Many people in England have felt flat roofs, for the simple reason that felt is a product that lasts a long time and it is still one of the most cost effective ways to protect your flat roof.
Main Problems We See:
20-30 years ago chipboard was used often by large developers instead of structural plywood for cost reasons. This should never have been used for the very simple reason, once it is wet, it looses its strength. This can lead to sink holes in your roof under your felt, making your roof not water tight and dangerous to walk on and inspect.
Many flat roofs we see have not been touched for 20 years or more and are in need of full replacement or have been covered over with aluminium paint or liquid tar by a cowboy builder who has charged the customer for a new felt roof, when that has not been the case.
What You Should Not Do
If your roof structure ie plywood or chipboard has lost its strength do not cover your roof with with aluminium paint (solar paint), liquid tar or another layer of felt. This will not change the fact that your roof is structural compromised and needs a completely renewing. Covering the problem will not make your roof watertight in the long run, and will eventually lead to a either a roof failure, or a major water leak.
What You Should Do
Replace your flat roof completely, if the roof is structurally compromised. This means stripping everything off back to the roof trusses to inspect the strength and condition of the existing roof structure.
Things to make sure your contractor does when renewing your flat roof:
Add Gradient and Strength
Most Important: Make sure you have a good gradient on your roof 4 inches which runs away from the house. A good gradient means that water moves quickly off your roof and ensures you will never have standing water. This creates the best conditions to extend the life of your flat roof. Standing water is bad for your felt as it causes damage to it when it freezes.
Cowboy roofers will not add gradient or extra strength, this because it is time consuming and it means much more work for them.
Add insulation to your roof whilst it is uncovered. This is important to make your house more efficient as 25% of your heat is lost through your roof. Your existing insulation will not be up to present day standards as building regulations have improved since your flat roof was last insulated, it may not even have any at all.
Again make sure this step is done correctly, if you can go up and inspect the work yourself, or ask to see photographs of the work. We have often found that this step will either be missed completely or done to a poor standard.
Add 18mm Structural Plywood
Make sure the roof is covered with thick structural plywood, and is screwed down securely, this will help strengthen the existing roof structure and ensure a long life for your flat roof. This material is designed for exterior purposes. Make sure thin plywood or chipboard is not used.
Add One Premium Layer of Underlay & One Premium Layer of Mineral Felt
What you want is a thick premium quality black underlay of rolled felt, and a thick premium quality mineral felt over the top, with a 4 inch overlap on each joint.
Make sure you see samples, cheap underlay is very thin and tears like paper, a premium product is 2-3 times thicker and much stronger; it feels rubbery and will have a sheen. Rolled mineral felt has replaced the gravel which used to be placed on top of plain tar roofs, mineral felt is a far superior finish to solar paint, aluminium paint or gravel and will outlast them all.
Life Span of a Properly Constructed Flat Roof
If your have a roof with a good gradient, with all the specifications mentioned above your flat roof should last 20-25 years or even longer.
More Advice: If you do not know your builder or contractor
Ask for samples and get in writing which felt product your builder will be using, and make sure that this matches the sample you have been shown. Ask to see the felt before it goes on your roof when it arrives on site.
We have found that in some cases customer have been shown a premium product and then on their roof a very thin underlay has been used which is then painted over with aluminium paint (dubbed as Solar Paint), this is a very poor way to do a flat roof we would not recommend using this method.
This method is done because it is very cheap and very quick for the person doing it, and most likely you will be charged for a new premium flat roof, which it is not. Make sure that your roof is not done like this unless you have asked and agreed upon these specifications.