Increasing Rafter Tie depth

[Ben565]

Hi, I'd like to install 300mm of insulation in my loft and board over the top of it. Problem is the ceiling joists (which are actually just rafter ties) are only 3"x2" with a 12' span.

A couple of friends suggested running lengths of timber at right angles accross the top of the ceiling joists. Wouldn't that just add extra weight to already rather thin ceiling joists though?

I don't see why I cannot just add 9"x2" joists above the old ones and attach each end to the rafters. Basicly adding new joists above the old ones, but leaving the old ones in place.


The loft space is far to small to be used as a room or any kind of conversion, just light storage and maintenance access to the underside of the roof.

 

[catlad]

I would put a length of 3x2" as close to the eaves as you can get on both sides of your roof then one in the middle of your loft that would normally sit over the central wall use some lengths of 6x2" to build a new floor using the 3x2" to support the new floor and splinting the 6x2"s in the middle as they cross the central 3x2" that would give you more room for insulation.

 

[Tony13579]

I just upgraded my insulation using 50 mm celotex boards. Then I refitted the loft boards over the celotex. The loft floor is now fantastically level. The load is spread. The celotex has twice the rating of Fiberglass type insulation.
I used 90mm screws into the joists. I used a plaster board saw £3 wilkinsons.
I converted all the down lights to mr16 led down lights. (No heat)

You could achieve almost the same thermal value with 3" fiberglass and 100mm closed cell insulation boards.

 

[noseall]

The celotex has twice the rating of Fiberglass type insulation.

....and only a fraction of the practicality where filling gaps is concerned......

 

[Tony13579]

I wouldn't put it between the rafters but over the rafters worked well and was easy

 

[noseall]

I wouldn't put it between the rafters but over the rafters worked well and was easy

How did you go on at the eaves and celing binders etc?

 

[Tony13579]

There are no A frames in this loft, but you could use silver foil to create an envelope
I cut notches for the some rafters. But in general I went within 4" of the edge of the loft. I still put around 40 square meters of insulation down with chipboard floor over the top with staggered joints. I only needed to screw down the loft hatch boards and outside boards. If there was a nail or piece of metal work in the way a good thump and the board made way for the hanger.

The upper loft now has 4" fibreglass + 50mm foam + 18mm chip board

The side walls of the loft room have 4" fibreglass, +heavy polythene membrane + 50mm foam boards + silver taped joints.