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Opening up roof structure to living space to roof pitch / removing ceiling and ceiling joists
We wish to open up the kitchen / living space to the rear of our property by expanding the space into the existing roof space. In other words - remove the ceiling and ceiling joists and allow the pitch of the roof to be the 'new ceiling'.
This is a single pitch roof which slopes from party wall to our own external wall. The space covered by the roof is 3.3m approx width in plan x 5.7m in length. The highest point of this roof space as existing is 1.5m approx in the attic space above ceiling joists.
There are vertical struts every 1.5m or so at mid point in the pitch giving support to the roof - but the roof / ceiling joists are not prefabricated - the property is Edwardian circa 1900.
My hope is to avoid steel work, but simply replace existing timber roof joists with joists deep / structurally strong enough to span from party wall to external wall. The new roof structure will simply need to support its own weight and concrete roof tiles above / plaster board & insulation below and potentially some roof lights. Any advice / help on this matter much appreciated - in theory can the above proposal work?
2 Answers from MyBuilder Roofers
Steel isn't a practicable option here as it will sag over a span as long as 5.7m, which will result in a saggy roof. So without intermediary support which will see your costs rocket, forget it.
225 X 45 treated, milled (for width) rafters between new wallplate at party wall and wallplate on your external wall.
Position these at 400 centres with doubles either side of your rooflights.
The vertical struts you mentioned are actually hangars and are holding up the ceiling joists rather than supporting the rafters.
I would suggest a level collar tie of treated 150 x 45 at 3.6m from your finish floor . Fix this running level from another wallplate on your party wall to each Rafter.
This cuts down the effective span of your rafter to a very strong 2.6m. It gives you ample room to position your rooflights on your sloped ceiling and gives a level ceiling of just under a metre out from the parity wall where you can fit light fittings, smoke/heat detectors etc.You couldn't do this if you left the ceiling to slope all the way up to the ridge which would be 4 m from your floor. This would be cold, echoy and a nightmare to finish and clean.
There is no risk of the external wall spreading under the roof load because at your pitch of 26.5 degrees the level cut on the birdsmouth will be comfortably long enough to transmit the load vertically into the wall.
Fit and tape 100m celotex type insulation sitting flush with the bottom edge of the Rafters and Collars ,then a vapour barrier, then 9mm OSB nailed @150mm centres to provide racking resistance .
After that, it's insulated plasterboard and dry line or skim.
15 no. 225 x 45 treated,milled rafters 4.8m long(before doubles)
5 no. 150 x45 treated 4.8m long for collars and collarplate
2 no 225x 45 treated 3.6 long for Ridge
2 no. 100 x 45 treated 3.6 m long for bottom wallplate
8 no. 8x4 sheets 100m celotex
8 no. 8x4 sheets 38 mm insulated plasterboard
8 no. 8x4 sheets 9mm OSB.
I hope this opinion has been of assistance to you,
all the best with the project,
Answered 17th Jan 2014
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