Question on roof ventilation with sarking boards...

[damanpunk]

Hi all,

At the moment my house is being re-roofed. The current build up is rafters, sarkin boards, felt, battens, concrete tiles.

The new build up is rafters, sarkin boards, breather membrane, vert battens, horiz battens, concrete tiles.

My question is, if I insulate the roof to suit current building regs are the gaps in the sarkin boards sufficient for ventilation or will I need to add Eaves and ridge ventilation?

 

[freddiemercurystwin]

Do you have a room in the roof? Or a loft conversion? What happens above the sarking board is unimportant with respect to ventilation.

 

[damanpunk]

Hi, it is like a loft conversion although the upstairs has been there since it was built. The house is like a wide bungalow with two small lofts at either side.

 

[freddiemercurystwin]

Then the ventilation needs to be between the insulation and the underside of the sarking board, with eaves vents and continuous ridge vents.

 

[damanpunk]

Here it is

Doh, for some reason I was hoping the gaps in the sarking boards would be sufficient for ventilation.

 

[damanpunk]

Last Q... what kind of air gap can I get away with? Some places say 50mm but others say as little as 13mm as long as a breather membrane is used?

 

[KenGMac]

Bluntly, I am unsure as to what gap is needed at the eaves, a call to building control in Inverness will dictate?

Given "up here" and you are a lot further north than I am, getting this right is important, recall the "big freeze" we had a few years ago? I saw a load of damp ceilings caused by condensation in loft spaces that had been recently "Insulated to standard" BUT had no ventilation !

As for ridge vents Yes get them in, a continuous vent is a good idea, that way there are no "dead areas" where stale air can condense.

CAUTION ! what about the walls of the rooms in the attic? any vertical surface within the loft space that is now [well] ventilated and a heavily insulated ceiling space to the ground floor could [will] result in the attic rooms becoming cold [if not VERY cold] in certain conditions will require that all of these surfaces be "heavily insulated" to make these rooms "habitable" ?

Hope this assists?

Ken

 

[Alastairreid]

Last Q... what kind of air gap can I get away with? Some places say 50mm but others say as little as 13mm as long as a breather membrane is used?

25mm minimum with breathable membrane but that would only apply without sarking boards.

In your case it is 50mm.