T4676/JCT/krrt 2nd May 2012 Mr and Mrs T Crowther Lansor, Nr

T4676/JCT/krrt
2nd May 2012
Mr and Mrs T Crowther
Lansor,
Nr Caerleon,
NP18 1LS
Dear Mr and Mrs Crowther
14 New Market Street, Usk
Structural investigations
Brief
In accordance with instructions, I attended the above property on 25th April
2012, in order to assess certain structural elements in the context of the
proposed alteration works. These were as follows:
i)
Replacement roof structure over Garden room, and consideration of party wall
implications.
ii) Assessment of existing roof and wall structure to first floor sitting room.
iii) Investigation of deformed ceilings to first floor bedrooms and hallway, and an
assessment of the main roof structure.
iv) Advice on proposed garage foundations.
Intrusive investigations took place, including localised removal of wall and
ceiling finishes, but in all cases, this related only to ‘modern’ plasterboard
linings and did not include any historic fabric.
i) Roof over garden room
I was asked to advise upon the party wall implications of replacing the
existing flat roof structure, particularly as a triangular ‘bulkhead’ was
apparent in the rear corner against the party wall. Having looked at this in
some detail, I can confirm that the roof of the adjoining property does not
project past the party wall face, but this is in fact an insulated timber panel
which was constructed as part of the flat roof installation. This is effectively
a small section of external infill wall between your flat roof and the
adjoining pitched slated roof. As part of the re-roofing proposals, this will
simply be removed.
The party wall was also investigated to establish the structural alignment,
as this is concealed behind plasterboard linings. The plasterboard was
removed locally to expose the masonry wall behind. The wall was found to
‘step’ on plan, which appears to correlate with a pre-existing rear access
passage. The distance from plaster face to wall face varies from 40mm –
130mm.
ii) Structure of first floor sitting room
The existing roof, walls and floor were investigated.
Description of roof
The roof comprises slates on battens over reinforced sarking felt. Common
rafters (50 x 75) are provided @ 450mm c/c approx, spanning from eaves
to ridge and taking support on a mid-span purlin. The purlins are 80 x 145
painted timbers with a max clear span of 2150mm. The purlins span from
the end wall onto intermediate timber ‘trusses’. These trusses comprise 50
x 160 principal rafters, with 50 x 140 ridge collars. At the eaves, the
principal rafters are bevel cut to bear upon 45 x 75 posts and triangular
haunches.
Where exposed, the existing wall plate was found to be severely damaged
by a combination of by insect attack and damp penetration.
Commentary on roof
The roof structure can be characterised as ‘lightweight’, and appears to
have originally been contrived to support corrugated sheet coverings.
Misalignment of the ridge line is clearly apparent externally, indicating
deflection of the supporting structure. Notwithstanding this, it may be reused on the basis that new common rafters are installed, and that we will
be permitted to adopt artificial slate to minimise the weight of the
coverings. I am aware that natural slate would be the default specification
for a listed building, but this would involve greater replacement of the
existing structure. We will need Rachel to advise upon this aspect.
The replacement rafters will be considerably deeper than the existing,
simply to provide sufficient insulation space, and this will be the criteria for
rafter depth rather than their structural capacity. When the new rafters are
installed, these should all be tied at the ridge with softwood collars, which
will result in a narrow section of ‘flat’ ceiling along the centre of the room,
at roughly the same level as the existing truss collars. This is already
indicated on the proposed cross section.
Along the outer wall, a studwork panel will be introduced in the plane of the
existing posts. This will be located over the stone wall at ground floor level
and will be perfectly adequate to transmit roof loads from the new rafters
and existing trusses. The size of stud may be dictated by insulation
requirements, but for structural purposes, 47 x 95 C16 studs @ 600mm c/c
will be sufficient. The studs should be set to match the new rafter centres,
and should be ‘paired’ at each of the existing post locations, being bolted
through the posts with M10 bolts @ 300mm c/c. A layer of 9mm OSB
sheathing should be fixed to the inner face of the panels with 3.35 x 50
annular ring shank nails @ 150mm c/c, following installation of insulation.
