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    • CommentAuthormarsaday
    • CommentTimeJul 9th 2017
     
    I am close to getting the foundations in for my extension. Things are rolling along slowly.

    I am still worried about my roof design and have done a drawing to help you understand what i am trying to do.

    I have about a 45cm pitch over a 5.9m length, so i think using furrings to give me a fall is not practical. I am wanting to use my oak purlins slightly staggered as they move away from the back wall. These will go in straight.

    These purlins will carry the joists / insulation / osb boards and fibreglass finish.

    I need to make sure the joist sits nice and flush with the oak so it works correctly. This will involve planing the oaks a little to get a slight triangular fall on them.

    I enclose a diagram to make things a bit clearer.

    Is this method flawed, involving a huge amount of planing and possibly knotching of the joists instead. Could it be done more easily ?

    I did initially think about building up the block internal wall with this fall on it and then laying a wood or thinner steel bearing plate. The purlins would then sit at a slight angle. I don't think this is as good an idea.

    I also have to work out how to get these purlins in place. I think i am going to have to hire some type of smaller lifting crane to hoist the beams into place. They are 6.5m in length and will weight 300kg each.
    • CommentAuthorowlman
    • CommentTimeJul 10th 2017
     
    Do you already have the Oak purlins? Are they solid or engineered i.e. Glulam?
    As for installing and if you're fit, try using builders extending trestles, the odd Acrow prop and a couple of blocks and tackles. one at each end and slowly raise each end by small amounts until its level with the final resting place and then slide in place.
    • CommentAuthormarsaday
    • CommentTimeJul 10th 2017
     
    They are solid but I haven't bought them yet.

    If I do it by hand I was thinking about hiring those hand turn lifts x2. The issue I have though is the beam will be wider than the wall layout and so you have to do it all at an angle and when proud of the wall you straighten up.

    That's why I thought a small crane or one of those jcb lifters might be useful.
    • CommentAuthorowlman
    • CommentTimeJul 10th 2017
     
    What prompted my question, and not intending to muddy the water, is that large Oak beams, (250 x 150 x 6500 ) presumably green or at very least air dried, in time will move. You seem concerned about precision construction and IMO, long term you'll be fighting that.
    If you want permanent precise structure you'll need to use engineered wood.
    Alternatively go for bark stripped sanded rounds in some form. then allow them to crack and mature nicely. From a design point of view IMO the contrast rustic, natural/simple, modern interior works very well.
    • CommentAuthormarsaday
    • CommentTimeJul 10th 2017
     
    They are air dried 3-5 years, but still will dry further. The movement will occur, but i don't think it will be excessive. Any cracks over the next few years in the plasterboard ceiling can be dealt with quite easily.

    Glulam is ok and i looked into it, but i want the real oak in the room.
    • CommentAuthorowlman
    • CommentTimeJul 10th 2017
     
    I built my partially glazed sunroom roof structure using a mixture of softwood glulam and plain redwood. The floor to wall glazing below is engineered Oak. Now two years down the line colour wise and interior wise they've equalised, and to the vast majority, I'd say 99% of visitors, really don't know the difference. The exterior visible softwood is painted and the exterior Oak is left natural.
    Oak is nice, but overall design is more important IMO.
    •  
      CommentAuthordjh
    • CommentTimeJul 10th 2017
     
    Posted By: marsadayAny cracks over the next few years in the plasterboard ceiling can be dealt with quite easily.

    Your drawing doesn't show any plasterboard?
    • CommentAuthormarsaday
    • CommentTimeJul 10th 2017
     
    ok, so the PB is under the roof joist and between the purlins.

    What i would like to know is has anybody trimmed and planed oak like this before, or notched the joists so i get a snug fit of the joist onto each purlin. For e.g., how much can the joists be notched out to fit.
    • CommentAuthorowlman
    • CommentTimeJul 10th 2017
     
    Best practice usually is to tilt the purlin to offer a flat surface to the rafter underside. This offers max resistance to bending stresses. Bird-mouthing the rafters at each purlin intersection may well reduce their strength.
    • CommentAuthorbarney
    • CommentTimeJul 10th 2017
     
    Agreed - all your problems are solved by tilting the purlins (as would be usual in a pitched roof)

    If you really want to put them in as you say, then birdmouth the joists and upsize the joist to account for this

    Or use a few manufactured wedges, if you won't see them again - least favourite option though

    The birdmouth need only be big enough to provide the same surface area bearing on the purlin if it were installed tilted - so not much at all

