The short answer
Built-up felt is a flat roof covering made from several bonded layers of bitumen-based sheet, usually a base layer and a cap sheet, laid over insulation and a deck. HomeBuilding gives felt a life of 10 to 15 years, and Less gives 10 to 20 years. A sound roof can be overlaid, and a wet or failed one has to be stripped.
How a built-up felt roof is made
The name describes the method. Instead of a single membrane, a built-up roof is assembled on site from layers that are bonded together, so that each layer covers the joints of the one beneath it. A flaw in one sheet is then backed up by another.
A typical system has three parts. At the bottom is the deck, which may be timber, concrete or profiled metal. Above it sits the insulation, if the roof is insulated, and a vapour control layer where one is needed. On top go the felt layers: a base layer or two that are fixed or bonded to the substrate, then a cap sheet that forms the exposed surface, often with a mineral finish to protect it from sunlight.
Torch-on felt is the best-known form. A gas torch softens the bitumen on the underside of the roll, and the roofer presses it onto the layer below. Other systems use cold adhesive or mechanical fixing for areas where flame is not suitable. Hot work needs strict precautions, since a torch near combustible material is a fire risk, and a firm that uses one should say how it manages that.
Details count as much as the open field of the roof. Upstands, edges, outlets and pipe penetrations are the usual leak points, so they get extra layers and careful dressing. Our guide to single-ply roofing compares the alternative approach.
How long felt lasts
Felt is at the shorter end of the flat roofing range, and two published sources give slightly different figures. HomeBuilding (Sep 2025) puts torch-on felt at "10-15 years", and Less (updated October 2026) gives "10 to 20 years". The sources overlap but do not agree, so both are worth knowing.
Both are typical spans, never promises. A felt roof laid well, on a sound deck with good drainage, can reach the top of the range or beyond. One laid in poor conditions, or left with ponding water and blocked outlets, can fail in less time.
The things that shorten a felt roof's life are familiar. Standing water puts constant load on the joints. Sunlight and heat age the bitumen, and foot traffic from maintenance workers wears the surface. Movement of the roof at joints and edges can split the felt, especially where it was fixed without allowance for expansion.
For a commercial owner, the practical lesson is to inspect regularly. A look twice a year, and after storms, finds blisters, splits and blocked outlets while they are cheap to put right. Our guide to a commercial roof maintenance plan sets out what such a routine covers.
Falls, drainage and the standard
A flat roof still needs a slope, called a fall, to carry water to the outlets, and the fall is what keeps felt dry. BS 6229:2025 is the current code for this type of roof, covering flexible membranes laid over a continuous deck, built-up felt included, up to a 10 degree pitch. The 2018 edition was withdrawn in December 2025.
The standard keeps a minimum finished fall of 1:80 across the main roof and in internal gutters, and manufacturers advise designing for a steeper 1:40, because the structure settles and construction is never exact. A roof designed to 1:40 still drains when it ends up shallower.
Where the deck is flat or the fall is lost, water ponds. A pond still lying there well after the rain has stopped is a warning sign. It adds weight, speeds up the ageing of the felt and, over time, finds a path through.
Drainage detail is the other half. Outlets must be clear, large enough and set at the low points, and gutters need their own fall. When a felt roof is renewed, the roofer can often improve the falls with tapered insulation, which fixes a drainage fault as well as renewing the covering. The survey should check both.
Overlay or strip back
When a felt roof is failing, the choice is between laying a new layer over the old and stripping it back to the deck. Each has a place, and the survey decides.
An overlay suits a roof whose existing layers are sound and dry, and whose insulation and deck are in good order. A new layer or system goes over the top, which saves time and the cost of removing the old material. The extra weight has to be acceptable, and the old surface must be prepared properly.
Stripping is the answer when the roof is wet. Water trapped in the insulation or the old felt cannot be sealed in, because it will damage the deck and the new covering. A moisture survey, or cores cut from the roof, shows whether the insulation is saturated. Blistered, split or badly ponded areas also point to stripping.
Renewing a flat roof's waterproof layer counts as renovation under Part L, and the roof is then upgraded towards a U-value of 0.16, with 0.35 as the threshold, where that is feasible. That is a further reason to look at insulation when the covering is renewed.
You see the evidence and the reasoning, and receive a single written price. For the rest of our commercial work, start at the commercial roofing hub.
