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ROOFS

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  • ROOFS

  • 1. General elements

    2. Classification

    3. Functional requirements

    4. The roofs forms

    5. Structural elements of the roofs

    6. Roof coverings

  • 1. General elements

    The roofs are parts of the building, placed at the top,and they create together with external walls and someelements of the infrastructure the outer cover system orthe envelope of the building.

    The fundamental purpose of the roof is to cover thebuilding and to create an internal space protected fromthe aggression of the external atmospheric factors as:weather (sun, rain, snow, wind) action, dust, noxiousemanations.

    It receives rain and snow directly and in much greaterquantity than the walls. It must therefore present abarrier to the entry of rain and rigorous waterproofing isessential in roofing design.

  • The other vital factor in roof design is thespan. In addition to supporting the dead loadof the roof covering, the roof structuremust also resist loads imposed by windpressure and snow. Most roofs will also haveto carry the loads generated by mean workingon their construction or repair, and flat roofsmay have to carry still greater loads resultingfrom their use.

    Other requirements which must be satisfiedare the stability, strength, thermalinsulation (often greater than that neededfor the wall), fire resistance, durability andeconomical and esthetical requirements.

  • As a principle the roof is made of the following mainelements that must satisfy defferent functionalrequirements :

    the covering which must be a water and dampnessinsulation and consists of a covering of tiles, or metalsheets or bitumen cardboard layers;

    the thermal insulating layer that must protectagainst the thermal gains due to the direct sun-heatand against heat loos in the winter;

    the vapour barrier (with diffusion layers orchannels), that must prevent the water vapourmigration inside the thermal insulation;

    some other auxiliary elements such as eaves gutterto collect and evacuate pluvial waters, ventilationchannels, lights etc.).

  • The construction and the function of all theselayers require some supporting and protectingelements. Most important are the structuralelements of the roof that support andtransmit the loads to the vertical structuralelements of the building.

    Consist of:

    the structural floor slab over the last or toplevel in the case of flat or terrace roofs;

    structural elements as rafters, ridges,purlins, posts or props or structural frames inthe case of pitched roofs.

  • The roof makes a major part of the

    building shell.

    The design of the roof structure depends on the loading on the roof ... including wind loading, rain/snow loading, ..., choice of constituent building materials, the type of finish desired on the roof, roof insulation considerations, ...

  • 2. Classificationa) Considering the angle of the slope or pitch:

    pitched roofs with steeply slope (fall between 21 . . .150%) and with medium slope (between 8 . . . 21%);

    flat (or terrace) roofs made of concrete, timber ormetal framed platforms or slabs which are inclinedwith the fall between 1.5 ... 4% ( if there can be usedfor circulation) or with the fall between 2 ... 7% , ifthere are not destined to be circulated.

    b) According the architectural forms roof:

    with plane surfaces;

    with curved surfaces.

  • d) Considering the span of the roof there are: short span up to 7.60 m; medium span 7.60 to 24.40 m; long span over 24.40 m.

    e) According the hygrothermal behaviour there are:

    cold roofs, known also as ventilated roofs;

    warm roofs, unventilated known as compact roofs.

  • The cold roofs present in their structure a layer ofair that is connected to the atmospheric air by acontrolled on an uncontrolled ventilation system. Thislayer of air divides the structure of the roof in twoparts:

    one is composed by the thermal insulating layer andthe support layer that is usually the slab over thetop floor (or a suspended ceiling);

    the other part consists of the waterproof membraneand its supporting layer.

    In this type of roofs, the water vapour coming (bymigration) from the building rooms of the top floor,are evacuated in the atmosphere by the circulatingair layer from the structure of the roof. In this way,the risk of vapour condensation is reduced as much aspossible.

  • The cold ventilated roofs present greater hygrothermalefficiency than the unventilated warm roofs.

  • The simple and unventilated or warm roofs areconstructed as flat roofs. This type of roofs does nothave any intermediate air layer for ventilation. Toimprove the hygrothermal characteristic of the warmroofs the internal layers beneath the covering are linkedto the external air by a network of small channels.

