Raising height of loft joists - can't find complete answer!!

I know there are several posts on this but I don't feel I quite have the full answer for my situation so please bear with me. My terminology may be wrong so please excuse that also.

I have the Warmer Home people coming to lay 270mm of loft insulation on the 16th Dec. I want to board a section of the loft and to allow for the insulation I need to raise the height of the joists.

The diagram below shows a cross section of my roof space.



The blue section I believe to be known as the roof trusses and these are fixed to the front and rear outer walls of the house(?). All my first floor internal walls appear to be partition walls so I don't think there is any support to the ceiling. I can only assume therefore that the rest of the structure i.e the orange sections (name?) and the joists (brown hatched sections) are all 'hanging' from the trusses - is this correct?

The 2 small red rectangles are timbers that I originally thought ran from one side of the roof to the other but on emptying the loft I discovered they stop a few feet short of one wall - what are these for?

The part I want to board is the mid section of the diagram to cover the width of the house and I will be using the chipboard flooring packs from B&Q. This will be for storage only but probably to be accessed on a fairly regular basis.

My original plan was to fix 100mm x 50mm directly on top of the existing joists (which are 100mm x 30mm) using something like mending plates along one side (couldn't do other side as new timber will be wider and over hang) but with the apparent lack of support from internal walls I'm wondering if this is not a good idea.

Can I go ahead with this original plan or should I consider strengthening first and if so please advise how?

Many thanks for your patience on this subject that does seem to keep popping up!!!

Are these a gang-nail plate truss frame as shown above? (approximately 38mm thickness).

The red section are ceiling binders to stop the ceiling bowing or twisting.

The truss frame which rest on the outer wall are wallplate, so it's best to run another joist from there and to your next loading bearing wall so the stress load is coming off between the 2 wallplate.

If you have 4"x2" traditional roofing then get back to us, looking at the design I'm guessing you have gang-nail roof truss frame.

Thanks for taking the time to reply masona.

The dimensions of the timbers are as follows. The main part of the truss (blue in diagram) is 120mm x 35mm. The 'inner' structure (orange in diagram) is 70mm x 35mm. The horizontal joists (hatched brown in diagram) are 100mm x 35mm (not 30mm as previously stated).

These are all connected together with plates which I guess are what you are calling gang-nail plates (see pics below).



Could any of my internal walls have load bearing properties? A quick tap of each them and it is obvious they are plasterboard. If they do not add strength surely I cannot run continuous new joists from the front to the back of the house as it is a length of about 9m?

The current joists are in 2 lengths joined in the middle with plates. Could I do this with the new ones and run them along side the existing? If so, I need to clear more than 270mm so as not to squash the insulation - would that mean I'd be looking 270mm x 50mm or there abouts and I guess attach them to the existing joists (side by side) for stability?

Many thanks.

Actually thinking about I can't run any new lengths that are resting on each wallplate because the ceiling binders are in the way! They would just end up sitting on top of the ceiling binders rather than resting on the wallplates.

Would it be sufficient to attach new larger joists to the side of the existing but only between the 2 ceiling binders?

Don't worry about the ceiling binders for now, you need to find out where are the walls are underneath, are they blockwork or timber studwork with plasterboards? The walls you're looking for are in the same direction of the ceiling binders, then use a very small thin philips screwdriver or thin masonary drill into the top corner of the wall to locate the wall position through the plasterboards ceiling (be careful of pipe or cable etc) these holes can be fill in easily, if you have coving then it will be easier to have a long thin masonary drill bit for this through the centre of the coving! Working 2 people is also a bonus (1 in loft and 1 underneath) for tapping sound location. I never owned a stud detector tool so not sure how good they are.
Let us know how you're getting on then we can tell you the next step.

Your new insulation will be great, but it's going to swamp all those timbers. Will they be laying it as a huge blanket?

Unless you squash it under the new floorboards, you'll need to extend the height of the "joists". This is separate from the weight-bearing properties of the roof trusses.which you're already looking into.

JohnD said:

Unless you squash it under the new floorboards, you'll need to extend the height of the "joists". This is separate from the weight-bearing properties of the roof trusses.which you're already looking into.

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Yes, you're right but no need to insulate under the new raise floorboards area as long as there's air-flow underneath and the floorboards will act as a insulation and storing rubbish on top of it .

From the loft I can easily see the posistion of the wall that splits the front and back bedrooms as there is a timber that is visible that runs the same direction as the ceiling binders. The joists run across this timber which is pretty much down the middle of the area I want to board.

The wall is definately timber studwork with plasterboard.

JohnD said:

Your new insulation will be great, but it's going to swamp all those timbers. Will they be laying it as a huge blanket?

Unless you squash it under the new floorboards, you'll need to extend the height of the "joists". This is separate from the weight-bearing properties of the roof trusses.which you're already looking into.

Click to expand...

I am trying to work out the best way to raise the height of the joists (see the posts above) to at least 270mm which is the thickness of the insulation. I'm hoping they will be laying the insulation in between the joists and because it's so cheap (£250 for cavity wall AND loft insulation!!!) I will end up with insulation beneath the new boarding.

I think you'll find they just flop it over the top. Think of the labour costs if they did any fitting...



I did mine myself (though I'm a very slow worker) and ran the extra joists at right-angles to the existing. This meant I could leave 100mm between the old timbers, and put 100mm between the new, and they also spread the load of me trampling about and storage within the loft when I boarded over them. Those gold bars are pretty heavy, you know

The insulation comes in rolls that they can just easily unroll between the joists so it's not exactly 'fitting'. A surveyor from the company has already been round and looked at the loft so they know which width rolls to bring.

If I raise the joists to 270mm before they come they can't just flop it over the top without it being a very poor job and if they do that they will be getting a phone call from me to get them back round to do it right! I'm probaly going to take the day off work to be there when they come round anyway so I can keep an eye on them!

Raising the "joists" as you describe will double the beam depth, and make them much stiffer and stronger, which is a good thing. Take care not to have joints in the new corresponding with joints in the old timber.

lovinit said:

The wall is definately timber studwork with plasterboard.

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Okay, when you locate the studwall top wallplate cut out a small section of the plasterboard from the loft say 3"x3" above the wallplate and put in a 3"x3" 18mm plywood or a number of thinner plywoods packing so it's above the plasterboards or past it. Need to do this at each end for the new bigger joist to rest on so it not touching the plasterboards. You can do number of joists this way or just on each end and use joist hangers in between the outer joists, don't know if I'm making sense here. Where the joist hit the ceiling binder, cut out the joist width for it to go through then use L-shape bracket to join the ceiling bracket from the binder to the new joist. The floor joists is ready for the flooring. This way the flooring is supporting of the walls and not off the truss frames. The new insulation butt up to the outer floor area and not underneath. Also the water tank should be done the same way as it's very heavy.

masona said:

The new insulation butt up to the outer floor area and not underneath. Also the water tank should be done the same way as it's very heavy.

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What do you mean by the new insulation butting up to the floor area? Are you saying the insulation should not go under the new boards I lay? Is there a problem if it does? No water tank to worry about as we have a combi boiler.

The rest of your advice makes sense - thanks for that.

lovinit said:

What do you mean by the new insulation butting up to the floor area? Are you saying the insulation should not go under the new boards I lay? Is there a problem if it does?

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As long your leave a small gap under the floorboards for air-flow to prevent condensation you should be okay. Don't forget the floorboards will be acting as a insulation.