Designing my loft boarding and supporting joistwork

dlockers said:
need to mess around then. Get new wood, lay it on top of existing wood, use structural screws and clad with OSB.
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I don't think it's a good idea to do this blind or assume it can be done without knowing what the original 3x2s are themselves sitting on or supported by.

The span in the large section to the right of the hatch is 3.6m. the span in the section to the left of the hatch is 3m. That's quite long to be putting load on 3x2s in the centre of that span.
 
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dlockers said:
OK well job one is to check that out surely?
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I've tried to find some information on how to get to the wall plate in the eaves, but can't find much. If I could access the solid 9" external walls of the house it would be much easier to run new joists directly off this to the internal supporting walls.

Any ideas whether this is possible?
 
danlightbulb said:
I've tried to find some information on how to get to the wall plate in the eaves, but can't find much. If I could access the solid 9" external walls of the house it would be much easier to run new joists directly off this to the internal supporting walls.

Any ideas whether this is possible?
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Can't you just follow the joist with your hand?
 
jpaul said:
probably many like it https://www.mycarpentry.com/joist-span-tables.html
showing, for uninhabitable space, celing deflection as a function of ceiling weight, joist span/size/spacing, wood type
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I've been using these tables which is for floor joists but if I design it for floor joists I know it will be plenty strong enough for loft storage. I then might also be able to reinforce the roof rafters as well and remove some of the purlin supports to get more usable space.

Free UK Span Table for Domestic Floor Joists to BS 5268-7.1 (C24, 1.5 kN/m² load) - Timber Beam Calculator

www.timberbeamcalculator.co.uk www.timberbeamcalculator.co.uk

The issue I have is not the spans or what size joists to use, it's how to actually support them i.e what are the joist ends actually bearing on.

I have plenty of supporting walls in the middle of the loft if you look at my diagram, but they are at different angles to each other so I can't just make a nice simple square section of loft boarding.

I could really do with putting joist ends on the external walls but I can't see how to do this as the roof rafters come down in a triangle shape making the eaves very tight/inaccessible.
 
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dlockers said:
How come you are assuming the current joists are not adequately supported?
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Because I can't see any large beams up there. I'll know more when the insulation is lifted.

If the 3x2s are spanning between brick walls then sure I could add to them. But I have a feeling that in some places they are at least partially supported by a timber hanging from the roof rafters. You can see this in the picture in a couple of places.
 
danlightbulb said:
I found a YouTube video of what I would like to do ideally, which is to access the brick wall under the eaves and fit new joists directly onto that.

youtu.be

Creating Loft Storage Floor Space UK - Part 2A (How To Fit Floor Joist Under Eaves)

Welcome to Part 2A of my loft storage space project.In this short episode, I'll show you how I prepared to fit floor joists beneath the eaves (sloping part) ...
youtu.be youtu.be
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At the point you find out the ceiling joist is sitting on the wall plate though, you aren't achieving anything other than cost.
 
dlockers said:
At the point you find out the ceiling joist is sitting on the wall plate though, you aren't achieving anything other than cost.
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It would be costly whatever I do though, even if I double up on the existing 3x2's, to get to 270mm depth for the insulation I'd still need to fit 8"more joist on it.

Pretty crazy joists needed. If I go direct off the wall plates that would mean I'd need 11x2 joists which can't be right.
 
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danlightbulb said:
It would be costly whatever I do though, even if I double up on the existing 3x2's, to get to 270mm depth for the insulation I'd still need to fit 8"more joist on it.

Pretty crazy joists needed. If I go direct off the wall plates that would mean I'd need 11x2 joists which can't be right.
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If the 3x2s are good just use loft legs (y).
 
dlockers said:
If the 3x2s are good just use loft legs (y).
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I'm not happy with using loft legs on some of those bigger spans - some of my spans look to be over 3m, I'm not comfortable boarding direct on 3x2s over those kind of spans. It's way off what the span tables say is acceptable.
 
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danlightbulb said:
I found a YouTube video of what I would like to do ideally, which is to access the brick wall under the eaves and fit new joists directly onto that.
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seemed like the guy compromised the strength of the new joists because they are shamfered down/diagonal cut, to fit on top of the packers on the wall plate.

