Roofconsult Website

Flat Roofing The Options by David Roy

Home
Contact Us
Calculations
Industry News
Articles
Standards
Search
Check out our web directory of the UK roofing and cladding industry www.roofinfo.co.uk

Sign up for our monthly news letter.
Name
E-mail

Re-Roofing
The design of re-roofing specifications follows the same principles as new work, but you have added complications arising from the need to adapt, reuse or overlay existing materials, or to strip and replace the existing roof specification.
First if all, you will need to do an inspection to determine the limiting factors which will influence the design and selection of materials.
Your inspection must take into account the condition of the supports, the roof deck, the insulation and the waterproofing.
If you find the existing insulation is moisture sensitive, it is likely that the parts near to a source of leakage are wet or degraded and you will need to replace them. Treading the area concerned may reveal soft patches which will require attention. Otherwise take moisture readings, or make test cuts near the source of the leakage, or at random over the roof.
Electronic, infra-red and other proprietary methods are also available to survey large roof areas. They give an indication of water content by temperature variations or by radiation methods. This work must be carried out by trained specialists who will submit a detailed report on the possibility of water in the insulation.
When considering the design of the specification, you must understand the reason for the failure of the original. If old age is the reason, the re-roofing will be straightforward, as there will be little to change. If the roof has suffered premature failure, the re-roofing specification must take account of the factors involved.
Having established the condition of the existing roofing specification, you can then consider a number of alternative approaches to roof renewal:
A Single layer torch-on waterproofing applied over existing waterproofing
The single layer torch-on system fully bonds a new thick surface layer onto the existing waterproofing. The old waterproofing will act as a backing to the new and provide a measure of lap security. Make sure the old roof covering is basically sound, rot free, not saturated with water, and the insulation or deck below is in a generally dry condition.
A new waterproofing system applied over the existing waterproofing
You may re-roof old waterproof coverings with the direct application of the full range of built-up roofing materials, but you must ensure the existing insulation is sound and effective, and the surface of the existing waterproofing is firm, clean, and sound. You must remove chippings and blisters to leave a satisfactory surface.
New insulation and waterproofing applied over the existing waterproofing.
You can re-roof old roofs with confidence using an overlay of new insulation, even if the old roof has suffered major movement or fatigue failure. The only criteria is that the existing insulation must be in sound condition.
There is usually enough room for a thin layer of insulation to be added, but check the height of skirtings, position of dampcourse, height of kerbs and thresholds to make sure that the thickness of the new insulation does not rob the skirtings of essential height. If the existing roof has poor falls and drainage with ponded areas, consider introducing tapered insulation or crickets so that the new roof can be well drained.
You will need to completely strip and relay the existing insulation if
  • it is saturated or degraded
  • it is necessary to inspect the roof deck
  • the structure will not support the weight of an overlay in addition to the original materials
  • the height of upstands limits the depth of extra covering, or
  • the attachment of existing materials is faulty
The need for you to strip and replace the structure of a roof will arise only because of the severe deterioration or inadequacy of the roof deck. The design of the insulation and waterproofing specification is as for a new roof.
Specifications
Use a fully bonded specification on all insulation boards other than rigid urethane and phenolic. Fully bonded specifications normally comprise two layers of high performance material, bonded in hot bitumen.
Use a partially bonded specification on rigid urethane and phenolic insulations or direct to concrete, cement screeds, plywood and particleboard. Part bonded specifications normally comprise three layers of material. The first layer BS 747 type 3G, or similar vented base layer, followed by two layers of high performance material.
Materials
Polyester felts were originally made with oxidised bitumen coatings, but the majority are now made with modified bitumen coatings. The addition of a polymer to the bitumen improves its properties as a roofing material in almost all respects, and in particular the flexibility, strength and fatigue resistance.
The most commonly used modifying additives are SBS (styrene butadiene styrene) and APP (atactic polypropylene).
Overall I would say that Flat Roofing, properly designed will last well in excess of 20 years and give you exceptionally good service.
Home > Articles