  • 3. Functional requirements

    The roofs has to protect theartificial indoor climate from the influence of the outdoor meteorological conditions.

    This specific location of the roofs, in the envelope of thebuilding, at the border between two different climates; andthe fact that its position is very close to the horizontalplane, emphasise some specific functional requirements.

  • a -Tightness against meteorological water actions

    The roof must provide the collection and evacuationof meteorological waters acting on it as rain or snow.The evacuation of the water must be complete,without any infiltration throw the roof internallayers.

    To satisfy these conditions the roof presents someslopes created by inclined plans or curved surfaces.The slope of the roof is chosen to determine a fastevacuation without water stagnation on the roof .

    Roofs must also provide enough tightness againstwater infiltration in windy rain conditions.

  • The roof covering constructive type and component material

    Slope (cm/m)

    Minimum Usual Maximum

    Burnt clay tiles : plain tiles:- distributed in single layer ;- distributed in double layer; grooved tile:- rolled;- pressed;

    60454553

    70 . . . 9055 . . . 7050 . . . 7045 . . . 70

    275275120120

    Plane metal sheet- with simple horizontal and double

    vertical seams;- with double horizontal and vertical

    seams;

    157

    30 . . . 5015 . . . 60

    Vertical

    Profiled metal sheets- one sheet high slope breadth;- more sheet high slope breadth;

    512

    6 . . . 1215 . . . 40

    Vertical

  • The roof type &slope

    The structure of thetightening layer

    Recommended values of thetightness pressure limit

    Flat roof with theslope 20o

    Waterproof membrane incontinuous layers

    The pressure of a water layer ofminimum 2 4 cm

    Roofs withthe slope 20 o

    Continuous layers, with tight joints, constructed by overlapping and sticking A pressure grater or at least

    equal to the wind dynamic pressure estimated by standards for the building placement area.

    Discontinuous covering of overlapped materials or elements with pen joints

  • b - Hygrothermal protection

  • Considering the hygrothermal aspect, the roof is submittedto more severe conditions than other building elementsbecause:

    in the summer time building must be protected against theaction of the external hot air, the temperature of theroof is much grater than the temperature of external airdue to the accumulation of direct sun-light;

    in the winter the heat loss throw the roof are greater tanheat losses thou other elements of the building envelope(30% form the whole heat loss) because the warm air frominside has the tendency the rise up to the ceiling;

    the water vapour when migrate from spaces with higherpressure to those with lower pressure (usually form insidethe building to outside) has to pass through the componentlayers of the roof. Therefore, the component layers ofthe flat roof must permit this transfer of the watervapour without condensation between the layers.

  • The position of the thermal insulating layer is important !!

    The temperature differences supported by the rigid roof due to the seasonal changes can cause cracks in the building structure

  • c. Tightness against air infiltration

    In order to avoid the excessive thermal loosesand the decrease of the thermal comfort,emphasised by the air circulation, the roofmust provide a minimal resistance to airpermeability (Ra, min) that can be establishedby the expression:

    Ra, min 0,1v2R0, nec [m/s],

    where : v - is the wind speed, in m/s;

    R0,nec - is the necessary thermal resistance of theroof, in m2K/W.

  • d. The function of acoustic protection

    The roofs have to contribute to the phonic insulationof the functional areas placed under them againstairborne or impact sound.

    The requirement of insulation against airborne soundis fulfilled by internal configuration of the roofmade as a system of multiple layers of differentmaterials.

    The requirement of insulation against impact soundis not a major condition for the roof because justthe raindrops can cause this type of noise. In thiscondition, the metal sheeting coverings are themost sensitive.

  • e. The function of natural lighting

    It is possible to include in the surface of the roof sometranslucent or transparent elements to provide naturallight in the spaces situated below the roof.

  • 4. The roofs forms4.1. Pitched roofs with plane surfaces

  • 4.2. Shell roofsThe term shell is usually applied to three-dimensional structures constructed with acurved solid slab or membrane acting as astressed skin. This kind of roof is usedespecially for industrial and cultural buildingsbecause of their span and height.

