Loft access, purlin adaption?

Discussion in 'Floors, Stairs and Lofts' started by Keith Fuery, 25 Nov 2021.

  1. Keith Fuery

    Keith Fuery

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    First time on here, so go gently with me.
    I already have a loft access and ladder, but I need to get a wider wooden ladder fitted.
    Where the original opening is, on the landing, is preferred, but there is a 7" X 3" purlin holding 3" X 2" rafters in the way which means up to now, I have had to crawl into the loft.
    I have took some photo's to try and help. My question is, could I remove the section of purlin marked, if I was to support the cut ends with braces? One side has a brace that is fixed to a x. member sitting on a load bearing wall. Could I do the same on the other side which would also rests on a wall?
    Do I need to fix a 4" X 2" along the rafter after cutting out the section to give it more support? IMG_20211125_180030.jpg IMG_20211125_175914.jpg IMG_20211125_175857.jpg IMG_20211125_175841.jpg
     
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  3. blup

    blup

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    You would need an SE to calculate the resulting loads and how to support them, even if your proposal was achievable in principle. But why go to that expense for an extra 3" of head room?

    Blup
     
  4. Keith Fuery

    Keith Fuery

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    Thanks for your speedy reply.
    It would give me an extra 6" of head room, would mean I could get into the loft on hands and knees instead of crawling, and I wouldn't have to make good the hole in the ceiling. These openings were not intended as access to storage when the houses were built, I know that. Have you any idea how much an SE would cost me, if only to explain the fallacy of my proposal?
     
  5. Dereekoo

    Dereekoo

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    Looking at the photos, does your purlin rest against the rafters in a perpendicular fashion or are they in a vertical orientation so in effect there is just a point contact between the 2 members
     
  6. RandomGrinch

    RandomGrinch

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    Hi,

    I'm sorry, I know it's not what you asked about.
    But having a properly opening loft hatch and ladder, that you can carry large stuff up safely, completely transforms how a loft can be used.
    Now that I have a proper hatch, it doesn't take a moment's thought to pull down the hatch and pop upstairs. Before it was an ordeal, getting out the stepladder and clambering through the tiny hatch.

    What you are considering is some pretty major structural work, that will not hugely enhance the ease of use of the loft space.

    I would really think about putting the hatch in another location.
     
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  7. Keith Fuery

    Keith Fuery

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    Dereekoo, The rafters are resting on the top of the purlin. There is a load bearing wall running parallel under the purlin from front to back of the house, and the brace is bird's beak to the purlin, and nailed to the 3x2 that rests on the wall plate, across the joists and onto another loadbearing wall. The distance between the rafters would be about 1 metre if I cut out the part of the purlin. I was hoping I could support the cut ends of the purlin with vertical supports off the load bearing wall, using notches to fit purlin, then fixed to a 4x2 running underneath, and beef up the solitary rafter using 4x2 screwed or bolted to it.
     
  8. blup

    blup

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    Yes 7” not 3. I would think a few hundred pounds for an SE to inspect and a bit more for the plans.

    Will a builder have the space to get in and fit the men and materials needed?

    Blup
     
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  9. RandomGrinch

    RandomGrinch

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    ...and I believe Building Control would need to be involved for a structural modification to the roof structure?
     
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  11. Dereekoo

    Dereekoo

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    Yes I realise that, but what I was trying to ascertain, as photos tend to skew things, is is the purlin and rafter connected flush with each other forming a T shape (say) or is the purlin at 45 degree to the rafter (or thereabouts, say)? I'm guessing its the latter which is at odds what is in my loft which seems to go against structural logic of having the purlin in the plane of the forces acting on it so that they are then transferred down the strut. (Think floor joists laid over at a 45 degree slant, they would still work but not in the most structurally efficient way). Notwithstanding the foregoing if as I understand it you can place a vertical strut adjacent the angled one and take it down to be supported off a load bearing wall then I feel this would provide the vertical stability of the cut purlin, I assume a similar construction at the other edge of the hatch. The outstanding issue would then be how to accommodate the resultant extra forces imparted on the central rafter due to its unsupported length being more or less doubled. Your problem here is is (having done a calculation assuming a certain roof loading for a 1.5m long 3 x 2 rafter and then on a 3m long rafter)it increases the deflection from 4mm to 56mm and takes the bending moment 3 times above allowable. Even quadrupling up the number of rafters in that area would still not satisfy structural requirements. Of course my calcs are based on my assumptions of load and span but should not be to far out?
     
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  12. Dereekoo

    Dereekoo

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    Only if you tell them, but issues may arise several years down the line when/if you come to sell
     
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  13. Keith Fuery

    Keith Fuery

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    Dereekoo, The rafters are "resting" on the purlin i.e. edge to edge, weird, eh? They built houses different in the 40's. Defies my understanding of load displacement. I read up regards the rafters and with the sizes and loads with reference to spacing of rafters, I was impressed with a web page, timberbeamcalculator, that said the same as yourself, (not that I doubted you).
    All references are to minimum 100mm x 47mm, with a rafter spacing maximum of 610mm. There's no point in fixing something that ain't broke. I've lived with it like this for 40 years, so, whilst my welders head says, "gussets, supports, braces", my common sense head says "leave it".
    I'm grateful for your input to this thread, you've raised the right concerns and I'm grateful to you and the other contributors.
     
    Last edited by a moderator: 26 Nov 2021
  14. blup

    blup

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    I believe so as this is an improvement not a repair.

    Blup
     
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  15. jacko555

    jacko555

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    Surely this is the answer?
     
  16. Keith Fuery

    Keith Fuery

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    I agree. New opening in bedroom, missing purlin altogether. Now have to look at fitting double trimming joists, as current joists run perpendicular to hatch placement. Need to find where current joists are supported over bay window area, to get the length and see if I can get joist hangers in?
    I presume there is a beam across from wall to wall supporting original joists, at entrance to bay then, I believe that under 1200mm I only need single headers,(hatch is 900mm) but I think it would be prudent to give as much support as poss. where most foot traffic will be, so I will fit double headers as well.
     
  17. RonnyRaygun

    RonnyRaygun

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    Your purlin is actually in the best orientation for the forces. Gravity forces (dead and live loads)act vertically downwards, regardless of the angle of the rafter, so the best orientation for the purlin is to be positioned vertically. That said, it's not ideal to have the rafters bearing onto the edge of the purlin as any calculation would show the bearing to fail. It would have been better to birdsmouth the rafters over the purlin or connect a wedge to the rafter to fill the gap.

    For your original question, if the purlin is propped at both ends where you plan to cut it, then you could follow the process usually used for trimming rooflights. Double up the rafters either side of the gap in the purlin, cut the unsupported rafter, and install a double 4x2 horizontally onto hangers to resupport the unsupported rafter.

    If you are planning to install a new prop beneath the purlin you just need to be sure that the prop is going down onto a wall that can take the load. Ideally you need a structural engineer to assess.
     
    Last edited: 27 Nov 2021
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