Coach Bolting spans of 6" x 2" joists - how long i

[grant_at_diynot]

I have a span from wall to wall of 6200 and I am using 6" x 2" beams as a 'sister joist' system to help support my new loft floor - these joists are basically laid next to the existing joists at 600mm centres.

I can only haul 4 metre lengths into the loft and therefore will need to overlap the new joists and coach bolt them together.

What I need to know is: how long must the overlap be to make up the full 6200 with my 4000 joists

My Structural Engineer will charge to supply the info so I have been hoping to gain a 'rule of thumb' as this is purely an addition to a very sound structure - I also believe this will eliminate any potential deflection and help with muffling footfall noise etc.

Thanks

Grant

 

[Mr Winston]

spanning 6m+ with 6 x 2 for a floor is the stuff of fairy stories (probably need something approaching 12 x 2) - its time to speak to your eng. - if hes spec'd these as supplementry to the existing floor surely hell let you know how to lapp them


Sound wise if you have more solid contacts between floor an ceiling it will make footfall/impact noise worse

 

[grant_at_diynot]

OK, forgot to mention that there is a load bearing wall that runs the full length of the roof space in at 1500 so really the majority of the span is only 4700.

The main idea is to raise the new floor level and having most of the load taken up by the new system which will not be connected to the ceiling below as are the existing 8" x 2" joists at 300 centres.

Would bolting the new joists to the existing joists help in their capacity to bear the load? This would obviously lessen my aim to eliminate direct impact on the exist. joists, but I am trying out a way to figure how I can do this without the obvious route of installing an rsj...

Grant

 

[masona]

Any photo?

 

[Mr Winston]

6 x 2 floor joists bolted to the existing 8 x 2 joists (overlaping the bottom 100mm of the former and the top 100mm of the latter) - is this what you mean?

 

[grant_at_diynot]

The existing 8"x2" joists are currently at 300 centres. I aim to place a new joist of 6"x2" next to every existing joist but 1.5" higher than the existing joists as I wish to raise the floor level slightly.

Thus the new joist system will mimic the exisitng, except they will not be complete spans but will have to be coach bolted at some point as I cannot get more than 4 metre spans up through the hatch (the stairwell will be cut last, hence everything has to go through the hatch).

Further to just laying them from one wall plate to the other. I am wondering if I have to bolt the new "sister joists" to the existing joists as you have indicated they may not be robust enough for such spans as I have mentioned...

Yes, I will take a photo as soon as I can

 

[Mr Winston]

To clarify

Q. Is the loft space already a room? (If not there are masses more implications than just the floor)

If not it sounds like the 8 x 2 are just ceiling joists (not floor joists which need to take much more loads)(8x2 can normally span approx 4m for just floor loads - hence these look to small for 4.7m but this is based on 400c/c rather than yours @ 300c/c). This doesnt allow for any loads imposed by studwalls picking up rafters etc)

If you bolt the new joists to the existing this will just put the load directly onto the 8x2s via the bolts - if you need to raise the floor level how about laying 2x2 across the top of the existing at 90 degrees (just need to check the joists are ok for the extra loads) The span of the 2x2 would only be 300mm between each joist.

 

[grant_at_diynot]

Mr Winston

I am in the process of developing the loft into a bedroom with en-suite.

I think you have hit the nail on the head!

2x2 across at 90 degrees sounds like the right way to go. I was also recommended this by a timber yardsman at the builder merchants down the road.

So, to make sure I have understood, I space these 2x2 at 300mm centres?

How can I deminish footfall / impact noise going through to the rooms below?

Many thanks - I am seeing the picture now...

Grant

 

[Mr Winston]

you can reduce speech sound by adding fibreglass quilt between the ceiling joists - but footfall would require something like an independant ceiling below or adding a resiliant layer below the floor or adding extra plasterboard to the ceiling below making it denser.

What are you doing for fire regs? You do have building regs to address - im not convinced that the 8x2 will be ok as floor even at 300c/c.- your engineer must calc this out. theres no real rule of thumb as every calc is specfic to the job. do you fondations need underpining for an extra floor?

get in touch with your local authority bco for advice

 

[grant_at_diynot]

Yip! I had a SE do the calcs and all is well - I can put 18mm T&G chipboard directly on these exist. joists.

I can go ahead with 100mm (60kg density) wool fibre suspended on chicken wire between the joists and my local Build Control have accepted this. This apparently takes care of Sound and Fire Regs.

I am simply trying to go the extra mile with soundproofing as I am still at the stage where the floor is not down yet and thought that raising the floor would help - do you think I should place the 2x2 timbers sitting on the exist. joists on rubber washers?

My idea was also to have 6x2 beams (not bolted to the existing 8x2 joists) running next to every second existing joist . These new 6x2 beams will be raised to the new height of 2" above exist. joists.

In other words, these will be 2x2 trimmers on top of the existing beams, as well as new 6x2 running alongside the existing joists and the 2x2 trimmers will also be mitred into these - I am thinking of dropping this latter idea though and just going for the 2x2 trimmers running perpendicularly to the existing joists...
Any guidance would be great!

Grant