Greenhalgh & Co.

Chartered Building Surveyors & Valuers

 

boltonsurveyors.org.uk

Greenhalgh & Co.

Chartered Building Surveyors & Valuers

21 Towncroft Lane

Bolton  BL1 5EN

Tel: +44 (0)1204 845382

Mob: +44 (0)7813 439196

Email: greenhalghco@btconnect.com

 

RICS Company No. 002973

 

Pitched roof structures

Pitched roof structures

 

There are two main types of pitched timber framed roof in conventional low-rise property, namely :

 

q       The traditional purlins, rafters, ridgeboard.

q       Roof trusses and wind bracing.

Traditional purlins rafters and ridge board construction

Bowing of the roof structure is very common. 

This is due to stress creep in the timber and can cause lateral thrust on the walls. 

Particular points, which exacerbate the problem, are :

The longer the span and smaller the section of timber, the more likely this is to occur.

 

Purlins being offset from the vertical.  This was undertaken by the builder to achieve a flush fixing for the rafter.  Unfortunately it reduces the purlin strength and allows it deflect towards the external wall.

 

Hip end roofs having insufficient vertical support.  Up to around 1930-35 it was often the case that hip end roofs simply relied on the hips and rafters to transmit the weight of the roof to the walls.  However the pressure induces lateral thrust on the walls and causes the walls to be pushed out.

 

After around 1930 vertical posts were added in the roof space.  These ran from the tops of the internal partitions to the underside of the purlins, and in some cases the meeting points of the hip.  This vertical support allowed the weight of the roof to bear down the through the walls, thus the problem of pushing walls out was overcome.

 

The large section of timber used in purlins is often affected by shrinkage splits when it is gradually drying out.  Sometimes such a split will seriously weaken timber and cause eventual failure.  It is common and affects around one in a hundred properties.

 

Deflection of ceiling joists.  This again due to stress creep in the timber.  Sometimes when the roof above the joist hanger deflects, it can cause downward pressure on the ceiling joists and this again will cause uneven ceilings.  The ceiling joist hangers can therefore act to push the ceilings down if the roof structure above deflects. 

 

Punched metal banding (available from most builders merchants) forms a very effective replacement for conventional timber joist hangers.  The banding will act to support the ceiling joists, but will not transfer downward thrust from the roof to the joist.  In addition if the screw fixing is angled it is possible to put the banding into tension at the point of fixing.

 

 

Roof trusses

 

Roof trusses do not tend to suffer major problems.  However the following are to the most common defects :

 

Insufficient lateral bracing.  Lateral bracing is strips of timber used to connect the trusses together.  It is often formed with 100mm x 38mm softwood.  This bracing is added to give the trusses lateral stability during the construction process and stability against wind pressure.

 

When roof trusses were first introduced, lateral bracing was often inadequate.  However the roof tiling battens act as bracing.  It is a very simple job to add lateral bracing to a roof if such does not exist in sufficient quantities.

 

The roof truss ends form the support for the perimeter roof boards (eaves and fascias).

The exposed position of the eaves and fascias leads to rot, and this can also affect the ends of the trusses to which they are fixed.  Again it is relatively straightforward to remove the ends of trusses and replace with treated timber when the eaves and fascias are replaced.

 

There are cases where owners of properties have decided to make the roof space into a usable storage area.  To this end, they cut out sections of the trusses and board out the roof space.  The end product is that the truss looses its strength and the roof and ceiling sag.

 

Needless to say cutting sections out of roof trusses can have disastrous consequences.

 

 

Weights of typical roof coverings 

 

Sometimes when it comes time to re-cover an existing pitched roof, a different type of roof tile or slate is used. If one uses a significantly heavier roof tile or slate the strength of the roof may need to be increased to combat the increase in weight.  However in most cases there is usually little change and therefore little alteration needs to be undertaken to the existing roof structure. 

 

Many people perceive that concrete tiles are heavier than natural slates and rosemary tiles, however as the table below demonstrates, this is not the case.

 

 

Examples of the weight per square metre of some typical roof tiles and slates.

 

Type

Pitch

Size (mm)

Lap (mm)

No. per sqm

Weight per Kg / sqm

Natural slates

 

 

30°

500 x 250

100

20.0

50.0

Natural slates

 

 

30°

600 x 300

100

13.3

46.5

Marley Modern concrete tile

 

30°

420 x 330

75

  9.7

44.0

Marley Roman

concrete tile

 

30°

420 x 330

75

  9.9

50.0

Marley Ludlow

concrete tile

 

30°

420 x 330

75

  9.8

45.0

Rosemary clay tile

 

 

35°

267 x 168

65

60.0

73.8

Fibre cement slating

 

 

30°

600 x 300

100

13.4

20.4