    Regards

    Barney
    • CommentAuthormarsaday
    • CommentTimeJul 10th 2017
     
    Ok so this is called bird mouthing. Thanks for the info. It was my original idea to create my walls with this fall and then set the purlins at this tilt but someone said it would be easier to set them in straight. I need to get some more advise on this but it is good to get some feedback on here.
    • CommentAuthorowlman
    • CommentTimeJul 11th 2017
     
    Setting the purlins straight is maybe easier at the purlin ends where they meet the walls. The difficulty comes when bird mouthing/notching the rafters exactly, so that the bearing surfaces meet precisely. Here are some of the problems:-
    1. the sheer number of notches, how many rafters?
    2. their exact alignment top to bottom on possibly not too perfect purlins.
    3. not the more usual one purlin but three, plus the wallplate at the bottom end.
    4. If you make a template for one end it may not work all the way along, ( see 2 above)
    5. as Barney says you'll have to increase the rafter depth
    6. the fixings into Oak?

    It's do-able, anything is, but you are making a lot of work for yourself, IMO.
    • CommentAuthorCWatters
    • CommentTimeJul 11th 2017
     
    I used a single Genie lift to raise a similar size oak 6m long beam but it was a very unsafe way to do it. Two lifts,
    one each end recommended.

    Are you putting more insulation between the rafters and plaster boarding the underside? If so then you can use some form of packing between the rafters and purlins. Fix battens and the plasterboard will hide everything.

    You could try asking the saw mill to cut the required slope on the top of the purlins that way the packing doesn't have to be tapered. However most likely the beams will still have a bow/curve requiring packing

    If they won't cut a slope on the top you might be able to make a template and cut the same birdsmouth notch on all the rafters and use packing. I think the birdsmouth would only need to be about 11mm deep. For packing perhaps use a selection of different thicknesses of plywood?
      Oak Purlins.jpg
    • CommentAuthorbarney
    • CommentTimeJul 11th 2017
     
    It would be far, far easier to tilt the purlings and fit all the rafters conventionally - and then pack the purlin ends within the wall structure

    That way you ensure purlin and rafter mate properly and it's easy to prop internally if you need to make final adjustments to the purlins - ie you can preload the roof and allow it to settle back into position when you apply all the relevant layers

    As the purlin supports are in blockwork, a bit of internal propping and then packing with a big gob of cement, bit of slate or similar all round them is perfectly normal

    Regards

    Barney
    • CommentAuthormarsaday
    • CommentTimeJul 11th 2017
     
    <blockquote><cite>Posted By: owlman</cite>Setting the purlins straight is maybe easier at the purlin ends where they meet the walls. The difficulty comes when bird mouthing/notching the rafters exactly, so that the bearing surfaces meet precisely. Here are some of the problems:-
    1. the sheer number of notches, how many rafters?
    2. their exact alignment top to bottom on possibly not too perfect purlins.
    3. not the more usual one purlin but three, plus the wallplate at the bottom end.
    4. If you make a template for one end it may not work all the way along, ( see 2 above)
    5. as Barney says you'll have to increase the rafter depth
    6. the fixings into Oak?

    It's do-able, anything is, but you are making a lot of work for yourself, IMO.</blockquote>

    14 rafters

    I have 4 purlins in total. One will be bolted to the rear wall of the house. This will be only 75-100mm in width.

    Then 3 purlins spanning in the room and finally the lintel over the patio door.

    The fixings i think have to be stainless steel screws.
    • CommentAuthormarsaday
    • CommentTimeJul 11th 2017
     
    <blockquote><cite>Posted By: CWatters</cite>I used a single Genie lift to raise a similar size oak 6m long beam but it was a very unsafe way to do it. Two lifts,
    one each end recommended.

    Are you putting more insulation between the rafters and plaster boarding the underside? If so then you can use some form of packing between the rafters and purlins. Fix battens and the plasterboard will hide everything.

    You could try asking the saw mill to cut the required slope on the top of the purlins that way the packing doesn't have to be tapered. However most likely the beams will still have a bow/curve requiring packing

    If they won't cut a slope on the top you might be able to make a template and cut the same birdsmouth notch on all the rafters and use packing. I think the birdsmouth would only need to be about 11mm deep. For packing perhaps use a selection of different thicknesses of plywood?<div class="Attachments" id="Attachments_259645"><ul><div><img src="/newforum/extensions/InlineImages/image.php?AttachmentID=6784" alt="Oak Purlins.jpg"></img></div></ul></div></blockquote>

    Yes it will be 2 genie lifts, but i think a machine with a special lifting belt will be the best thing here. My mate in the past lifted up a 9m long piece of steel on his tele handler when i converted the loft. This was at a height of 6m 5.5m and it stuck out into the road, but we got away with it. I was up a scaffold tower and pushed the thing through a small hole i made in the wall.

