Air gap under tiles

[marsaday]

Do i really need a 50mm air gap under my tiles. they are just back pointed tiles with no felt. all in great nick.

i will be adding kingspan between the rafter and would prefer to leave just a 25mm air gap. will this be ok. my BCO seems ok with it. he is also happy to pass the insulation with only 100mm of kingspan.

 

[freddiemercurystwin]

If you have no felt you do not need any gap at all, although it would be good practice to leave say a half inch or so.

 

[noseall]

he is also happy to pass the insulation with only 100mm of kingspan.

The rafters will be fine with 100mm betwixt, but they will also require 40mm underdraw to deal with 'coldspot' or 'rafter shadow' issues. I am surprised the b.c.o. has not highlighted/enforced this.

 

[marsaday]

The officer is happy with 80mm in the rafter and 20mm on its underside so stopping the cold spots. so in total he is happy with 100mm.

i am going to go for 75mm in the rafter and then 35mm on the underside.

His thinking is not to be too strict with insulation laws because you are dealing with an old building which is pretty poorly insulated. so 100mm in the roof is fine for him.

 

[marsaday]

one other thing.

i am going to be relaying the ridges. should i incorporate 2 ridge vents when i do this?

 

[noseall]

You could....but, the tiles are likely to be draughty enough.

 

[masona]

my BCO seems ok with it. he is also happy to pass the insulation with only 100mm of kingspan.

If you got 100mm rafters then the next problem would be, if and when the tiles need replacing they will fit the roofing felts on therefore no air gap, so I would always make sure there's an air gap flow to allow for this.

 

[marsaday]

yes, possibly i may add felt in the future, but i really dont think it will be needed.

So lets say i will, does a 25mm gap sound ok if i use breathable membrane?

 

[freddiemercurystwin]

yes, possibly i may add felt in the future, but i really dont think it will be needed.

So lets say i will, does a 25mm gap sound ok if i use breathable membrane?

No you don't need any gap with a breathable membrane, if the membrane is breathable then it allows air to flow through it therefore its as if no membrane is fitted at all like you have now.

 

[marsaday]

ok that makes life even better because if i do come to upgrade the roof i would definitley use breathable membrane.

in this case i will still leave a 10mm gap though and fill the rafter with 90mm.

 

[masona]

yes, possibly i may add felt in the future, but i really dont think it will be needed.

So lets say i will, does a 25mm gap sound ok if i use breathable membrane?

No you don't need any gap with a breathable membrane, if the membrane is breathable then it allows air to flow through it therefore its as if no membrane is fitted at all like you have now.

How breathable are they and do they clog up with dirt? You can't beat a proper air gap