FENSA, Replacing Vertical Sliding Sashes, Double Glazing UK

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Sliding Sashes

FENSA Limited is the Fenestration Self-Assessment Scheme for companies that install windows and doors in dwellings.

FENSA Guidance notes:

In order to assist its member’s interpret various aspects of the approved documents, FENSA has issued a series of guidance notes.

Special Note: These are the interpretations being used by "FENSA registered companies". This is important because some of the local authorities may interpret some of the building regulations slightly differently. If you are not using a FENSA member and are going to apply to the local authority for permission rather than use a FENSA company you would be well advised to seek clarification before an installation commences.

Replacing Vertical Sliding Sashes

When replacing vertical sliding windows it is important that the compliance to Approved Documents F (Ventilation), N (Safety) and B (Fire Egress) is either maintained, or that the replacement windows are not worse than those being removed.

The surveyor is called to a house fitted with a wooden vertical sliding sash window in a room measuring 4m by 4m


Other Information:


Diagram 1

The customer requires the external appearance of the window to be fundamentally the same and requests a PVC-U top hung vent over a fixed light.

Diagram 2 – This solution will fail (Fire Egress)

FENSA Guidance Notes (2)

Replacing Vertical Sliding Sashes

Approved Document F - Ventilation

1. Rapid Ventilation

The original window provided 0.45m2 of rapid ventilation (less than required for new build regulations 4m x 4m x 1/120 = 0.8m2) however a portion of the opening sash set out in Diagram 2 is 1750mm above floor level and therefore, does not make the situation worse and is acceptable in this respect.

Note: Rapid ventilation needs some part of the ventilation opening above 1750mm.

2. Background Ventilation

As the original window did not contain trickle vents, nor was it capable of being locked in the open position, it did not provide background ventilation. The design of the window in Diagram 2 is acceptable in this respect. If a background ventilation facility was available then the replacement window would need to be fitted with either trickle vents or a lockable night vent so compliance would be maintained.

FENSA Guidance Notes (2)

Replacing Vertical Sliding Sashes

Approved Document N – Safety Glazing

The replacement window is less than 800mm from floor level and therefore the lower sash needs to be fitted with safety glass to BS 6206.

(Diagram 3 – window on a staircase).

Note: Diagram 3 shows how the distance from a staircase to the bottom of the window is to be measured.

Diagram 3

FENSA Guidance Notes (2)

Replacing Vertical Sliding Sashes

Approved Document B – Fire Egress

1. Fire Egress

The original window provided a clear opening 600mm high and 750mm wide at height of 600mm from the floor. This more than satisfied the size requirement for a fire egress window in a new build situation (i.e. 0.33m2 with a minimum dimension of 450mm). The replacement window (Diagram 2) can, dependent upon the type of hinge used, maintain the size of opening required for egress (taking into account an external stone cill if in place). However because symmetry is required the transom height will be above the 1100mm level. (This height is the maximum required for a fire egress window to comply with the legislation.) Therefore the window in Diagram 2 fails fire egress compliance (it makes matters worse).

Options available:

  • Revert to vertical sash design.
  • Lower the height of the middle transom to below 1100mm (top opener becomes a fire egress window)
  • Redesign the window to be two top opening vents, one above the other. (Diagram 4)
  • Redesign the window to be one single top hung opening, measuring 1200mm x 750mm, if suitable hinges are available.

Diagram 4

Our Thanks to FENSA for permission to reproduce these notes here:

http://www.fensa.org.uk FENSA Limited is the Fenestration Self-Assessment Scheme for companies that install windows and doors in dwellings


For Information, please visit:

Conservatories | Sliding Sashes | Inner Rooms | Fire Escape




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