Old Felt Flat Roof Problem

Old Felt Flat Roof Problem

Old Felt Flat Roof Problem

Hi Steve,

As discussed on the phone, we are looking to have our flat roof (extension at the back of the house) insulated and the roof covering replaced, as we’ve had a leak into our son’s bedroom below – probably where the drain enters the roof. A builder said the roof covering should be below the black bit around the bottom of the drain, not above.

The roof is roughly 2.25m x 6.35m. I’ve just realised that it will be dark by the time I get home tonight, so I’ll take pictures tomorrow morning.

We have been considering high-performance mineral felt or EPDM as a roof covering. What would you suggest?

As I said I looked at the Energy Saving Trust website which says a flat roof should preferably be insulated from above, by installing a layer of rigid insulation board on top of the timber roof surface with a new weatherproof layer on top of the insulation.

Thanks for discussing the options for the roof in detail. could you please clarify a few things:

Would the first (overlay) quote include any insulation? Would the liquid system in itself provide some insulation against the cold?

We are not sure whether the flat roof is insulated at the minute; that room (which is our 4-year-old’s bedroom) gets quite cold in the winter. Would the problem with the low window sills occur with any type of insulation, or are there ways around it? Will we need some kind of insulation? Would the first (overlay) quote include any insulation? Would the liquid system in itself provide some insulation against the cold?

No, and No

If it has any insulation, its nothing like the quality that we use these days. 120mm of solid insulation laid continuously over a flat roof forming a warm roof is many times more effective at keeping in the warmth. It’s not just the depth and quality of the insulation; it’s how it’s installed.

Attention to details, stopping thermal bridging and the movement of air accounts for a large percentage of the gains. It’s interesting the feedback we get from our customers; they don’t necessarily say its warmer in the winter they say its cooler in the summer, in fact, it’s like having air conditioning.

This is because in the winter the central heating keeps the room at the correct temperature, so you don’t always notice how the insulation work unless you look at the heating bills or the heating never kept up with keeping the room warm. In the summer the insulation keeps the radiant heat out and because the room was never air conditioned before people tend to notice its a lot cooler.

To get to the correct building standards we need to install insulation to a depth of 120mm. Building control also dictates that window cills or door cills should be at least 150mm above the finished level of the flat roof. Like I say in the video this is not always possible. Part 1Lb of the building regulations Conservation of fuel and power also states that where it too expensive to meet the above criteria and the cost of doing so outweighs the fuel saving over a ten year period building control will except variations to any regulations.

On a small project like yours, nobody is interested as its all too small so depending on the paperwork trail you want depends on how you go about dealing with this problem. Common sense kicks in we contact building control, set out the problem, do the best we can.

In your case, I need more info, measurement and possibly to see the problem to comment, but basically, the options are.

  • Move cill up.
  • Use extremely expensive thermally broken thin insulation panels (very expensive better off moving the cill).
  • Use thinner insulation.
  • Don’t have a 150mm upstand and go for slightly smaller such as 100mm.
  • Put in a water channel below the cill.
  • A combination of all the above.

Hopefully, that helps.

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