Roof terraces – flashing height, level access, insulation, drainage, construction of story partition
Experience Sheet (23) 07 12 29 Translated by: HAI Via University College Content
Introduction Level access Increased height of storey of the building Roof terraces Isolation Method and materials Prestressed betondækelementer Storey heights and insulation thickness Fall and flashings Drainage Conditions
By roof terrasses there has often been identified water intrusion due to low flashings against doors and facades. Heavy rain fall over a short period of time makes need for quick and efficient drainage.
This sheet outlines the need to increased height of the storey partition and the floor structure of the building to achieve adequate flashing and to use robust flashings, proper insulation and ensure efficient drainage of water. The requirement for levelfri access must be satisfied with the solutions.
The photo shows a roof terrace with a low - almost no – hight of the flashing against a window section, which has led to water intrusion into the building. Introduction
In recent years we have identified several cases of water intrusion by the flashings on roof terraces. This is in many cases a result of the design of the level access. It is designed with a to low flashing where the coating on the roof terrace meets the door or facade component.
When designing a building with roof terraces it implies a risk of water intrusion at the enclosure, therefore it is precisely here very important to be aware of the geometrical conditions for the establishment of appropriate slope and height of the flashing.
Furthermore, it appears that roof structures are not always done so that drainage from the terrace surface can function well. Level access
The top of the insulation on the roof terrace and the connecting to door and facade element must, under current practice usually be performed with at least 150 mm flashing height.
Figures 1 and 2 show two ways to design a terrace floor, which brings the terrace surface in level with the door - in this way the establishment of the Building Regulations requirement for level access is fulfilled.
Figure 1 Warm roof and high floor buildup (dimensions in mm).
Figure 2 Varm roof and ceiling lowered and suspended ceiling.
Increased height of the storey partition in the building
Since the surface of the floor in the building and the terrace will be located in the same level to achieve levelfri access specific measures is required to make up for the height difference of approximately 500 mm witch is nomaly the thickness of the terrace construction.
In Figure 1 the concrete deck level everywhere the same, resulting in approximately 670 mm total height of the storypartition.
Figure 2 shows a solution where the concrete deck underneath the terrace is lowered to avoid a high interior floor construction. You can use an extended an ceiling to hide the jump in deck/ceiling height.
It appears that even this solution requires a total height of the story partition of approximately 670 mm to achieve the required insulation thickness [1] and levelfri access [2] from the terrace.
Note that the increased floor height can result in a exceeded height limit in the local plan for the building - and therefore this has to be taken into account already in the planning phase.
Figure 3 Schematic diagram for construction of roof terraces respectively unreversed "hot tag", "duo" tag and "inverted roof".
The decrease can be built in isolation or created with plaster on concrete deck.
The roof is constructed as duotag use wedge-cut insulation under - and plan, extruded polystyrene over - roofing membrane. Tiling must be installed on wood feet. Floor Building and door / front lot is not illustrated.
Roof terraces
A roof terrace - typically related to terrace house building and penthouse apartments - in this context means a storey partition of the building with outdoor climate on the upper side and indoor climate on the underside. To comply with the energy frame [1] the insulation thickness is usually about 300 mm (previously about 200 mm) if not otherwise compensated for in the building's energy loss of the building.
The terrace floor protects the underlying roof surface against mechanical influences. Isolation Method and materials
The construction of the roof terrace can be both a warm roof, an inverted roof or a duo roof [3, 4] (Figure 3). Advantages and disadvantages of the three roof types and the membrane is shown in [4].
The flashing (minimum 150 mm) must - regardless of isolation method – be calculated from the top of the waterproof layer or drainage layer, ie. from the top of the roof membrane in a warm roof or the upper side of the insulation in an inverted roof or a duo roof. Choice of insulation depends on whether it is a warm roof, an inverted roof or a duo roof [4]. Normally used is the cell glass - or foam - with a compressive strength of 150 kN / m 2 - and mineral wool cannot be used! In an inverted roof - or as a top layer in a duo-roof - only extruded polystyrene (XPS) with a compressive strength of 250 kN / m 2 can be used .
Note that the support of the terrace floor must be adequate to transfer concentrated loads (eg from flower pots) without roof damage. Pre stressed concrete slab elements
If you are using precast, pre stressed concrete slab element it may be necessary to increase the floor height by about 40 mm - equivalent to the height (total bow) which elements come with if the span is large.
