GRP or EDPM over existing flat roof felt?

[JohnAbbott]

Short question: Can GRP or EDPM products be successfully applied onto the top of existing felt substrate?

Background: We have a relatively large 'L' shapped flat roof (48 square metres in total) as part of our house (converted garage one side, single story extension on the other).

Existing covering is felt that is approx 10 years old and has had minor patching (leaks) done at some point in the past and the roof does suffer from ponding.

Looking at products like polyroof185 I had understood that the old substrate would need to be completely removed with a new board base put in place.

I have, however been recommended to not go to the expense of replacing the whole roof (the substrate is 'sound') and instead have an EDPM layer applied over the top of the existing felt, with a number of mushrooms vents installed through the existing flet/boards in order to allow any existing trapped moisture to escape.

Can this overlay option really give a comparable life extension to the flat roof or should I push for the whole roof to be replaced? Obviously there is going to be a significant cost difference.

Thanks

 

[SurreyRoofingEstimator]

If the boarding below is ok over-roofing will be fine. dependant on the finish of your existing flat roof an EPDM such as Rhepanol or Rubberbond have compatible adhesives or you could mechanically fix the membrane, we are about to use Rhepanol with their Gripfix (like velcro) system to over-roof an existing felt roof and this is fully guaranteed by FDT (the manufacturers). www.singleply.co.uk

A GRP system such as EasyFit made by Specialised Roofing Systems (SRS) is also mechanically fixed so would be fine www.specialisedroofingsystems.co.uk

Hope this helps

 

[Xenon]

You can also do a felt overlay

 

[^woody^]

You can't fit it over existing felt and the manufacturer will not guarantee it unless you lay it as instructed onto the timber decking boards.

There may also be differential movement, interstitial condensation risk, lack of adhesion and bubbling of the existing felt which will affect the EPDM top layer and its life.

If there is a specific system which allows this, then that's fine, but you need to confirm the manufacturers fitting guide