Trimming joists

[duffsparky]

Hi All,

I've got a few questions about floor joist trimming and wondered if someone could help.

Attached is a drawing detailing the existing joist layout, the trimmed joist and additional strengthening joist layout required as a general beefing up of the floor prior to making a partition wall and bathroom mods.

I have attached a drawing which hopefully will make answering my questions a lot easier. Each joist on the drawing is labelled with a lower case letter starting at (a) and info for each joist is listed below.

Joists:-

(a) is a new stub joist to support the centre of joist (b)
(b) is a new trimmer joist to support the end of a new section of partition wall to be built over it.
(c) is a new trimmer joist fixed to joist (e) to support trimmed joist (j) and new joists (h), (i) & (k).
(d) is an existing 38 x 190 joist to remain as is but may require sistering.
(e) is a new trimmer joist fixed to joist (c) to provide support for trimmed joist (j)and new joists (h), (i) & (k).
(f) is an existing 38 x 190 joist to be beefed up by installing a sister joist.
(g) is a new 47 x 175 sister joist installed to beef up joist (f) and generally strengthen the floor prior to making mods as previously mentioned, there will be an additional 4 off such joists, to the right of the drawing, all for a similar purpose.
(h) is a new 47 x 175 joist installed to provide support for a new section of 75mm cut timber with 2 off 12.5mm plasterboard partition wall to be built over it.
(i) is the same as joist (h)
(j) is the existing 38 x 190 joist to be trimmed and beefed up by installing a sister joist.
(k) is a new 47 x 175 sister joist installed to beef up joist (j) and generally strengthen the floor prior to making mods as previously mentioned.
(l) is an existing 38 x 190 joist to remain as is.
(m) is a new 47 x 175 joist installed to provide support for an exiting 50mm honeycomb wall or a new section of 75mm cut timber partition wall to be built over it if the honeycomb section proves too difficult to modify.
is an existing 38 x 190 joist to remain as is.
(o) is an existing 38 x 190 joist to remain as is.
(x) is the trimmed section of joist (j)


I want to install a trap in the floor, as indicated on the drawing, that will probably be fixed to the left-hand part of trimmer joists (b) & (c) as seen on the drawing, the upper part of joist (d) as seen on the drawing and a noggin fixed to the underside of the new partition wall. This will give access to a shower waste trap located on the far side of the new wall.

Please could someone look over this layout and advise if anything is missing and or anything that need not be installed?

Should I install a sister joist along side joist (d)? If so, could I install it on the LHS of joist (l) and put in very short full height noggins at each point I nail or bolt it to joist (d)?

When fixing the trimmer joists in place can I simply skew nail them through the other joists or must they be fixed into place using joist hangers? If so, what size should I use?

All of this is probably as clear as mud, but any help would be much appreciated.

Regards,

duffsparky.

Joist trimming

• duffsparky
• 14 Aug 2009
• 2

 

[Norcon]

My advice is get a joiner in.
My eyes glazed over on the first sentence.

 

[^woody^]

Try and summarise the post

From what I can make out, you don't need any double joists, nor the trimmer doubled as the spans are very short.

But do you need a double joist to support the stud walls above?

Nailing joists into the end grain is not an acceptable means of support, so you need to put the joists on hangers. And double joists should ideally be bolted and use timber connectors (ie spurs) or use coach screws, but for small loads, you could bang a few nails in instead of bolts.

 

[duffsparky]

Thanks woody,

Part of the reason for doubling up the joists is:-

• The building control officer wants the floor strengthened due to relocation of the bath.
  Deflection in the floor appeared to be high in the area of the original stud wall.
  The floor appears to bounce when walked across which makes things rattle.
  The original bathroom and en-suite floorboards are chipboard but not bathroom grade and over the years have become rotten in several places. Rather than just replacing the rotten parts I'm replacing the lot for bathroom grade P5(?).

The BCO agreed doubling/sistering the joists should be relatively easy if the floor was being replaced anyway, and it should improve/solve the various problems.

If I bolt the joists together what size bolts/studding should I use???
If the timber connectors/dog tooth washers come in different sizes, what size should I use???

Regards,

duffsparky