replacing purlins

[WyteNyte]

I have been advised on the loft forum to seek assistance from "shytalkz" on this issue

I have a 1930s semi with a large hip roof. It has purlins on each of the 3 slopes supporting the 4x2 rafters. These purlins are themselves supported by centrally positioned diagonal timber struts to load bearing walls.

I would like to make an "occasional use" hobby room in this loft, and to provide more clear space I would like to remove the purlins and associated struts. To provide the required rafter support my idea would be to insert 100x50 collars fixed between each rafter across the roof space at about 2.4m above loft floor level (The ridge is some 4.2m above the floor). Additional support would be provided by 100x50 timbers from the rafter to the floor at about 1.2m above floor level.

Is this a feasible and structurally sound solution? If so, what form of connection should be used between the rafters and the new timbers? (I am assuming bolts)

This is not meant as a loft conversion. I just need a bit more clear space in which to potter around making my models, so it needs to be an inexpensive solution that I can maybe tackle largely myself, but which is still structurally sound.

Any advice would be welcome from someone with experience of doing such a thing

 

[^woody^]

Collars and purlins have different structural functions, and i can't see how replacing the purlins with collars is going to help if the roof loads are not transfered to the walls below

 

[noseall]

'twas i who suggested the move to the building section and i echoed your points previously, woody.

there is a fair bit going on with a purlin - remove it and you will open a can-o-worms.

 

[Shades]

Collars and purlins have different structural functions, and i can't see how replacing the purlins with collars is going to help if the roof loads are not transferred to the walls below

Your Purlins don't only support your rafters, they support your roof covering also and any snow that may rest on your roof.

they are a structural support and removing them would be detrimental to your roof structure.

The purpose of a roof does not only provides covering to the dwelling it also needs to support to itself as well as withstand the forces and stresses brought on from storms, wind and snow.

You would be surprised with the amount of downward force on a roof, when it is just sitting there with out snow etc.

It is possible to reposition the struts so that you can gain movement to the area inside the purlins.

It is possible to move purlins but I don't think your interested in that as it is an expensive job.

Hope this helps.

 

[Shytalkz]

What height are the purlins above the ceiling level?

 

[Shytalkz]

Collars and purlins have different structural functions, and i can't see how replacing the purlins with collars is going to help if the roof loads are not transferred to the walls below

Correct, it will stop the rafters moving together horizontally, but it won't help with resisting vertical deflection. The diagonal props limit both horizontal and vertical deflection. If you put a collar in, and remove the props, the roof will drop vertically, the rafters will bend markedly and you're likely to get some lateral movement at the eaves.

Everything else that Shades says is 100% correct.

 

[Joe Malone]

I was going to talk about maximum allowable bending stress and redundancy but then I realised I'm only a building surveyor and it wouldn't be sensible

 

[WyteNyte]

Thanks for these opinions. I realise that purlins and collars have different purposes; that is why I suggested taking the horizontal load via the collars, and the vertical load by means of struts beneath the rafters supporting them from the joists below (probably actually from a cross-beam supported from the load-bearing walls, rather than on the joists themselves)

The purlins are 1.7m from top of joist. I was thinking of the collars at 2.4m high to give good headroom, and the vertical struts at about 1.2m high to make most effective use of standard plasterboard sizes, but they could be moved inwards and upwards, if necessary, to say 1.6 with some loss of floor area.

The design of a loft conversion to produce a clear "Room in Roof" must be based on similar techniques as they have totally clear space, so this cannot be an impossible dream, can it?

 

[Shytalkz]

Those dims put the purlins less than halfway up the slope, no?

To be honest, if you moved the line of support to the rafters outwards by a foot or so (birdsmouthing the rafters over the top plate), built a stud perimeter wall on that line to the three sides off the joists and put the collars in, then removed the existing purlins and props, it would not collapse in a heap. I've seen and had to sort out far worse "conversions" which somehow stayed standing, than what you are proposing. However, doing it the way outlined above would not necessarily be proveable by calculation, if it ever needed to be and therein lies the rub.

The problem would come if you ever came to sell, in that a surveyor would pick up the alterations, do a LA search and know that no BRegs application had been made. Cue SE having to do retrospective design and your having to apply for a regularisation certificate. Lots of fees, both engineer and LA and possible costs of strengthening as well.

But...if you don't put a fixed staircase in, you won't have to comply with any other elements than Part A of BRegs, no thermal, means of escape etc considerations. I have been known in such circumstances to take a view on how the work has been done, taken into account that, regardless of number-crunching, the fact that it's standing there quite happily and given it the ok, with either none, or just some very simple alterations to what's been done. Other engineers might take a more prescriptive approach though, so there remains a risk to you in not doing it "properly" in the first instance.

Just a thought: if you did it all in compliance with BRegs, you'd gain a legitimate extra room and add some value to the property.

Does this help?

 

[^woody^]

What would this stud wall rest on?

 

[Shytalkz]

Joists most likely: but it's only taking one rafter load per stud/joist that way. Yes, it's probably overstressing it (by modern design standards), yes it might deflect and cause a minor ceiling crack or two. Collapse, though? Nope. To be even surer, clad one side of the studwork with ply.

However, I did forget to add double up the joists running parallel with the line of the new studwork and supporting this (probably the hip end element of the wall).

 

[^woody^]

I wonder if a UB or such like could span somewhere at joist level and then some posts fixed to this to take the purlins?

Or again, a beam and then at 90 degrees box beam instead of the stud wall or another beam and then studwork?

As its a hipped roof, the problem is going to be getting a beam to sit on the wallplate with poking through the roof

 

[WyteNyte]

I could possibly put a new beam under the struts, tied into the party wall and supported at one of the load bearing walls toward the other end.
If I did this, and put a plywood facing on the rear of the struts, this should spread the load sufficiently to avoid too much joist deflection. Its worth looking at.

Now I have a bit more confidnce with the eventual solution I can consider things in more detail, and come up with a firm proposal.

Should I try talking to my local building control guy to see if I could get some form of approval, or would it be better to just go ahead.

As for the survey, stats show that 80% of buyers don't get one, and if the job looks good, with no obvious deflections, even a surveyor should give it the nod.

Thanks for all the help guys. I'll let you know how it pans out if I decide to go ahead.

 

[Shytalkz]

Woodster: For a pukka loft conversion, then that's a possible solution for sure, but the guy doesn't (or didn't!) want to go to that extent. Steelies can be chamfered to suit the roofline for such circumstances at the wall bearing without too much trouble.

Far be it from me to tell him to just do it regardless, but with a bit of fine-tuning of his proposals and cautioning him as to the limitations of his approach, it isn't going to turn into a flat roof all of a sudden.

 

[Shytalkz]

WN: if you go to LA, you'll definitely have to make every structural element check out design-wise and you're almost at the point of having to go the whole hog: access, thermal, MoE etc etc. Not that I'm advocating that you don't involve them even though it's a structural alteration, of course...