New floor joist advice

[jonnyg2k6]

Have removed a large wardrobe/stud partition underneath a supported purlin in my bedroom to open up the room into the eaves on the 1st floor of my dormer bungalow, and gain around 1.5m of floor space either side. I now need to add floor joists as there are currently only ceiling joists where the wardrobe was. My plan was to attach a wall plate to the two supporting walls and hang the new floor joists along side the ceiling joists on jiffy hangers. See pics for the wall plate plan for both the left and right sides of the room.

My main issues are this:

1. How to bolt wall plate to wall – especially on the left side where I have to go around a chimney stack so wall plate will have to be 2 small lengths.
(I plan to cut out turrets so it sits on the ceiling joists as well as bolted to the wall – advisable?)

2. Should you screw the old ceiling joists into the new floor joists to give the ceiling joists support (previously a few hangers provided the support.)

Thanks and hope someone can advise or maybe even come up with a better solution…..??


View media item 43399 View media item 43400

 

[tony1851]

You don't say what the span of the proposed new joists is (ie distance wall to wall).

 

[jonnyg2k6]

they will be 3.2m span. they will be 175mm x 50mm joists. Its getting the joists stuck to the wall is my main concern......

 

[tony1851]

I only asked re. the span of the joists because the bigger the span, the greater the load the new floor is carrying, and then the more care needed in fixing your short (supporting) timbers to the walls.
The blocks look like aerated concrete, so instead of putting bolts in them, i would consider using 4" framing screws, perhaps 8 or 10 on the flat length of wall; this spreads the load over a bigger area of blockwork and reduces the stress on the weak blocks.
Caution; you cannot screw a timber support piece directly to the chimney shown in your r.h.pic. unless the chimney is not in use and permanently blocked up downstairs. You have to maintain a 2" gap between the chimney face and any structural timber (Building Regs). Suggest putting a joist-hanger on to the existing floor joist (the one with the cables on - they would have to be re-positioned) and a trimming timber across to your new joist adjacent to the chimney, to maintain a gap.
Strictly, this is a Building Control matter.

 

[jonnyg2k6]

OK great thanks tony, so the left Pic looks fairly straight forward with timber screwed to the block with 4” screws and hangers for the joists.

The other side I have attached a diagram of what I think is what your saying???? and ensuring the timber is a good 2” from the chimney stack….. how does this look?
View media item 43450

 

[tony1851]

Yes, that's it. Good luck.

 

[jonnyg2k6]

Thanks tony. Any suggestions regards insulation of the roof in pic one?

 

[tony1851]

If you insulated it to comply with Building Regs. you would loose a lot of headroom.
An alternative could be; 1" Kingspan board between the rafters, with the board-face level with the room-side face of the rafters, and then 1" or 2" thick Kingspan running across the underside of the rafters. The plasterboard (preferably foil-backed) would then go over that.
It is important that you maintain a 2" air gap between the felting and the outer face of the Kingspan to allow adequate ventilation. Your rafters look to be 3" deep so a 1" thick board will maintain that gap.

 

[jonnyg2k6]

Thanks again. Would your suggrstion be inline with the building regs? If not, what do they specify......

 

[tony1851]

No; Building Regs would require the equivalent of;

3" insulation between the rafters and 2" below the rafters. To maintain your 2 " airgap, you would then need to add 2" thick battens along the underside of the rafters, giving them an effective depth of 5" (ie 2" + 3").
Then there is the board underneath, and the plasterboard. All this adds to the thickness and reduces the headroom.
There are other insulants available which might appear to offer reduced thickness, such as multi-foil products, but these require air gaps on both sides and in the end probably save comparitively little space.

In some circumstances, Building Control can accept a lesser standard of insulation where space is limited, but you need to be able to prove this by calculation. Personally I'd steer clear.

 

[catlad]

To help with ventilation before you insulate I would buy some plastic 22mm over flow pipe and cut them up into 6" pieces, and stick them between the overlaps in the felt.

 

[jonnyg2k6]

Thankyou all for your help and advice, very much appreciated.

 

[ThamesDesignandBuild]

I might be missing something, but I'd have thought you'd be better off hanging your new floor joists on masonry hangers. Then there's no load being applied to the ceiling members

 

[jonnyg2k6]

Which in your opinion are easier to install? The block to the right and left is cinder block would this have any influence?

 

[tony1851]

It's possible to use joist hangers direct into the wall, though you might have a problem getting them level and also making sure that they are firmly mortared in. Also, if the blocks are cinder blocks, its possible they may be hollow with quite thin walls (possibly not much more than an inch) and the joist hanger might cause them to crumble where it rests into the wall.
We've specified joist hangers many times when situations warrant them and they are fine but, in this particular instance, I think you will find it easier to level and fix your short piece of timber (ie the one fixed to the wall) and fix the main floor joists to this with the bendy joist hangers. (If the blocks are in fact hollow, several screws spread over the length of the bearing timber will help distribute the load over a wider area of block and hence reduce the stress).