Description of walls
The first floor wall on the garden side, consists of a 215mm (9”) bonded
brick wall, surmounting the ground floor stone wall which is 450mm (18”)
thick. The brick wall is constructed flush with the outer face of the
stonework, with the previously noted timber posts set inside the brickwork
to bear upon the inner edge of the stone wall below. The wall containing the
chimney breast on the opposite side of the room is also masonry behind a
plasterboard lining.
Commentary on walls
The walls will be structurally adequate, and the garden side wall will benefit
from the addition of an insulated stud wall panel on the interior.
The damaged wall plate, seated on top of the brick wall, will require
replacement as part of the works.
Description of floor
The first floor comprises50 x 130mm joists @ approx 450mm c/c. The
underside of the joists are exposed above the dining area, but concealed
above a ceiling over the kitchen. The joists clear span approx 2900mm.
Commentary on floor
The existing floor is not unsafe, but does display considerable deflection.
Strength and stiffness are separate structural considerations, and even
though timber joists may be adequate to safely sustain a given load, they
may still be too flexible to prevent undue deflection (sag) and a ‘springy’
feel. It is not a requirement of the building regulations to retrospectively
upgrade floors to reduce deflection, but it is advisable and desirable to
improve the floor characteristics whist undertaking a refurbishment project.
There is currently a ‘ridge’ across the existing floor which correlates with
the line of the arched kitchen partition below. Although this wall is non-load
bearing, the excessive deflection of the floor has resulted in some load
being transferred into the wall and a ‘hard’ spot appearing in the floor
boarding above. The removal of the partition will therefore exacerbate the
current deficiencies. It is recommended that additional joists are installed.
If these are to provide a level boarded surface, and a flat plane for a new
ceiling, these would need to be deeper than the existing joists, as these are
already distorted. I would therefore suggest 47 x 170 grade C16 joists are
installed alongside the existing.
iii) Deformed ceilings and main roof structure
The deformations of the ceiling above the first floor landing and bedroom 3
are significant, and are of a degree that suggests failure or detachment of
the supporting joists. An inspection of the roof void indicated that the
supporting joists run parallel with the front wall of the property, as we had
envisaged from the pattern of deformation, but the form of the main roof
was entirely unexpected. The roof contains a ‘room’ space with a boarded
floor and plastered perimeter walls. The approximate profile is indicated on
the sketch below.
Intermediate trusses (2 No)
Common rafters
Purlin
Plastered stud
Boarded floor on joists
Ceiling joists
Main roof profile diagram
not to scale
The structure comprises common rafters spanning over intermediate
purlins, which in turn are supported on 2 No intermediate timber trusses
spanning between front and rear walls. Ceiling joists run parallel with the
front wall, and are supported on the bottom edges of the truss tie chords.
Separate joists are then provided level with the top of the truss tie chords,
and boarded, over the centre section of the roof space. Lathe and plaster
stud walls then delineate the front and rear edges of the room, isolating the
triangular eaves voids. Narrow access openings are provided through the
plastered stud walls into the eaves voids, but the restricted access space
and the possible failure of these ceiling joists, precluded safe access for
inspection.
With the above in mind, it was not possible to inspect the ceiling joists in
the areas of the worst ceiling distortion. The only means of establishing the
nature and extent of the problem with the ceilings will be to remove the
existing ceilings at this end of the house. The hallway has polystyrene tiled
finishes, and the substrate above is unknown. The bedroom ceiling is
boarded but it is not clear whether this is fixed directly to the ceiling joists
or used to ‘sandwich’ a previously failing ceiling. The ceilings that would
need to be removed are those which will need to be replaced in any event,
due to their excessive distortion and clear failure.
Once removed, it will be possible to define a repair and have this costed by
your contractor.
Unfortunately, further exploratory works are required before the exact
scope and detail of the repairs can be ascertained. These investigations are
significant and will require consent of the conservation officer. Rachel is of
course better placed to advise on matters of listed building consent. If the
programme of work permits the ceiling removal to be done in advance of
commencing the main contract works, this should be done. The exposure of
the structure should also include the removal of the existing insulation,
which is loose-laid over the ceiling joists in the eaves voids, and directly
over the boarded floor area.
