Building Pathology (2) Flashcards
(1) North Caldeen Road - What is cut edge corrosion and why does it occur?
- Corrosion that affects profiled metal roofs typically at the sheet laps and ends.
- Profiled metal sheets (typically galvanised steel) are coated with a factory finish. Once coated, the galvanised steel sheets are rolled into coils before being sent to be cut and profiled.
- Where they are cut, this leaves an untreated metal edge.
- Over time, the treatment begins to peel back from the edge.
(2) North Caldeen Road - How to treat / prevent cut edge corrosion?
- If not too damaged, the surface level corrosion can be removed with an angle grinder. The edge can then be treated with a silicone based paint (such as Seamsil). These products can often come with a guarantee, where Seamsil offers a 15-year guarantee.
(2) North Caldeen Road - What are the different types of profiled steel finishes and how do you identify them?
The three main types of profiled metal steel finishes are;
- Plastisol (PVC). This can be identified with a slightly leathery, raised finish.
- PVF2 / PVDF - Paint based, this has a smooth finish.
- Powder Coated - Polyester based (synthetic resin) very smooth yet almost slightly plastic feeling. Typically used on windows and doors.
What is corrosion?
Deterioration of materials (usually metals) by chemical / electrochemical reaction with their environment (oxygen / water).
What are the stages of failure to a paint finish of profiled steel?
Loss in colour, delamination, rust.
(2) North Caldeen Road - What other defects did you identify and the causes?
Leaking gutters - failed gutter linings
Drainage issues due to build up of gutter sediment
Rotten window frames
Impact damage / damage to cappings.
(2) Trystings Close - What would be the typical construction of a 1960s bungalow?
Timber trussed roof
Cavity wall (early) with brick outer leaf and blockwork inner leaf
Strip foundations
Casement windows of steel / aluminium / timber
Concrete slab floor with sometimes a DPM of polythene / bitumen.
(2) Trystings Close - How did you conduct your inspection?
I first did a desktop study, where I found borehole data of the local area (Claygate). I then did some research (OSMAPS) to determine the age of the premises.
I arrived on site and discussed with the owner to establish the timeline of the cracking.
I asked whether any as built drawings were made available but there were none; however I was informed that the extension was carried out in the 1990s.
I had a walk around the site to familiar myself. I recorded the construction of each element, and noted the location of the services.
I observed the topography of the site.
I carried out a sketch noting; crack locations, widths, types of cracks. I used a level, and discovered that the walls were not deflecting.
Internally, I recorded the location of the internal cracks. I checked for any floor sags / gaps in the skirting or doors jamming.
After the survey I wrote up my notes and determined that the cracks were affecting the serviceability of the premises, however at present it was not deemed to be structurally unstable / dangerous.
As per BRE digest 251, I determined the damage as ‘Category 3’ - the cracks could be repaired by a skilled trade but my main concern was future progressive movement. This is because the ground conditions were likely clay based, and I observed a large Oak tree to the rear of the garden within a proximity of 5-6 metres of the extension.
This would warrant further monitoring (Tell-Tales) and further investigation by a structural engineer. However, the buildings insurance provider should be notified as they would likely monitor and investigate the problem if subsidence is covered within the policy.
What are tell-tales and how would you use them?
It is a crack monitoring gauge that monitors horizontal and vertical movement across a crack on a flat surface. It consists of two plates which overlap for a part of their length.
The bottom plate is marked in MM and the top plate is transparent and marked with a hairline cursor.
This is fixed into the wall using screws and adhesive
You fix the tell tale across the crack. As the crack opens or closes the cursor moves relative to the calibration scale
You then record the opening and closing of the crack to be able to tell if the movement is structurally progressive.
10 mm horizontal - 20mm vertical
LVL 1 - What is differential settlement, subsidence and heave?
Differential Settlement - Unequal foundation pressures arising from such factors such as extensions added to existing buildings or concentrations of load.
Subsidence - A movement down in the ground level. In clay soil, the growth of large trees which remove water from the ground can cause this. This can lead to vertical cracking in the centre of the building or diagonal cracking towards the corner of the building, which is stepped and gets wider at the top due to rotational movement. Generally leading from ground level.
Heave - Opposite to subsidence, an upward movement. In clay soil the removal of a tree will lead to an increase of moisture content which will cause the soil to swell. This causes wider cracking at the bottom and thinner at the top.
I believed the action upon the building was differential settlement, mostly because the cracking was along the joint line. This would likely occur naturally, but due to the shrinkable soil conditions, heatwave and the presence of the mature Oak Tree, this caused a great deal of settlement in this instance. In any case further input should be taken from a structural engineer, the cracks should be monitored and remedial work carried out if required.
