LET'S GET STARTED - Levelled thresholds
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In this blog I address access to and use of buildings.
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Building regulations specify access requirements to make sure that both ambulant and wheelchair-bound individuals can enter and exit a building with reasonable ease. The main risks related to level thresholds, such as thermal bridging and water ingress, are summarised in this blog. Please consult the references listed in this publication's footnotes for more details.
Always seek out expert advice from the designers you have chosen, as well as from our surveyors at Warlies Park House, as they can point you in the right direction.
THE REQUIREMENTS
Part M of the Building Regulations in England and Wales outlines the legal requirements for access to and use of buildings. "Reasonable provision shall be made for people to a) gain access to; and b) use the building and its facilities," according to Requirement M1.
Guidance to meet the M1 requirement is met by the clauses in Approved Document M. Where the entrance approach is level or ramped, clause 6.19 states: "An accessible threshold at the entrance should be provided."
The threshold design must also comply with other requirements of the Building Regulations or be compatible with them, particularly those that pertain to: reducing the risk of water ingress or damp entering the building (Part C); providing adequate sub-floor ventilation (Part C); requirements for the provision of soil/gas membranes; and thermal insulation requirements to prevent thermal bridging.
• the provision of long-lasting solutions while taking into account craftsmanship standards.
NOW THE ESSENTIALS
1. A level or accessible threshold consists of three main components:
The external landing's drainage system, first -
The external landing needs to be big enough and level for wheelchair users and ambulant disabled people to approach and, if necessary, turn to face the door. Designing it should prevent standing water and reduce the amount of surface water that crosses the threshold.
Checkout the Technical Handbook - Domestic or Approved Document M should be used as a guide for the landing's size in Scotland (Section 4).
• To provide surface water drainage, external landings on a ramped approach should be laid to fall between 1:40 and 1:60 in a single direction.
• Where surface water is likely to be blown towards the threshold, a drainage channel should be provided between the landing and threshold.
• In extreme circumstances, additional field drainage should be provided at the intersection of the ramp and the landing, in order to avoid ponding and to keep storm water well away from the threshold.
• Additional field drainage to adjoining land may be required on low-lying or steeply sloped sites, in order to prevent the drainage channels from flooding.
• The channel should discharge to a drainage system or permeable field drain. Both a site-formed slot above a discharge channel and a custom drainage channel can be used to create the drainage channel. In either scenario, the drainage slots should be no wider than 18mm to maximise drainage and lower the possibility of walking sticks and wheelchair wheels becoming entrapped.
Consideration should be given to additional steps to ease access in addition to specific guidelines for threshold design. For instance, a porch roof will offer protection to wheelchair and ambulant disabled users while they search for keys or access codes, helping to lower the risk of direct water entry.
2. The external landing and its intersection with the threshold sill
The threshold profile should minimise the risk of surface water entering the home while still allowing access for wheelchair users and ambulant disabled people.
It is challenging to strike a balance between attaining a nearly level threshold and maintaining adequate weatherproofing to lower the risk of moisture ingress at the threshold sill to external landing interface.
• It is preferable for the surface of the landing/drainage channel to be level with the door sill where a drainage channel is situated in front of the threshold sill. However, the landing can be up to 10mm below the threshold sill in situations where doing so is impractical due to exposure to wind-driven rain. To make the transition easier for wheelchair users in this situation, the leading edge of the threshold sill should be rounded or chamfered.
• To prevent water intrusion and promote runoff, threshold sills should slope at a maximum angle of 15 degrees and a minimum angle of 7 degrees.
• The door threshold unit's upper leading edge should not rise above 15mm. The exposed edge should be rounded or chamfered if it is higher than 15mm. Directly in front of any timber sills or related components should be created a drained and ventilated void due to the risk of moisture ingress and the subsequent deterioration of wood components.
• The floor insulation should be applied all the way around concrete slabs and wood floors, and it should be continuous with any wall insulation in order to significantly reduce the thermal bridging risk at the threshold level.
• In addition to the level threshold, there should be sufficient space provision. To accomplish this, insulate the edge of the slab, set the door frame back so that it laps with the edge insulation, and specify an insulated door. An entrance should have a minimum width of 800mm and ideally 900mm.
3. The point where the internal floor finish and the door threshold meet
While allowing occupants to choose the type and thickness of floor coverings, the transition between the lower threshold unit and the internal floor level should permit accessible transfer for wheelchair users and ambulant disabled people.
• To make it simpler to enter through doors, the internal floor next to the threshold should be level or gradually sloped using an internal transition unit.
• Where the expected finished floor level is intended to be less than 15mm below the level of the door threshold unit, there is no requirement for a graded platform or internal transition unit. When a graded transition is specified, it should have a maximum slope of 15° and a slip-resistant surface. There should be enough room within the internal lobby area to allow full turning for wheelchair users and unhindered entry for other disabled users. This clearance should be reduced to 10mm where the floor covering specified is an uncompressed soft pile carpet.
ADDITIONAL GUIDANCE
Level thresholds should only be used to make homes accessible to people with limited mobility, which is their intended use. The positioning of external ground levels flush with internal ground floors should not be justified using the principles adopted here.
References
• Thresholds that are accessible in new homes. Advice for Home Designers and Builders. The Stationery Office published this under BRE Level External Thresholds: Reducing Moisture Infiltration and Thermal Bridging (Good Building Guide 47)
The Building (Scotland) Regulations 2004 and accompanying Technical Handbooks. Building Regulations Approved Document M.
Example construction details for level thresholds to dwellings are provided in Technical Manual Section 11.1.6.
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