I understand that felt roofing done properly - three layers, including a good mineral topping (light-coloured stone chippings) - has a lifespan of up to 25 years.

I see some firms are offering a 40-year guarantee for glass fibre flat roofing.

The obvious advice is to get some quotations and guarantees for the work and decide which is the best value in your case. As a guide, it seems fibre glass is roughly twice the cost, per square metre, of felt.

Points like how long you intend to stay at the property will come into the equation.

Hello scot, Is it vital that your shed roof is installed to a habitable standard? If not, then there are several cost effective options you can consider. To have it felted with hot bitumen or heat applied felt is one option. This means removing the existing felt. Both types of heat applied felts go straight down onto the roof boards, which need to be in good condition. If the boards have suffered from water ingress then they need replacing. All this is a cost for only a 'shed'.

Alternatively you can cover the existing roof material. I put steel profiled sheets over mine, with an edge trim. They screw down into the roof boards or joists with self sealing screw heads. This cost me about £180 for a 3M x 2.5M shed. I had considered laying new felt onto the existing using cold bitumen but the cost was marginally more and the job would have taken longer and would have been very messy. Been on since last July and no problems at all and not expecting any. It is my workshop and tool store so I like it dry. Consider this option.

Cheers concrete