  • Single curved shells - these are barrel vaults of whichthere are two main forms: long span barrels and short-span barrels

  • Cantilever barrel vaults

  • Double curvature shells

    - Domes

  • - Hyperbolic paraboloids

  • 5. Structural elements of the roofs

    1. Pitched roofs in timber

    Usually pitched roofs are: singlepitched, double ( or purlin) roofs, ortriple ( or trussed) roofs.

  • 1.1 Single pitched roofs

    The lean-to roof is a

    monopitch roof with the tops of

    the rafters pitched against

    a wall. The upper ends of

    the rafters are supported by a wall plate fixed

    to the wall by corbel brackets

  • Couple roof -this is the

    simplest from of ridge roof with

    pairs (or couples) of

    rafters pitched one against

    other at theirheads and

    bearing on the supporting wall at their feet.

    In this arrangement of the rafters, P- the load transmitted by the raftersgives a vertical component (V), well supported by the walls, and a horizontalcomponent (H) which is an outward thrust action of the rafter that tendsto overturn the walls. In order to keep the roof stable this outer thrustmust be resisted by sufficiently heavy supporting walls.

  • Close couple roofs

    This form of roof

    consist in the

    iIntroduction of

    horizontal members

    to tie together the feet

    of each pair of rafters

    and to prevent the

    outward thrust or

    spread tendency of the

    rafters. This forms

    simple triangulated

    structure and produces

    vertical loads in the

    bearing walls.

    The maximum

    economic span of this

    roof is about 6.00 m

  • 1. 2 Double or purlin roofs

    When the span is greater than 6 m and couple types of roof areused, the required rafters will have an uneconomic cross section, muchgreater than 10 cm. In this case, instead of using large rafters, it ischeaper to introduce a longitudinal beam under all the rafters, from oneslope, to reduce the span.

    The introduction of this beam or purlin as a second stage ofsupport makes the roof to be considered as double roof. In this type ofroof the main structural elements are the purlins which run parallel tothe length of the roof. The purlins may be supported directly by crosswalls or partitions at sufficiently close spacing or by props spaced at 3~5m, along the length of the purlins.

    Rafters are secondary structural elements and are placed inpairs, or couples of rafters, pitched against each other at their heads andwith their feet resting on opposite walls.

  • Timber purlin roof elements: 1- wood boarding, 2-

    rafter, 3- purlin, 4- prop, 5- pad, 6-collars

  • 1.3 Triple ortrussed roofs

    When the roof span islarge (more than 6,5 m)and multiple purlins arerequired these impose alarge amount of props tosupport the purlins. Inthis case an alternativemethod of supporting thepurlins and the roofcovering is needed.

    The structural elementsthat can support thewidth of the roof are theroof trusses.

  • Truss can be made only using timber for spans of 6 to 10 m, or can combine

    timber for the compressed members and steel for the tensioned members for

    larger spans (of 10 30 m). Another alternative for spans of 6 15 m is to use

    plywood web beams instead of trusses.

  • 2. Metallic pitched roofs - used forindustrial construction with large spans

  • 3. Reinforced concrete roofsThese roofs can be constructed as a monolithic element or made of acombination of precast reinforced concrete elements.

    3.1 Monolithic reinforced concrete roofs - used only forspecial buildings where precast elements cannot be used.

    Plates with ribs and beams

  • Vaulted and arched roofs, made of a continuous curved shell ofreinforced concrete with constant or variable thickens.

  • 3.2 Roofs of precast concrete elements

    The main structural precast concrete elements used for roofs are :

    a) Precast rafters with on end supports. of precast orprestressed concrete ( T or double T sections) beams - withconstant or variable cross section on their length.

  • b) Precast or

    prestressed

    concrete trussed

    roof beams.

    As secondary elements

    of the concrete roof

    there are used:

    - reinforced concrete

    ribs to support the roof

    covering.

    - reinforced concrete T

    or elements, precast

    or pre-stressed

    caissons, etc.

