope is 20 degrees.
Joists are 150*38 and rafters are 100*38 @ 600mm centres.
If someone could give me an idea what RSJ is required be great just to see the options.
im thing a 152 * 152 * ~30 might be ok.

Cheers and thanks in advance!
Marty.

 

[freddiemercurystwin]

A 'rule of thumb' for steel beam sizes is 1/20th X span therefore 1/20th X 7.3m = 365 so assume a 356x171 or thereabouts.

 

[markie]

The steels we use for doing an full loft conversion are 6"x10" and they carrie the new floor and support the roof.

 

[martyc]

Good christ!
That RSJ (~356*175) would be ~ half a tonne!...probably heavier than the roof!
If i supported it half way (can be done no bothers) cani get away with less?
Marty.

 

[freddiemercurystwin]

In that case a 203X133 would probably be about right. To clarify these are strictly guesstimates and would need to be designed by an engineer.

 

[Bowness72]

254 x 146 x 37 UB. The mid-span deflection will probably be too much if you use the 152 x 152 UC 30 or 37.

 

[martyc]

Cheers Guys for the input.
Its tres expensif! (and heavy!)

Ill probably just go with a load bearing timberframe wall instead as ill have to stud underneath it in any case so a much cheaper and easier option.
Ill build up a 4*2 stud wall @ 600 c/s and double up the header and that should do the trick me thinks..
Marty.

 

[Bowness72]

Personally i would probably reduce the centres of the the timber studs to 400mm but you also need to bear in mind what the `load bearing' partition is sat on, i.e. will there be something below it to transmit the load into the ground without any settlement. One other option is to increase the size of the timber rafters so that they span the full width and do away with the need for intermediate support.

 

[^woody^]

Yes 400 centres for structural studwork.

And you need a suitable beam below the whole length of the studwall if it is load bearing - otherwise you only need posts off any loadbearing walls.

In anycase, the section size to build a stud wall off, would be the same size as if just inserrting a beam as a purlin - so you have not saved anything with this idea, you still need a beam!

I don't tend to specify large steel sections in roofs or lofts - too heavy and awkward.

Flitched beams, spliced UBs or box beams are much better for the builder.

 

[martyc]

Cheers Guys,
All very helpful info.

Cant span any further with the rafters unfortunately as it is a parapet arrangement and i need to support inside the external block wall somehow...if i ran my joists out to the external block wall it would raise my gutter and further reduce my 20 degree pitch as i have no flexibility on the ridge side due to a window.(which i am already moving up 18 inches to achieve this pitch!)

If i go 400 c/s on the studwork should i also go 400 on the rafters and joists and keep all in line?

The studwork would be sitting on a 4 inch slab which i assume is Ok...i could always put a course of block or brick under it to strengthen the whole unit??(or is that nuts!)
I was just going to use a 4*2 sole plate.

what are these?
Flitched beams, spliced UBs or box beams.

which is the lightest/cheapest that would run 7 metres or so?

Thanks again,
Marty.

 

[Static]

Out of curiousity who designed the structural side of this in the first place? And shouldnt you be consulting them with queries changing the plans.