Loft Conversion advice. Purlin strut removal.

Poe

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Hi all,
I've asked for advice previously both here and on Screwfix forum so sorry to be bothering you again.
I have been in contact with two structural engineering companies who (after contacting numerous others with no interest or response) have promised to get back to me with quotes but both seem to have forgotten to do so.
In the mean time though if get some feed back on our plan here.
We have narrowed down what we wish to do to putting a stair case in one of the two large bedrooms and creating two rooms in the loft (no dorma) which will be either two bedrooms or a bedroom and a bathroom.
We plan to leave the existing purlins in place and using wall plates off the gable and party walls to sort the floor joists which will be supported on the center off the spine wall.
This should be the most cost effective way and avoid the need for steel beams to sort the roof.
I did wish to run flitch beams to support the roof in place of RSJ's but the shear mention of this sends the local structural engineers running for the hills (or valleys here in Wales).
I know the floor joist spans are quite long but should be achievable.
So the main issue and I guess point of this thread is how to remove the purlin struts and re-support them at the spine wall.
No work will be undertaken without an engineers calculations and the thumbs up from building control but I need to start getting my head around how this is to be achieved and also get some designs planned out.
Thanks,
Poe.
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Can you provide a sketch? You will probably get a better response if you post a drawing showing what you mean.

Which direction does the spine wall go? Between the front and rear elevations where rose purlin struts land now or from gable to gable?

What is proposed span of your joists?
 
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I can offer you no advice but only sympathise with you regarding design/plans/SE calcs.

I too am imminently starting a loft conversion myself. I simply need a few calcs on supports and have contacted tons of people who just don’t seem to want to provide the limited info I want. I thought I’d hit the jackpot at the end of July - chap came round twice, measured, checked, chatted his A game and I thought he’d grasped exactly what I was after. 4 weeks later nothing. Not answering phone etc. but I knew how hard it was to get anyone on board. Eventually got hold of him…. I was blagged off for a couple more weeks….. this process repeated till the end of October. Don’t get me wrong, I was looking elsewhere during the latter stages. Thank god no money changed hands.

just recruited a SE who has come round to measure up. He too is aware of my requirements. Just hope he comes through.

Bloody nightmare trying to get started. Too much work on for these people as everyone now wants bigger living accommodation and has a couple of years holiday money saved up.

Got the pain of material prices to deal with after!
 
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Are you altering the roof structure other than removing the purlin struts? If not, why do you need RSJs / flitch beams?

However, obviously the purlin struts are there for a reason (the purlins won’t span the full distance from wall to wall).

Usually the simplest thing to do is to support the purlins using a stud wall down onto a steel beam running above the existing joists. Then run new joists between the steel beams. Flitch beams usually end up prohibitively deep for anything but the shortest spans.

As the comment above, I didn’t fully understand what you were describing with regard to resupporting the purlins onto the spine wall so let me know if I am barking up the wrong tree.
 

Poe

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Sorry I thought I had attached all the images but clearly not.
The spans of the joist would be 3450 and 4120.
What I'm an proposing would be to put in the joists and using these and the existing spine wall plate build a structural stud wall to support the top two purlins. The bottom purlin struts can remain in place.
And through this wall have either a doorway or a walk through/landing.
I would possibly bring the stairs up from bedroom 2.
Hope that makes some sense.
Cheers Poe


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Sorry I thought I had attached all the images but clearly not.
The spans of the joist would be 3450 and 4120.
What I'm an proposing would be to put in the joists and using these and the existing spine wall plate build a structural stud wall to support the top two purlins. The bottom purlin struts can remain in place.
And through this wall have either a doorway or a walk through/landing.
I would possibly bring the stairs up from
Will the stud wall hide the existing purlin struts? The spine is on the line of the existing struts and the stud wall will be built directly above the spine wall?
If you need to remove the top purlin props in order to provide a doorway through, you should be able to take the load down to the spine wall using a 100x100mm post within the stud wall. Needs checking though.
 

Poe

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Pillsbury,
Exactly my thoughts to much work out there now to bother with smaller jobs.

Ronny Raygun,
The reason I mentioned flitch and rsj beams is originally I was going the more traditional route with dwarf purlin walls running the full length and may still require this but looking at it I think what I mention above would serve us well.
I just need an engineer would can do the specs for supporting and bracing the top purlins and for joists.
Or tell me it's completely out of the question and revert back too what I refer to as the traditional loft conversion.

Cheers Poe.
 

Poe

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RonnyRaygun,
Sorry I'm a bit behind your posts, you seem to be posting as I'm still responding to your earlier ones.
But yes you have got my idea.
 
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I assume you’ve got enough headroom to allow for 8x2 joists over the top of the existing ones, plus a gap between them to allow for deflection, plus floorboards, plus insulation and plasterboard below the existing rafters?

Probably be fine but should also check the rafters for the increase in load…
 

Poe

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Would 8x2 joists not be on the small side for a 4200 span?
I have 2700 from the top of the ceiling joists to the underside of the ridge.
The roof trusses are 3x2" so will need beefing up for the insulation and air gap.
I would guess that with leaving the purlins in and making the trusses deeper in-between the roof strength would more than strong enough and considerable stiffened but would get the SE to check over any final design to be sure.
Poe.
 
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Would 8x2 joists not be on the small side for a 4200 span?
Considering you won’t be supporting any ceiling load and assuming you haven’t got any perpendicular partitions, you’d get 8x2s to span 4.2m at 400mm centres, but as you seem to have plenty of headroom anyway no reason not to increase to 9”.
 
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You would need minimum 195mm deep C24 joists for a 4.2m span as per the NHBC span tables.

I think putting in timber posts in to support the purlins and hiding them in a partition would be okay. The wall below will be fine as it's already taking the load, so you just need the posts designing.
 
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Brain fart, obviously the wall panel will need checking due to the increased loads from floors.

What is the wall built from; clay bricks, concrete block etc.?
 

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Thanks SpaceWorm,
Can remember how I worked out the joist size as it was a while ago but I came out with 10x2 timber minimum.
I would be putting extra loads through the spine wall as the new floors would be wall plated to the sides of it.
Cheers Poe.
 

Poe

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Ha, ha, I didn't it again we both posted at the same time.
Both with the same idea.
The wall is brick, but I will need to check a few more areas to be sure as it's an ex council property and they could have made up the courses with whatever they had.
Poe
 
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