  • 3.3 Reinforced concrete flat roofs structureThe roofs with the slope of less than 7% are called flat roofs and in ourcountry are common in blocks of flats and social or cultural buildings.

    Flat roof structure: 1 - ceiling, 2 - reinforced concrete slab, 3 - cement mortar sloping layer, 4 - vapour barrier, 5 - thermal insulation, 6 - mortar protection,

    7 - diffusion layer, 8 - waterproof layer, 9 - protection layer (gravel).

  • 6. Roof coverings

    1 Thatched roofs

    Sheaves of reed are joinedon wood strips beginningfrom the eaves and endingat the ridge. The width ofsheaves is (more than) 35cm for reed and 25 cm forstraw and they are joinedwith cooper wires to thestrips.

  • 2 Timber roof coverings

    2.1 Timber board coveringThe wood boards can be fixed parallel to the ridge, directly on the rafters

    and in this manner results a horizontal weatherboarding, or can be placed

    down directly on the purlins, perpendicular to the ridge in a vertical

    weatherboarding system. The timber boards should be preservative treated

    against weather action and fungal attack before being fixed.

  • 2.2 Shingle roof covering

  • Wood roofs must be always equipped with skylights or roof lights

    or dormer windows for a better ventilation of the attic, in order to

    increase its life span.

  • 3 Pitched roofs covered with asphalt cardboardWhen it is used for pitched roofs, the asphalt cardboard is distributed in twolayers. The first layer of the cardboard sheets is spread parallel to the eaves andthe second layer across it.The sheets are fixed with nails and lapped 8~10 cm at joints. The last layer iscovered with a continuous membrane of hot bitumen mixed with sand poured onthe surface and spread to a thickness of not less than 3 mm.

  • 4 Slates4.1 True

    slatesTrue slates are obtained from

    metamorphic rocks which have the

    property that they can be split into

    thin sheets. They are impervious to

    water and extremely durable.

    Slates are manufactured in a variety of sizes in length from 60 to

    25 cm and in width form 35 to 13 cm.

  • 4.2 Cement diamond slates

  • 4.3 Clay roofing tilesA. Curved tiles1 - Pan and cover tiles - This type of covering consists in pairs of rounded pan-tile

    (or under-tile) and cover-tile (or over-tile), in the shape of truncated cone orhalf of cylinder. Their length is between 30...40 cm and they have nail holes orfixing nibs.

  • 2 - one-piece barrel tie

  • B. Plain tilesThe tiles are laid over the wood board and battens or directly on the battens.It is recommended to place under the tiling battens a waterproof membrane ofasphaltic bitumen on felt or cardboard sheets against the condensation thatmay occur on the underside of the cold, impervious underlay of tiling.

    Scale tiles - size 35 cm x17 cm x1 cm

  • Interlocking single lap tiles - One or more grooves in the vertical edges ofthe tiles interlock when the tiles are laid. The advantage of this side lock is that it

    excludes wind and rain more effectively than a simple overlap

  • Ridges - The usual form of ridge

    tiles is the half-

    round tile (but there

    are also V ridge

    tiles, see fig.

    5.39,b.) and should

    be bedded in fillets

    of cement mortar

    and have sufficient

    width to give right

    lap over the tiles

    below the top

    course. The top two

    rows of tiles should

    be nailed. Applying

    ridge tiles with the

    lap facing away

    from the prevailing

    winds helps the tile

    shed wind-driven

    rain.

  • Hips the hips of the roof can be covered with ridge tiles beaded inexactly the same way as the ridges. This constructive solution requires theuse of hip irons to prevent the whole row of tiles from slipping.

  • Corrugated cement sheets - 120~350 cm long and 67~102.5 cm wideused for non-heated industrial or agricultural buildings.

  • Metal sheet coverings for pitched roofs - The predominatecoverings for low pitched roofs are black metal, galvanised sheets andsometimes aluminium, copper or lead strips, which are comparatively light inweight and therefore do not require heavy timbers to support them and havealso a useful life as roof covering of many years and offer a very goodwaterproof insulation.