St Margaret’s


Overcladding Solutions:

Façade refurbishment is always challenging as it presents unforeseen risk and the element of surprise for works that cannot be accounted for dully in the tendering process as the above project demonstrates.

This type of overcladding solution is bespoke to this project and the reasons for using it primarily driven by economy. There are other approaches and solutions of course, working with old buildings is challenging beyond belief. This project was about extending the service life of the building and where possible improving its energy efficiency. Despite the drawbacks the outcome visually is acceptable and the construction risks well managed. Active and early engagement with the specialist façade contractor on projects such as these will allow the architect the opportunity of carrying out a thorough preliminary appraisal of different solutions, ie rainscreen, composite cladding, curtain walling , window walling etc.

The challenges:

This single storey building is typical of a facade that has outlived its service life. A close up inspection of the modular bays reveals degradation of surfaces and finishes. Vandalism, incorrect levels in the external tarmac and a poorly executed plumbing system has led to multiple penetrations through the external building fabric. The outcome was the gradual degradation of internal timber framing members.

The brief called for its repair and replacement with a face sealed insulated cladding system with high performance windows.

The works were to be carried out whilst the building remained in occupation, the occupants were to be temporarily decanted in a structured phased basis.

Design Challenges and Solutions

Interface management was a critical part of the successful completion of this project. Coordination of different workpackages within the façade package with the daily routine of the occupant was critical to the successful outcome of the project.

An interesting feature of this was the external shutters. These were to be retained and upgraded, a challenge in itself. Normally these would be replaced but cost restrictions and security issues required these to be retained. The assembly is demonstrated left.

Retention of the shutters reduces the quality of natural daylight internally and the reconstruction of the façade had to make provision for their replacement in the future.

Because of the adhoc assembly and construction of the shutters, the shutter layout had to be well documented so that they could be reinstalled in the identical location after refurbishment, ie sandblasting,galvanising and ral painting.

Façade deconstruction of the external precast skin was the initial core task as it exposed the primary structure.

An initial site investigation revealed a discrepency in the perceived structure of the building fabric and as a consequence the installation methodology had to be revisited.

This proved benefical to all parties as active design engagement between the architect, specialist façade contractor and client lead to minimun disruption to the internal activities.

The existing vertical window framing was the actual primary structure and its removal would have had serious unforeseen implications to the support of the roof, its finishes, existing fittings and furniture. The horizontal timber framing acted both as a secondary brace/ties and provided support to the above mentioned elements

The decision was to retain these members and install a rebated casement window system into the primary structure.

Retention of the existing vertical timber beams and horizontal cross members meant that the requirements of temporary props to either side of each opening as works were carried out were no longer required. It also minimised the amount of making good internally by reducing the works to the installation and painting of teak liners.

All existing timbers were treated with preservative and in some instances replaced. Secondary bracing was installed within each bay with galvanised angles strengthening each joint.

Many of the base plates timber rails were replaced with a suitable weathered upstand to prevent the possibility of potential decay.

The thermal efficiency of the wall was improved by inserting 100mm phenolic insulation in between the timber framing members. These approaches lead to linear thermal bridging and as part of the overall solution the metal rebated trays were insulated. To receive the new windows the intermediate rails were removed forming an opening.

Post Contract thoughts

It’s always important to get the installation methodology correct. The deconstruction and reconstruction of the fabric was completed without disturbing the internal activities. Likewise the process could be done in sequence. Devising a flexible strategy is always part of how a specialist façade contractor approaches a project.

Examining critical adjoining interfaces is of paramount importance. The repair of the wall above the windows was much slower than anticipated because removal of the overhead parapet flashing would possibly impair the satisfactory performance of the roof.

The existing base detail was more challenging; the level of the external ground finish had to be altered to incorporate a weathered upstand.

The decision to work with the existing timbers did present some challenges in respect of installing the water proof marine ply.

The building structure was not plumb vertically or horizontally and all of the openings were of square. Shimming was required to ensure that the external sight lines and the sealant joint was perceived equally throughout the buildings length. It was important that this was achieved as the marine ply formed the solid grounds on to which the rebated trays were fixed to.

Light reflections externally in the finished product would highlight such discrepancies if sufficient care was not taken.

One of the difficulties with this type of insulation is the level of accuracy. Working with an existing building can create linear discrepancies when working to very tight fabrication tolerances. A survey is recommended especially when there is a possibility of creep.

The installation of the windows must be coordinated with the carpentry to ensure that the zone between the finished face of the new and the cladding and the position of the replacement window remains consistent throughout. Setting up a survey line/grid is therefore required to ensure quality.
Sitting the windows in between the existing framing members and overlapping the external marine ply in effect formed a rebated jamb and head detail making it easier to weather.
The base cill flashing projects over and extends beyond the proposed metal tray allowing the installer to slide the panel up behind the cill.

The efforts made in the restructuring of the wall resulted in a flat surface onto which the prefabricated welded trays could be installed. Each panel corner was welded and the perimeter fold was designed to add stiffness to these 3mm panels. The joint detail was maintained at 6mm but occasionally this increased nominally. The sealant joint concealed the fixing detail.

And what about the shutters, well the answers is simple they were reinstalled, upgraded and installed to the same pattern and level allowed the new top hung casement sashes to open and close without catching the shutter. The setting out of the window was dictated by the design of the existing shutters. Not the best approach as the occupant has to reach to open the internal windows.

Sashes at lower and higher level would give better natural ventilation to these one sided ventilated rooms.

The reinstallation of these girds reduced the daylight into the rooms but afforded the client security and achieving a very tight budget.


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