To allow construction of the insulating layer from a horizontal surface a concreted screed layer can be made.
Note that the floors height must be increased accordingly. There could be construed a screed layer which the desired slope. Storey heights and insulation thickness
It is possible to reduce the insulation thickness - and therefore floor height - for example by improving insulation elsewhere in the building (energy frame). This will compensate for the lack of insulation in roof terrace.
However the U-value must not exceed 0.25 W / m2 K (corresponding to approximately 150 mm insulation thickness of the deck).
Insulation thickness on the concrete deck can also be reduced by placing a portion of the insulation on the underside of the concrete deck. This reduces, however, the ceiling height in the underlying space. If you insulate on the underside of the concrete deck, you must use a completely impermeable material such as cellular glass.
To avoid a thermal bridge to the underlying space the insulation placed on the underside, must not exceed half of what is placed on top of the concrete slab. Slope and flashings
In addition to the minimum requirement of 150 mm flashing height , you must also:
establish necessary slope of the roof surface, 1:40 on the warm roof and 1:100 in duo roof and inverted roof. In addition, the valleys and the like [3] must have at least the slope of 1:100, take into account dimensional tolerances, take into account the possible total bow height on a pre stressed concrete slab element.
These factors may cause the building height to be further increased to achieve at least 150 mm flashing height above the highest point of the drainage layer.
Enclosure to protect the membrane must be constructed as a stainless steel cap, so that chair legs from patio furniture do not wear out or break the waterproof layer. To prevent splashes it is recommended to establish a steel grate along the doors and facades. Drainage conditions
To guard against water damage in connection with a possible blocking of a terrace drain there must be established "an emergency drain" - either in the form of overflow from the terrace edge or Spewing.
If it is not possible to establish a "an emergency drain", there must be at least two drainage wells.
Rainwater from the overlying roofs and terraces must be led directly to "own drain" - they must not run down to (and accumulate on) underlying terraces!
To avoid ice formation, which can block drainage from the roof terrace there can be mounted electric heating cables (eltracing) in the gutters and drain pans. Operation and Maintenance
Wells and any gutters must be easily accessible so they can be cleaned regularly.
In connection with the operation there must be ensured that snow does not build up in front of doors and windows. This snow must be removed so that water intrusion is avoided when the snow thaws.
Since the waterproof membrane is well protected, it is usually not necessary with regular inspections by the general climate influenced roofs.
Lifespan Considerations for roofing material are reviewed in [3].
The condition of the flashings (the stainless steel caps), however – must at least once a year – be inspected for damage.
The waterproof membrane should never be pierced, to secure furniture with bolts or lighting and plant stands - the possibilities for attachment of such objects must be resolved in connection with the planning.
From an operational point of view it can be an advantage to used easily removable floor coverings like wooden-slatted floors or "tiles on feet."
Terrace floors of tiles on a "draining" base of gravel (built as a warm roof) is not recommended because:
it is almost impossible to trace - and difficult to repair - any leaks in the waterproof membrane it is very difficult to maintain water channels, the "draining" layer can freeze into and destroying the roof membrane. In [4] identifies a wide range of other conditions such as choice of membrane, the construction, drainage and details.
References
[1] Insulation Requirements. Building Regulations 2008, Chapter 7
[2] level access. Building Regulations 2008 chapter 3.2 1 paragraph. 2
[3] The design of roofs with roofing felt and roofing membranes, 2001 TOR-instruction 22nd Roofing Industry Information. 2001
[4] Dense roof terrace Byg – Erfa Experience Sheet (27) 99 04 20
Prepared by:
Jens Dons Technical Manager, Architect MAA Building Defects Foundation Studiestræde 50 1554 Copenhagen V http://www.bsf.dk Phone: 3376 2000 Georg Christensen Engineering Construction and Environmental Engineering A / S Dr Neergaard Road 15 2970 Hørsholm http://www.byggeteknik.com Phone: 4566 2922
Tommy Bunch-Nielsen Engineering Construction and Environmental Engineering A / S Dr Neergaard Road 15 2970 Hørsholm http://www.byggeteknik.com Phone: 4566 2922