| For Roof Space Ventilation
Refer To: |
Building Regulations Approved Document F2: 1995 ‘Condensation
in Roofs’ and subsequent amendments.
Building Standards (Scotland) Regulations 1990 Part G. (where
applicable)
BS5250:2002 Code of Practice for control of Condensation in
Building
BS5534:PT1:1990 ‘Slating and tiling design’ |
 |
| For Soil Vent Application Refer To: |
Building Regulations Approved Document H1 1990 ‘Sanitary
pipework and drainage’ and subsequent amendments.
BS5572:1994 ‘Code of Practice for Sanitary Pipe work’ |
|
| For Passive Stack Ventilation Refer To: |
BS5925:1991 (1995) ‘Code of Practice for ventilation
principles and designing for natural ventilation’
Building Regulations : Approved document F1 1995 ‘Means
of Ventilation’
BRE Information paper 13/94 ‘Passive Stack Ventilation
systems: design and installation’ |
|
| For Mechanical Ventilation Refer To: |
| Building Regulations Approved Document F1 1995 ‘Means
of Ventilation’ and subsequent amendments. |
|
| For Further
Guidance Refer To: |
| NFRC Technical Bulletin No.20 ‘Roof
ventilation products’ |
|
| Roof Ventilation |
|
Approved Document F2 (1995) Building Regulations and BS5250:2002
‘Control of Practice for Condensation in Buildings’
describes the causes, problems, and practical methods for avoiding
condensation.
Here are the key points of these requirements: |
|
| Condensation |
| The requirement of Approved Document F2 is that condensation
is reduced in order that it will not cause damage to the structural
or thermal properties of materials in a roof. Dampness is caused
by: weather, interstitial condensation, surface condensation,
and construction water (in wet constructions). |
|
| Roof Ventilation |
Roof ventilation is necessary to avoid the problem of condensation.
Adequate cross ventilation is required, with openings placed
on the longer sides of a typical rectangular roof. This eaves
to eaves roof ventilation relies on wind power. In most cases the
roof ventilation system is improved by utilising the natural thermal
upflow of air in a roof void. This eaves to ridge ventilation
also avoids the problem of stagnant air pockets due to inadequate
through-flows.
Warning: high level roof ventilation should never be used on its
own as the suction effect created could increase water vapour
transfer into the roof void.
Roof ventilation should provide a continuous weatherproof path from
roof void to the outside. Openings must not be blocked by dust
or debris, and ingress of rain, snow, birds and large insects
must be prevented. Mesh size of 4mm is recommended by BS5250,
it is small enough to prevent entry by nesting insects, birds
etc. yet is large enough to prevent blockage, provide adequate
air movement and avoid excessive airflow restraint.
Roof ventilation is recommended in all circumstances. NRFC bulletin
20 states: ‘any water vapour transmission benefit of a
vapour permeable roofing underlay cannot on its own eliminate
roof space condensation. Any water vapour transmission benefit
should be treated as fortuitous.’ Where a vapour permeable
underlay is used, it should therefore be in addition to, rather
than in place of, ventilation of the roof void.
Ventilation openings can be sited at intervals, they should
be of equivalent area to a continuous opening:
5mm air gap = 5,000mm² /m ventilation
10mm air gap = 10,000mm² /m ventilation
25mm air gap = 25,000mm²/m ventilation |
|
| Applications Of Building Regulations |
|
| Pitched Roof - Ceiling and Insulation Horizontal |
|
| Open Roof Void |
| Building Regulations state: where the void is open, eaves
to eaves air flow is effective, along the longer sides of the
building. Brett Martin recommend the use of high level ventilation
in addition to eaves ventilation in all cases – as it
utilises the natural thermal uplift in a roof void. Eaves to
eaves roof ventilation relies on the wind conditions which can result
in poor through flow and stagnant air pockets. |
|
 |
|
PITCH 15º or less
USE: eaves vents OR: low level slate/tile vents 25mm air
gap |
 |
|
PITCH 15º or greater
USE: eaves vents OR: low level slate/tile vents 10mm air
gap
In addition BS 5534 recommend:
USE: ridge vents OR:high level slate/tile vents 5mm air
gap |
 |
|
PITCH 35º or greater OR:
WIDTH 10m or more
USE: eaves vents OR:low level slate/tile vent 25mm air
gap
AND: ridge vent 5mm air gap
OR: high level slate & tile vents
5mm air gap each side |
|
|
| Steep or Wide Buildings |
| In addition to eaves vents, increased ventilation
must be provided by high level openings. These are necessary
to avoid stagnant air pockets due to inadequate through flow.
In particular for roof slopes steeper than 35º , or for buildings
more than 10 metres wide, high level roof ventilation is required. |
|
| Single Pitch Roofs |
| Use ventilation at the eaves and at the abutment. |
 |
 |
PITCH 15º or less
USE: eaves vents
OR: low level slate/tile vents 25mm air gap
AND: high level slate/tile vents 5mm air gap |
|
 |
|
PITCH 15º or greater USE:
eaves vents OR: low level slate/tile vents 10mm air gap
AND: high level slate/tile vents 5mm air gap |
|
|
| Air Flow |
| Where eaves ventilation is provided care should be taken to
prevent insulation blocking off air flow to roof. |
 |
|
| Pitched Roof - Ceiling and Insulation Inclined |
Where the insulation follows the line of the roof, it is necessary
to ventilate both at low and high levels.
An air gap of at least 50mm must be maintained between the underlay
and insulation all the way along the inside of the roof in order
to prevent air resistance in this area. Where joists run at right angles to the air flow, use counter battens. |
|
|
Air Flow Between roofing underlay and
insulation
USE: eaves vents OR: low level slate/tile vents 25mm AND: ridge
vent 5mm OR : high level slate / tile vents 5mm each side |
|
| Pitched Roof - Obstruction in Roof |
| All isolated parts of the roof should have ventilation provision.
Where an obstruction in the ventilation path occurs, such as
at roof lights or at changes in pitch, the roof void should
have additional ventilation openings. |
 |
|
 |
Obstrution
outside inclined ceiling
Immediately below the obstruction 5mm
Immediately above the obstruction 10mm |
|
Obstruction
within inclined ceiling
Immediately below the obstruction 5mm
Immediately above the obstruction 25mm |
|
|
| Pitched Roof - Dormers |
Pitched type dormer roofs should be ventilated from eaves
to eaves.
Flat type dormer roofs should be ventilated from eaves to ridge
of the main roof. |
 |
|
 |
Pitched
Type Dormer
USE: eaves vents OR:low level slate/tile vents 10mm |
|
Flat
Type Dormer
USE: eaves vents 25mm |
|
|
Main Roof
USE: ridge line 5mm OR : high level slate / tile vents 5mm each
side AND: eaves vents / low level slate/tile vents 25mm |
