Download - Cap4 Roofs

Transcript
Page 1: Cap4 Roofs

ROOFS

Page 2: Cap4 Roofs

1. General elements

2. Classification

3. Functional requirements

4. The roofs forms

5. Structural elements of the roofs

6. Roof coverings

Page 3: Cap4 Roofs

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.

Page 4: Cap4 Roofs

• 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.

Page 5: Cap4 Roofs

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.).

Page 6: Cap4 Roofs

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.

Page 7: Cap4 Roofs
Page 8: Cap4 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, ...

Page 9: Cap4 Roofs

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.

Page 10: Cap4 Roofs

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.

Page 11: Cap4 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.

Page 12: Cap4 Roofs

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

Page 13: Cap4 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.

Page 14: Cap4 Roofs

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.

Page 15: Cap4 Roofs

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.

Page 16: Cap4 Roofs

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

Page 17: Cap4 Roofs

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

Page 18: Cap4 Roofs

b - Hygrothermal protection

Page 19: Cap4 Roofs

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.

Page 20: Cap4 Roofs

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

Page 21: Cap4 Roofs

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 0,1·v2·R0, nec [m/s],

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

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

Page 22: Cap4 Roofs

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.

Page 23: Cap4 Roofs

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.

Page 24: Cap4 Roofs
Page 25: Cap4 Roofs
Page 26: Cap4 Roofs
Page 27: Cap4 Roofs
Page 28: Cap4 Roofs

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

Page 29: Cap4 Roofs
Page 30: Cap4 Roofs

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.

Page 31: Cap4 Roofs

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

Page 32: Cap4 Roofs
Page 33: Cap4 Roofs

Cantilever barrel vaults

Page 34: Cap4 Roofs

Double curvature shells

- Domes

Page 35: Cap4 Roofs
Page 36: Cap4 Roofs

- Hyperbolic paraboloids

Page 37: Cap4 Roofs
Page 38: Cap4 Roofs
Page 39: Cap4 Roofs

5. Structural elements of the roofs

1. Pitched roofs in timber

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

Page 40: Cap4 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

Page 41: Cap4 Roofs

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.

Page 42: Cap4 Roofs

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

Page 43: Cap4 Roofs

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.

Page 44: Cap4 Roofs

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

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

Page 45: Cap4 Roofs

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.

Page 46: Cap4 Roofs

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.

Page 47: Cap4 Roofs

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

Page 48: Cap4 Roofs

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 for

special buildings where precast elements cannot be used.

Plates with ribs and beams

Page 49: Cap4 Roofs

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

Page 50: Cap4 Roofs

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.

Page 51: Cap4 Roofs

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 π elements, precast

or pre-stressed

caissons, etc.

Page 52: Cap4 Roofs

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).

Page 53: Cap4 Roofs

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.

Page 54: Cap4 Roofs

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.

Page 55: Cap4 Roofs

2.2 Shingle roof covering

Page 56: Cap4 Roofs

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.

Page 57: Cap4 Roofs

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.

Page 58: Cap4 Roofs

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.

Page 59: Cap4 Roofs

4.2 Cement diamond slates

Page 60: Cap4 Roofs

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.

Page 61: Cap4 Roofs

2 - one-piece barrel tie

Page 62: Cap4 Roofs

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

Page 63: Cap4 Roofs

Interlocking single lap tiles - One or more grooves in the vertical edges of

the 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

Page 64: Cap4 Roofs

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.

Page 65: Cap4 Roofs

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.

Page 66: Cap4 Roofs

Corrugated cement sheets - 120~350 cm long and 67~102.5 cm wide

used for non-heated industrial or agricultural buildings.

Page 67: Cap4 Roofs

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.


Top Related