Purlin position - or not at all !

[glangri2]

All

I am in the process of designing the roof construction of my extension, and I need some advice regarding what is and isn't acceptable.

The extisting roof has got 2 big purlins in it, at roughly half height. That space will be staying as loft storage, but the loft space of the extension will be habitable.

My questions relate to how to cut a roof for a habitable space, and minimise the purlins encroaching into that space.

1. Does a Purlin have to be at mid height? By that I mean, could I position the purlin about 30% up from the bottom of the rafter, so long as the clear span of the rafter bwtween the Purlin and the rideg was still OK. That way I could hide the purlin away behind the low vertical walls .

Or,
2. Is there a way of putting a vertical member in instead of having a purlin (so it looks more like an open plan roof truss).

Like normal, I want as much info available first, so that I can sketch out what I want, and then get it approved, rather than pay someone to design it from scratch.

Cheers
Gary

 

[simonjay]

All

I am in the process of designing the roof construction of my extension, and I need some advice regarding what is and isn't acceptable.

The extisting roof has got 2 big purlins in it, at roughly half height. That space will be staying as loft storage, but the loft space of the extension will be habitable.

My questions relate to how to cut a roof for a habitable space, and minimise the purlins encroaching into that space.

1. Does a Purlin have to be at mid height? By that I mean, could I position the purlin about 30% up from the bottom of the rafter, so long as the clear span of the rafter bwtween the Purlin and the rideg was still OK. That way I could hide the purlin away behind the low vertical walls .

Or,
2. Is there a way of putting a vertical member in instead of having a purlin (so it looks more like an open plan roof truss).

Like normal, I want as much info available first, so that I can sketch out what I want, and then get it approved, rather than pay someone to design it from scratch.

Cheers
Gary

The roof probably needs to be designed and/or checked by a structural engineer.
Anyway, its quite normal to have dwarf walls down to a steel to offer rafter support, combined with a ridge beam and / or tie beams at ceiling height (forming a type of A frame).
Or you can get loft trusses made up that are suitable for containing a habitable area.
Simon.

 

[theoldun]

Gary, Said I would look out for yer.
Forget cut roof.
Send plan to
Donaldson Timber Engineering, For the attention Ian Mathews, Senior Design Technician, Airfield Industrial Estate, Enstone, Chipping Norton, Oxon, OX7 4NP giving overall span, and overhang, and length of roof , show staircase position, pitch and roof lights or dormers as required. on plan and ask for quote for attic trusses.
If you still want to go cut roof, PM me. Will explain in detail.
oldun

 

[glangri2]

Cheers Oldun.

2 more questions then ................

Firstly, how do roof trusses compare price wise to cutting a roof on site?

Secondly, how on earth do you get them up and into position ?

Cheers
Gary

 

[theoldun]

Attic trusses at 600mm centres out of 47x172mm chords at 45 degree pitch will cost you approx £18.50 per truss per metre of span. VAT inclusive. This does not include any 25x100mm strapping or ancillaries. Bear in mind you do not require any purlins, steels, spline walls or labour to fabricate.
Can not give you a budget for cut roof, without knowing over all span, length of plate, pitch and whether there is a load bearing spine wall. Give that information and will give you budget, but of top of head trusses will work out cheaper, unless it is a very small roof.
A 45 degree pitch attic truss out of 47x172 chords will weigh in at approx 14kg per metre of span.
By adapting your scaffold three lads who know what they are doing can lift and place a 7m long truss. 7 to 9 metres long requires 4 lads or crane. 0ver 9 metres long half day hire crane job.
Attic trusses are heavy and if H&S caught you lifting by hand they would go ape sh*t.
oldun

 

[glangri2]

Well the clear span is about 6.8m and so including overhang that is approaching 8m.

And with this being a DIY job over many weekends, I haven't got scaffolding - Everything is being done from the inside out - So lifting them into place would be a nightmare.

I am going to have to get scaffolding in so that I can finish of the gable end and do the tiling, but I can't afford to hire it and have it sitting there not being used 5 days a week.

Price wise, 6x 8m trusses @ £18.50/m = £888.

I worked out that if I cut the roof, the timber will be about £500.

 

[noseall]

And with this being a DIY job over many weekends, I haven't got scaffolding - Everything is being done from the inside out - So lifting them into place would be a nightmare.

This is a wind-up, right?

 

[geraint]

And with this being a DIY job over many weekends, I haven't got scaffolding - Everything is being done from the inside out - So lifting them into place would be a nightmare.

This is a wind-up, right?

i guessed that when getting a manufactured trussed roof with all the rest....

 

[glangri2]

And with this being a DIY job over many weekends, I haven't got scaffolding - Everything is being done from the inside out - So lifting them into place would be a nightmare.

This is a wind-up, right?

Not at all noseall !

But like I said - I need to get scaffold in to finish the gable end and tile the roof. If I need to get it in earlier I will, but the budget is too tight for me to have the it sitting unused for 5 days a week.

My intention all along has been to cut the roof, and do as much as possible from inside before needing to get the scaffold in.

Back on topic though, have you got any advice relating to the purin position question?

 

[glangri2]

i guessed that when getting a manufactured trussed roof with all the rest....

Guessed what? Sorry I don't understand what you are saying.

 

[freddiemercurystwin]

i guessed that when getting a manufactured trussed roof with all the rest....

Guessed what? Sorry I don't understand what you are saying.

No, geraint is the Forum troll and above us all, his posts are often cryptic! Either that or he's a complete idiot!

 

[glangri2]

And with this being a DIY job over many weekends, I haven't got scaffolding - Everything is being done from the inside out - So lifting them into place would be a nightmare.

This is a wind-up, right?

Noseall, Geraint

Just for reference, here is a picture of how far I have managed to get without scaffolding.

No Wind Up, just grit and determination from an enthusiastic DIYer.

Untitled

• glangri2
• 3 Oct 2011
• 1

It isn't perfect, and I am sure that many Pro's would relish the opportunity to point out every flaw in the quality of the work, and the methods I have used.

From my point of view though, I have saved tens of thousands of pounds, have an amazing sense of achievment, and am close to adding 50m^2 to the floor area of my property!

Scaffolding should be turning up within he next couple of weeks.

Cheers
Gary

 

[noseall]

How come i can see floor joists up there but no rafters?

The joists are meant to go in the same direction as the rafters to offer triangulation restraint.

 

[glangri2]

The joists are meant to go in the same direction as the rafters to offer triangulation restraint.

I have discussed this with the BCO, and sufficient additional metal strapping means he is happy with arrangement that I have gone for.
I showed him the design proposal and he was happy with it.

Once it is all in place, I'll post another picture.

 

[glangri2]

It isn't going to be a million miles away from this photo I took of my neighbours extension a couple of months ago.......

View media item 37624
Add in 1.8m Twist straps from each rafter across the first 3 joists, to give the triangulation, and noggins in line with the straps from rafters 2 & 6.