(2) Trystings Close - Why would clay soils be an issue?
Clay soils - mouldable, smooth and greasy
Due to the open nature of the molecules, they readily absorb and let go of water. This means that clay soils shrink and swell depending on moisture content.
(2) Trystings Close - Why would the tree have an affect here?
Due to the heat wave, desiccation of the soil would occur, resulting in downward movement of the soil in the summer. The tree would be extracting moisture from soil in an already dry environment.
(2) Trystings Close - If the foundations were found out to be insufficient, what would be the options?
Three options:
- Tree removal (however there would be a risk of heave). Also the tree might be a protected under a Tree Preservation Order.
- Underpinning.
- The exact cause must be investigated further, and separated from the design process.
- BS 8004 ‘ Before underpinning is resorted to, the fullest possible investigation should be carried out by an experienced and competent person to determine whether an underpinning procedure will achieve the object intended’.
- The shape, size, depth and condition of the existing foundations of a damaged building are examined.
- Geotechnical investigation - trial pits to define the soil type, clay, silt, sand, gravel, silty clay, sandy gravel etc.
Consistency and strength
Structure and texture
Moisture conditions
Root areas
During the investigation it is imperative that the level of any water table is determined. These have a major influence on the choice of type of underpinning (standpipe used to monitor ground water levels) although for domestic it is not often used.
(2) Trystings Close - Can you explain the types of underpinning?
- Mass Concrete (most popular)
‘traditional method - extending an existing strip or pad foundation downwards to reach an adequate bearing stratum. Constructed in sequence of ‘bays’, where each section is excavated, concreted and pinned. Generally restricted up to 2m depth due to labour costs. Propping may be required - Beam and pier (second)
Reinforced concrete beam formed at or above the footing level spanning between mass concrete piers
Small areas of brickwork below DPC are broken out and props ‘stools’ are fixed into the wall (Steel) with precast concrete top and bottom plates
The remaining brickwork or existing footing is removed, reinforcement and formwork fixed and the beam cast into the wall; the stools are incorporated into the beam. The beam is pinned up to the existing brickwork. Isolated piers are then excavated to appropriate depth, concreted and pinned up to the underside of the beam
Used in most situations
Less risky
High labour costs in hand excavation tend to restrict their use to depths less than about 4m - Beam and pile (less etc.)
Where a suitable bearing stratum is not available within a depth economical for hand excavation (>4-5m) or through shallower depths through soft, loose or water bearing strata.
Similar to beam and pier, but beams are extended at corners and intersections to form caps attached to pile heads.
Piles usually augered or bored and cast in-situ.
Can be used satisfactorily in all circumstances but economical to depths of 4-6 m.
Less risky
Trickier detailing forming pipe caps after completion of the beams.
Piles / Mini pile
Loads transferred directly from the stricture to the piles either by needles or cantilevered pile caps, or by placing the piles directly through the existing footing.
Depends on inherent strength of existing structure
- Expanding resin injection
(3) Forge Cottages - What is timber decay?
The deterioration of wood cells via fungi - resulting in the loss of weight and strength of the timber
(3) Forge Cottages - Why is a mortar fillet detailing unfavourable? What would be the solution?
They are prone to cracking and moisture penetration
3) Forge Cottages - What is an impermeable sarking membrane?
A membrane that is fixed after the battens to provide a line of defence for moisture. Older felts are often bitumen based products which are non-permeable. Breathable membranes are synthetically made which allow for moisture to escape from one side. Often foil backed to reflect heat
3) Forge Cottages - What does 22% wood moisture mean?
> 22% wood moisture is the percentage of wood moisture that will allow fungi to develop in timber.
3) Forge Cottages - Talk me through how you used the electronic moisture meter
I used the electronic moisture meter to measure moisture contents throughout the whole roof area, not simply around the area of where the moisture is apparent.
I took note of the moisture contents of the timbers from the rafters, purlins, battens and joists where possible throughout all accessible areas.
3) Forge Cottages - How did you know condensation was also an issue
I noted that the moisture levels were excessive (70%^) around the rafters that were affected by the roof leak.
However, a consistent reading of around 20% was noted in all other areas.
This lead me to the conclusion that condensation was affecting the loft space, as originally it was a cold roof construction. Sarking membrane installed (probably around the 1960s-1970s), and loft insulated. Naturally, the ventilation would be provided by the gaps in the tiles and through the soffit.
Since insulation was provided throughout at the eaves lead me to this conclusion.
As a backup check, I measured the humidity levels using a humidity meter, which were measured around 70% Relative Humidity.
