Within the last few days, we have been asked by a warehouse owner to consider over-roofing rather than the cost of stripping Asbestos and re-roofing his roof which we had originally recommended.

It has become common practice to “over sheet” existing roofs using a spacer system and metal or G.R.P profiled sheets. We have carried out hundreds of over-sheeting jobs over the years and in many instances, it can be a very suitable option, either where cost saving is paramount the priority or where access issues can be difficult.

A case in mind was where we were asked to replace an old asbestos roof over a very sensitive site where exam papers were being printed. Working internally, for obvious reasons, was to e avoided. Over-roofing was the obvious choice.

Another factory we over-clad, was where guided missiles were being manufactured and it would not have been appropriate for asbestos dust or debris to fall into their clean rooms! Apart from the security aspect being at the start of the Falklands war!

Fragile asbestos or fibre cement roofs: Over-roofing v Strip and Re-Roof

The request this week from our customer was very reasonable. Cost saving must be looked at for everyone but sometimes a short-term gain can cost more in the long run. The points I raised during our meeting were:

  • Over-cladding asbestos will be leaving a problem which will need to be addressed at some point in the future, even if during demolition. The cost of asbestos removal has escalated over the years and becoming more onerous.
  • Even with careful planning and execution, disruption cannot be totally avoided. Nobody is allowed below where we are working without using safety netting, and in some instances, an internal crash deck.
  • There are existing roof-lights. These are old and partially obscured. If they are left insitu and merely over-roofed with new G.R.P the light reduction will be significant. If they are to be replaced, this will involve opening the roof to the elements and negating some of the reasons for avoiding disruption.
  • Securing a spacer support system involves drilling through the asbestos sheet and this will create dust which will be trapped above the old roof and any false ceilings. This may be a concern in the future.
  • Gutter girths may be compromised and will need careful consideration to allow future maintenance. The use of a designed system such as Unifold can increase the gutter dimensions. Cheaper coatings are often short lived and will not address the problem.
  • Ensuring correct insulation is installed AND good building practice requires the use of a breather membrane and or a vapour barrier. We see many failures where this has been omitted to save cost and condensation occurs creating leaks through the original roof defects.
  • A structural survey is required to ensure the additional weight loadings are within building tolerance.

Advantages of a new roof:

  1. A new, factory insulated composite panel avoids possible problems through poor design or labour.  Condensation will be avoided.
  2. An increased asset value. This might have a significant impact if the property is part of a pension plan or likely to be sold or let in the future. Many surveying practices are advising clients to avoid buildings with asbestos.
  3. A bright, white lining panel with new insulated roof lights positioned where required and not following the existing layout will give an enhanced internal working environment.
  4. The new roof-lights will be non-fragile allowing the roof to be regarded as “safe” and fully compliant to HSG33.
  5. New insulated gutters can be installed or the existing lined with Unifold to reduce exposure and the girth increased. To avoid condensation, they can be insulated from below.

By careful planning, disruption can be kept to a minimum.

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