    So i think this machine is going to be the best thing to get these beams into place.

    There will be 50mm insulation between the rafter and 100mm over.

    Don't think the saw mill can cut this for me. It has to be done on site.
    • CommentAuthormarsaday
    • CommentTimeJul 11th 2017
     
    <blockquote><cite>Posted By: barney</cite>It would be far, far easier to tilt the purlings and fit all the rafters conventionally - and then pack the purlin ends within the wall structure

    That way you ensure purlin and rafter mate properly and it's easy to prop internally if you need to make final adjustments to the purlins - ie you can preload the roof and allow it to settle back into position when you apply all the relevant layers

    As the purlin supports are in blockwork, a bit of internal propping and then packing with a big gob of cement, bit of slate or similar all round them is perfectly normal

    Regards

    Barney</blockquote>

    Tilting the purlins does sound like the best option, but how hard will this be ? IF i can hire a tele handler for a weekend i think it sounds like a good option because that way i can go slow and build each purlin into place as i fit them. So this will involve mortar having to set as well. Maybe fast set stuff is what i need for these bits.

    Writing this i have just had a thought that i can create my block line with its fall and leave out some chunky sections where the purlin will sit. Say gaps about 250mm wide (150mm wide purlin). I can then tilt the purlin in to place and provide support as you say. The rest of the block work will be strong and so won't budge as will be a few days old.

    Question: Do you think these purlins will need to be bedded on a concrete lintel or could i use a length of steel plate ? I am going to be using a fairly dense concrete block to be making the internal walls.

    Thanks everyone for the feedback, i am getting a better idea on how to tackle this now.
    • CommentAuthormarsaday
    • CommentTimeJul 11th 2017
     
    Just realised my second beam,after the beam screwed to the wall is to sit on a single skin wall on each side. The other 2 beams sit across the walls either side and so can be held in place both sides of the beam. These beams bisect the walls they sit on, but the second beam rest on top of walls which run under the purlin by the usual 150-200mm.

    This second beam cannot be tilted because it will need to sit fully on the 100mm wide wall. So this means i need to set all beams in straight i think.

    On a brighter note i think i can get a tele handler with driver for about £150, so not that much more than 2 x genie lifts. Am going to call tomorrow to get more details.
    • CommentAuthorbarney
    • CommentTimeJul 12th 2017
     
    Tilting the purlins is easy - as I said, connect the rafters and then set the ends of the purlins

    I used to put up a few bits of 5 x 2 (or a couple of scaff boards nailed together with a correct notch in the end of them) to carry the purlin

    Prop in place, cut out the roof rafters, fill in around the purlin ends as you bring up the taper at the top of the wall

    You can probably bed them straight on the blockwork without adding steel or concrete

    If the purlins run through subdividing walls, then they either make no demand on the wall - or continue as the ends of the purlins on the outer walls - same methodology

    Why does the second beam need to sit on the wall - is it acting as a load bearing structure - ie akin to being a wall plate - if so, treat it as such and spilt the rafters

    Again, tilt all the purlins, it'll be far easier for you - if it's not actually a purlin at position 2 and more like a wall plate, then treat it as such or just sit it at the angle dictated by the rafter falls.

    It's a simple monopitch roof basically

    Regards

    Barney
    • CommentAuthormarsaday
    • CommentTimeJul 12th 2017
     
    The no 2 purlin is structural. The design of the current kitchen means this beam has a much bigger span than the others. So i am reducing down the span by creating some return walls either side.

    This will give me a cupboard where the new services will come up in and on the other side it will be my door way into a utility. This door way was going to be at an angle, but i need the door frame to take this load, so it is going to be straight and made of block with a concrete lintel. The purlin will then bear on this door set up and the new wall at the other end.

    I just can't see how i can set this purlin at an angle when it is 150mm wide and bearing on 100mm wide block.

    The other purlins will run for the full 6.5m span, but there will be a utility wall which will break this span up. This is important as the purlins have been calculated to only span 4.5m safely by SE.
    •  
      CommentAuthordjh
    • CommentTimeJul 12th 2017
     
    Posted By: marsadaythe purlins have been calculated to only span 4.5m safely by SE.

    Were the purlin sizes calculated as installed upright or at an angle?
    • CommentAuthormarsaday
    • CommentTimeJul 12th 2017
     
    Flat/upright. But prob haven't accounted for them being planed slightly.
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