Cutting felt in roof

[Greg M]

Hi everybody.

 

I have a quick question which hopefully somebody will be able to help me with.

 

I bought some lap vents for the inside of my roof as we get condensation during the winter, however when I went to install them I noticed that the felt on the inside of the roof does not overlap, but instead is one continuous sheet of felt that is attached to the roof.

 

My question is, would it be possible to make a slit in the single layer of felt to be able to place the lap vents in or would that compromise the roof and allow water to enter?

 

Thanks in advance,

 

G

[nod]

If it’s one piece (tarp) That’s probably why you have condensation issues 

Probably needs redoing 

[Greg M]

Thanks for the reply Nod. Yeah I was given a quote for £1200 to instal new felt and roof vents however I can't afford this amount at the moment hence trying the lap vents.

[makie]

Can you take a picture of the felt? Just so we can clearly see what we are looking at.

[Temp]

Have you checked you have sufficient ventilation at the eaves. You typically need the equivalent of a 10 - 25 mm wide slot the full length. That's a lot of 80mm round vents, perhaps 4 - 5 per rafter bay. Sometimes you have to have two staggered rows to meet recommendations a one line won't fit. 

 

https://nhbc-standards.co.uk/7-roofs/7-2-pitched-roofs/7-2-15-ventilation-vapour-control-and-insulation/

 

 

[ETC]

21 hours ago, Greg M said:

Hi everybody.

 

I have a quick question which hopefully somebody will be able to help me with.

 

I bought some lap vents for the inside of my roof as we get condensation during the winter, however when I went to install them I noticed that the felt on the inside of the roof does not overlap, but instead is one continuous sheet of felt that is attached to the roof.

 

My question is, would it be possible to make a slit in the single layer of felt to be able to place the lap vents in or would that compromise the roof and allow water to enter?

 

Thanks in advance,

 

G

If you’re getting condensation in the roof space you should try and increase the insulation in the roof space making sure that you overlap insulation at the eaves and provide adequate ventilation.

 

However lap vents aren’t great. Where the insulation is at ceiling level you should be providing 10,000mm2/m ventilation at the eaves and - depending on the span and pitch - 5,000mm2/m ridge ventilation. For insulation between the rafters the requirement is higher.

 

Take a look at Glidevale’s website for products and take a quick look at the White Paper which explains a little bit about ventilation in buildings.

[Greg M]

20 hours ago, makie said:

Can you take a picture of the felt? Just so we can clearly see what we are looking at.

I will take a picture when I finish work and post it onto the forum 👍

[Greg M]

On 12/06/2023 at 17:57, makie said:

Can you take a picture of the felt? Just so we can clearly see what we are looking at.

 

[bassanclan]

Looks like old type bitumen felt rather than modern breathable membrane, never the less it is on a roll, so there are laps where one roll overlaps the next

[Greg M]

On 02/07/2023 at 23:07, bassanclan said:

Looks like old type bitumen felt rather than modern breathable membrane, never the less it is on a roll, so there are laps where one roll overlaps the next

Thanks very much. I really appreciate it 👍

On 12/06/2023 at 17:57, makie said:

Can you take a picture of the felt? Just so we can clearly see what we are looking at.

 

[makie]

19 minutes ago, Greg M said:

Thanks very much. I really appreciate it 👍

 

bassandclan already gave you the answer, it's got laps.

[Dave Jones]

what sort of tiles are on the roof ? Probably quicker and cheaper to just add a bunch of tile vents both sides of the roof.