how do you calculate number of air vents in a roof

Postby rabbit » Wed Oct 07, 2009 8:04 am


There is no ventilation in my roof and I need to put in air bricks. How do I work our how many airbricks are needed and where should they be positioned?
thank you.
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Simply Build It

Postby Perry525 » Sun Nov 01, 2009 6:37 pm

Why do you think that you need to add air bricks to your roof?
What do you expect them to do?
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Postby rabbit » Mon Nov 02, 2009 12:39 pm

When I had the place surveyed - the report said the loft needed vents. It was rather stuffy up there and there is also a lot of loft insulation. I assume that putting in tile vents help air the loft. I am not sure why vents are needed - some technical reason. It is just how many do I need that I am not sure about.

thank you.
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Joined: Thu Sep 24, 2009 10:36 am

Postby Perry525 » Mon Nov 02, 2009 6:50 pm

In this country there seems to be two reasons for fitting roof vents.
One is, if the roof is leaking, then the movement of air will help dry it out, the other, that water vapour created inside the home, will rise through the ceilings and holes in the ceilings and condense on the cold roof timbers, perhaps a dozen or so times a year, with the potential to create wood rot.
If you go up there with a bright light, you will see if the rafters have water marks on them from a leaking roof and if a lot of water vapour is rising from your home and condensing on the timbers.
If the roof isn't leaking and if there is no sign of wood rot, ignore the coment.
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Postby rosebery » Mon Nov 02, 2009 10:03 pm

A roofspace needs ventilation in order to breathe and most are built so that it occurs naturally. There will either be a ventilation gap at the eaves or more modern builds have ventilation built into the soffits. Either way someone may have been overenthusiastic with the insulation. Make sure that there is a clear gap at the eaves and that the insulation isn't stuffed right down as far as it will go. That should provide you with the ventilation you need without having to fit roof vents.

Many surveyors are operating the CYA principal these days in our increasingly litiginous society so they can easily make mountains out of molehills.

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Postby hereward » Thu Feb 25, 2010 7:26 pm

When I had a visit from the building inspector at the start of my loft conversion, he insisted that some ridge tiles needed to be replaced with vent tiles? to satisfy building regs.
I'm nearly at the stage of getting on my roof so if anyone knows the answer to original question.....

My B.I. is neither helpful nor approachable.
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Postby angelboy » Wed Mar 17, 2010 2:28 pm

Ventilation is defined in the Building Regs as 'the supply and removal of air (by natural and/or mechanical means) to and from a space or spaces in a building'.

In addition to replacing stale indoor air with fresh outside air, the aim of ventilation is also to:

• Limit the accumulation of moisture and pollutants from a building which could, otherwise, become a health hazard to people living and/or working within that building.
• dilute and remove airbourne pollutants (especially odours)
• control excessive humidity
• provide air for fuel burning appliances
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