How do I position extra joists to raise a loft floor

Discussion in 'Floors, Stairs and Lofts' started by jimjordan, 5 Jan 2011.

  1. jimjordan

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    Hi

    I'm planning to raise my loft floor by 150mm to accommodate more insulation under it.

    The existing joists are 100x47 and I'm going to put 150x47 on top of them.

    Is there a "correct" way of doing this? i.e. can the new joists just be placed on top of the existing ones (i.e. in line with them) with some bracing between them for lateral stability, or should they be installed perpendicular to the existing joists?

    Does it make any difference to the roof structure?
     
  2. freddymercurystwin

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    Everytime! ;)
     
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  3. 1two3

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    Sounds a strange thing to do (to me)? Why would you need to add joists just to add insulation? Does the loft area already have a habitable room up there, or need to support a "proper" floor?

    From timber sizes you've quoted sounds like a fairly new place, but regardless if there are any supporting walls below (i.e.brick or block), particuarly if the supporting walls run across the middle of the house (as is fairly typical in many an old terrace or 3 bed semi) then it's often a better job to install (or raise) a timber wallplate along the supporting wall(s) so that your raised floor level doesn't rest on the ceiling joists (else you'll very likely get cracked ceilings at some point)and run your joists perpendicular to that with the ends resting on appropriate rafters/purlins (or joist hangers may suffice if it's purely for storage).

    If it's all timber up there (apart from perimeter walls obviously) then the above doesn't apply but you may still be able to raise timbers off the top of stud walls providing it's for storage etc.
     
  4. freddymercurystwin

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    One assumes he's gonna add some boards to allow some storage up there! :idea:
     
  5. jimjordan

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    Thanks

    It's just for storage (roof pitch to shallow to suit full conversion for anyone over 4ft tall). The existing joists are well supported by the block wall structure underneath.

    Just want a minimum of 250mm insulation under 18mm chipboard panels so I've got somewhere to stash luggage, "heirlooms" and wedding presents we wish we'd never put on the list ;)

    There was already a poorly laid chipboard effort on the existing joists, but had to rip that up to get at the lame 50mm candyfloss insulation below to replace it.

    Out of interest - why the preference for perpendicular? Is it better structurally (better load distribution? stability?) or is it just easier to install?
     
  6. Static

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    Better structural as you say for load spread.. cos its all just dead weight you want to spread it over as many joists as possible..
     
  7. John1million

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    Hi Jim,

    I'm in the middle of doing the same job. I'm not a carpenter but I'm doing it myself after getting structural advice from my brother, who is an architect.

    All you need to do is to make sure the boards you are placing above the joists are of exact equal thickness... 35 mm in my case. Six inches is ok to raise them once you fasten them well. I chose to raise mine 4 inches (100 mm) and I managed to comfortably fit in double sheeting of insulation roll without squashing it.

    From my experience so far there are 7 points I would make.

    Firstly, you don't need to raise the joists all the way across. Only raise them where you will walk. So effectively, you are going to raise the timbers in the middle (between the rafter-supports); about 2 meters wide; or wider maybe, depending on your house. You wont ever be walking out to the extreme corners so you don't need to put in flooring there.

    Secondly, measure your joists meticulously. If you get the measurements right, then you've made an important start.

    Thirdly, remember to mark any spaces to cut out to allow any wires or water piping that might need to run through the joists (currently running over your initial joists).

    Fourthly, fasten the joist extensions onto the original joist below by using standard brackets. Two or 3 max each side will do.

    Fifth, and this is an important one. After putting in your first two joist extensions, put in what's called a strutt to connect them. By this I mean a plank of timber linking the two joists. Cut it to measure the distance between the inside of the two joists. A piece of timber four inch by one inch will be fine. Position the strutt on the inside of the two joists flush with the top of the new joists or even a centimeter or two below. Screw in a long screw throught each joist and into the end of each strutt to hold it in place. Strutts serves a special purpose... they stops your new joists from swaying. It will make them rock-solid. Stagger the placement of your strutts for easy access.

    Sixth. (and maybe I should have said this at the start) When you're doing your work up in the attic, have a wide board of timber up there to kneel on that will allow you to get relatively close to your work. And it also gives you somewhere to leave your screws, brackets, screwdriver-drill etc. Dont try and do the work by balancing on joists. That's messy and your knees will be in bits. Once you've three or more new joists in you can bring up some flooring and you'll begin to work a lot quicker (and you'll get a great sense of achievement seeing a few boards up). Don't screw them in yet though, you'll need to move these around as they'll be your movable floor, as you progress from one end of the attic to the other.

    Seventh. When you're putting in the actual insulation rolls, just roll them under the strutts. That's a very quick process. Then you're ready to place in all your floorboards.
    Let me know if that helps. I'm based in Dublin, Ireland.
     
  8. John1million

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    Hi again Jim,
    I may have used a misleading word at the start of that post. I meant to say, make sure your new joist extensions are the same thickness exactly as the original ones. I might have confused the issue by calling these boards.
    Good luck.
    John
     
  9. John1million

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    Jim,
    Just realised I didn't answer your perpendicular dilemma.

    I considered doing this but I decided against it after speaking to people I trust on the subject, for 3 main reasons.

    Firstly, if you are putting in perpendicular lines of timber, that means you will have to lay one layer of insulation fully, before coming along and putting in a second line of timber, and then rolling out a second layer of insulation in that direction.

    It is not a simple process but not impossible.

    Secondly, if you put up an elaborate network of criss-crossed timbers you might have a problem accomodating all the wires and pipes that are up in your attic. You could have to cut out a million and one little grooves to accomodate the wires. Basically, all the existing wires and pipes will run over your existing joists but under the joist extension. This is easily accomodated with by cutting out the odd groove here and there. But it's a lot simpler when all the timbers go in one direction. With perpendicular lines, you will effectively have a series of squares about 20 inches each way, that's a lot of little grooves to allow one wire cross the span of your ceiling. And if you have a leak with a pipe, your plumber might have to rip up a lot of timber to get access to it if the timbers cross each other.

    But thirdly, and perhaps the main reason I did not do this, was because of structural strength issues. The existing joists are laid to disperse the load onto the exterior walls of a house. If you cross these with perpendicular timbers, then they will run quite a distance before the load is transferred into a wall, in most cases.

    And you don't lose any strength by raising your existing joists, in fact all you are doing is giving additional strenght to the original joist.
     
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  10. jonwestuk

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    Apologies for resurrecting an old thread, but it felt better to keep information in one place.

    Can anyone point me to the type of bracket that is used to join the two joists together (on top of each other?). Are we talking about a camplate?

    Thx

    Jon
     

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