loft roof insulation twist of fate

2 Jan 2014
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United Kingdom
We have moved into a house which used to have a flat concrete roof. This was changed to a pitched roof, and the whole loft space insulated with rock wool ie no joists. We are now in the process of moving all the shart from the garage into the loft. I believe I am right in saying if I lay boxes on the rock wool I compromise the insulation, will create a cold spot and have my own indoor water generator. So I was thinking about those plastic feet, wooden frame with interlocking loft boards to build a kind of free standing raft so the insulation isn't compromised. However if I bought kingspan sheets, cut them to fit through the loft hatch and then taped them together back into a raft. This would obviously squash the rock wool, but would the kingspan more than compensate for the compressed rock wool. PS I am skint so cannot do anything more elaborate. Any advise gladly taken.
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Squashing the Rockwool will definitely comprimise it's insulation properties, and putting a kingspan raft over the top will pretty much do the same. You'd be better off taking the insulation out, and replacing it with 100mm kingspan instead. The plastic legs are damned expensive, so you'd be better off just raising the height with 50x100 joists, and then only squash the rockwool slightly. Depending on how much storage space you need up there, you don't need to do the whole loft in one go.
Thanks for the reply, bear with me here, to avoid removing the rock wool could I not just lie 100mm kingspan on top of it ? You are of course spot on probably only need a couple of meteres squared space for all our stuff.
Okay, let's clarify your intentions; what will you do after you add the 100mm of kingspan.
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i dont like plastic legs
with 2 parallel timbers at right angles at say 16"centers the greatest span[distance between the 2 closest timber centers is 16"] these timbers also help spread the load over all the ceiling timbers
if you have stilts at 16" centers the span is 45% greater as at the center off the legs in a square its the diagonal as the nearest support so as if your timbers where 26" apart
you also get point loading as the stilts don't spread the load sideways to adjoining timbers
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Thanks for the reply yes I think as doggit said the price is a major factor as well not having joists is a real pain as those interlocking boards are really cheap however I do have a solid concrete floor for a base just need to insulate it as cheaply as possible cheers
Am I missing something MD, I half get the impression that you're just going to put the Kingspan on top of the rockwall, and then put the boxes on top of that; if that is the case, then everything will just wobble, and fall over. Sorry, but I've just managed to work out that you kept the concrete roof, and then added a pitched roof above it to create the loft space; and if that's the case, then I'd take the Rockwool out where you want the storage, put down 125mm of Kinspan and just put the interlocking loft boards on top of the kingspan, but glue the joints for stability.
Doggit yes thanks it was a long initial post always had a concrete floor remove the rockwool where required put down kingspan board over the top perfect appreciate your help cheers
Sorry mate no floor joists I have what appears to be substantial king trusses ( they might not be called this) off the ground running the length of the roof nothing I would use to put additional weight on cheers
Screw some horizontal 2" X 1" to side of joists, poking out about 5'. screw some vertical 2 X1s so they act as legs to stand the free end directly on your concrete roof. Screw on horizontal 2 X 1s across new bits on joists,. Now you have a lightweight shelf to store some goods on while compromising .001% of your insulation. Add more insulation under to taste.
( have you insulated your loft trap?)

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