In advance of the further investigations, the nature of the ceiling failure
suggests that a number of the supporting joists (probably 50 x 75) have
fractured or become detached from their supports. Owing to the tendency
for timber to undergo creep deflection, members that have been distorted
for a long period of time permanently change shape, and this is likely to
mean that a number of ceiling joists will require complete replacement
rather than simple repair. It must also be recognised that the plastered
stud panels in the roof space represent a considerable weight, and that the
structure may not have been constructed with adequate strength to
withstand such a load in the long term, particularly if timbers undergo a
reduction in strength due to some deterioration. Once exposed, there may
therefore also be some work identified to strengthen the supports for the
plastered panels. I suspect that the conservation officer will be keen to
retain these features, but once again, Rachel is far more experienced in
dealing with consents.
My inspection revealed a fractured common rafter, and a number of joists
which would benefit from upgrading. Other defects are likely to be
identified, and these are best dealt with when the roof structure is fully
exposed.
iv) Garage foundations
There are difficulties associated with the construction of the garage building
within the influence zone of existing trees. The degree of difficulty depends
upon the exact nature of the sub-soils which is currently unknown. I have
made a preliminary enquiry with the local building control officer, who has
informed me that the soil is likely to be an alluvial material with variable,
and sometimes very poor, strength characteristics. In addition to soil
strength, there are two principal aspects to consider when building in
proximity to trees. Firstly, how the construction will impact upon the health
and stability of the tree, and secondly the potential effects of the tree(s)
upon the foundations. I understand that it has been accepted that the
partly felled mature tree in the adjoining garden to the North is no longer
protected by a TPO, but there is also a very large tree in the garden to the
South. The garage construction will inevitably impact these trees, and
confirmation of the protection status is therefore very important. The
potential effect of the trees on proposed foundations depends upon the
nature of the sub-soils. If the soil is cohesive (clay or silty clay) this may
shrink and swell in response to moisture content. The degree of
‘shrinkability’ needs to be established by testing, and foundations may then
be designed in accordance with NHBC guidelines to ensure these are of
sufficient depth to preclude influence of shrinking/swelling clay.
Bearing in mind the likely variability of soil conditions, and the effect upon,
and of, the trees, a soil investigation
will be required. This goes beyond the
scope of a simple shallow trial pit,
and will require formal investigation
by a geotechnical consultant. I have
issued enquiries to a number of
consultants to issue competitive
tenders for the investigation, and will
advise further n due course. There is
a possibility that the constraints of
the tree(s) will dictate a nontraditional foundation such as piling,
but the investigation will steer us
towards the most cost effective
solution to the garage construction.
Significant misalignment was noted of
the free-standing boundary wall along
the South East boundary, particularly
at the garage location.
A small partial failure of the masonry was noted towards the far end of the
wall and the ‘lean’ of the wall is significant alongside the existing garage.
Construction work will inevitably impact on this wall, and consideration
needs to be given to this through the mechanism of the Party Wall etc Act
1996. In the first instance, the ownership and responsibility for the wall
needs to be established. This will define the wall in terms of the Party Wall
Act and whether this actually applies in this instance. Once the ownership is
clarified, and approach will need to be made to your adjoining owners to
access and inspect the wall from the other side, and to commence
discussions about how this will be approached. Some re-construction
appears inevitable where the wall is already leaning at its worst point.
Conclusions
•
The existing triangular bulkhead against the party wall within the garden room
may be removed.
•
The frames and purlins of the existing sitting room roof may be retained
subject to using artificial slate and replacing rafters. Should natural slate be
essential, the roof structure may require complete replacement.
•
An insulated stud wall is to be constructed along the existing brick wall
overlooking the garden. The existing wall plate is to be replaced.
•
The existing sitting room would benefit from having floor joists supplemented
with additional new joists to reduce deflection.
•
The main roof structure unexpectedly contains a room. The restricted access
requires some ceiling removal to expose the structure and determine repairs.
•
A geotechnical investigation will be required to establish parameters for garage
foundation design. Clarification of the status of the existing trees and
associated protective measures is required in advance.
•
The boundary wall is likely to require partial re-construction, and ownership
needs to be established in order to progress proposals/negotiations with
adjoining owners.
I trust that this is of assistance. Pease do not hesitate to contact me should you
require further advice.
Yours Sincerely
O’Brien & Price Stroud
John Topp IEng AMIStructE AssocRICS