I agree 3x2 alone don't seem adequate, unless you will be very delicate about how you move around and what you place in loft space.

Hagar said:
Loft legs are good for 25kg per square metre. Weight of person 75kg. Also flooring boards laid directly on top.
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... so you are saying that is adequate , must depend on how well the floor boards are transferring that 75kg across themselves. ie need to be >1"
 
Hagar said:
Loft legs are good for 25kg per square metre. Weight of person 75kg. Also flooring boards laid directly on top.
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I think that would depend on the loft legs you are using but take LoftZone stuff for example, they are way stronger than the joists they are screwed on to. However they recommend storing no more than 25kg/m2 as that's the British Standard rating for modern truss roof houses. Older houses my have stronger roofs.

Another benefit of the plastic loft legs is they won't be a thermal bridge like timber will - this is a huge factor and I wouldn't underestimate it.

The previous owners of our house really messed up when they boarded the loft. They hid the bathroom vent under the loft boards (moisture nightmare, surveyor didn't even spot this), nailed the loft boards straight to the ceiling joists compacting what insulation there was. This created a huge thermal bridge for the warm air of the house to the cold air of the loft space to condense - this warped all the loft boards and caused most if not all the upstairs ceilings to crack. Some are cracked in the middle of the ceiling or where the wall meets the ceiling or coving, which can be a sign of "truss lift". I probably made it worse with my good intentions - I had new insulation laid to the two thirds of the loft which wasn't boarded but the installer covered all the soffit vents. Then had the leaking soffits and facia boards replaced and those installers again covered the soffits with the insulation. This all trapped the moisture from the thermal bridging which of course condensed and created more grief. I've since cleared the insulation from the soffit vents but the damage has been done.

All the existing loft flooring and insulation is being ripped out this spring and I'm intending to use plastic loft legs as I'm now acutely aware of the damage thermal bridging causes.

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Mysterae_ said:
I think that would depend on the loft legs you are using but take LoftZone stuff for example, they are way stronger than the joists they are screwed on to. However they recommend storing no more than 25kg/m2 as that's the British Standard rating for modern truss roof houses. Older houses my have stronger roofs.

Another benefit of the plastic loft legs is they won't be a thermal bridge like timber will - this is a huge factor and I wouldn't underestimate it.

The previous owners of our house really messed up when they boarded the loft. They hid the bathroom vent under the loft boards (moisture nightmare, surveyor didn't even spot this), nailed the loft boards straight to the ceiling joists compacting what insulation there was. This created a huge thermal bridge for the warm air of the house to the cold air of the loft space to condense - this warped all the loft boards and caused most if not all the upstairs ceilings to crack. Some are cracked in the middle of the ceiling or where the wall meets the ceiling or coving, which can be a sign of "truss lift". I probably made it worse with my good intentions - I had new insulation laid to the two thirds of the loft which wasn't boarded but the installer covered all the soffit vents. Then had the leaking soffits and facia boards replaced and those installers again covered the soffits with the insulation. This all trapped the moisture from the thermal bridging which of course condensed and created more grief. I've since cleared the insulation from the soffit vents but the damage has been done.

All the existing loft flooring and insulation is being ripped out this spring and I'm intending to use plastic loft legs as I'm now acutely aware of the damage thermal bridging causes.

embed
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Great post (y)
 
jpaul said:
seemed like the guy compromised the strength of the new joists because they are shamfered down/diagonal cut, to fit on top of the packers on the wall plate.
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Yeah true, but how else can you get an extra joist into the very tight triangular space to sit it on the wall plate. It would be good to find out how much strength this loses from the joist.

As per my diagram in post 1, I have the option of using a ledger board on the party wall and then spanning across to an internal brick wall. But, using this method I can't board a very big area and the loft hatch area is an issue because of where it is and the orientation of the internal supporting walls.

Hagar said:
Loft legs are good for 25kg per square metre. Weight of person 75kg. Also flooring boards laid directly on top.
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Exactly, and in the centre of a 3 m span on a 3x2 that is creating a very high point load in my opinion.
 
jpaul said:
... so you are saying that is adequate , must depend on how well the floor boards are transferring that 75kg across themselves. ie need to be >1"
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Far from it it is inadequate for most things other than empty suitcases or maybe with pillows, duvets in.
 
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