Roof waterproofing is the system that stops rain and snowmelt from passing through the roof build up into the structure. Different materials and different sections are used depending on the roof type and on whether the surface stays exposed or is covered. A wrongly chosen system leaks within a few seasons.
Solutions by roof type
Flat roof to be covered
Primer is applied over the screed to falls, followed by bituminous membrane or liquid applied polyurethane. A separating layer, thermal insulation, protective screed and finish follow. In an inverted roof the insulation sits above the waterproofing and a non absorbent board is used.
Flat roof left exposed
UV resistance is the deciding factor. Mineral surfaced bituminous membrane or synthetic TPO and PVC membranes are preferred. Synthetic membranes are mechanically fastened or loose laid under ballast.
Pitched roof
An underlay membrane is laid beneath tile or metal covering. This layer drains wind driven rain and condensation that gets past the covering. A breathable membrane allows the timber structure to dry.
Metal roof
Problems usually appear not on large surfaces but at fixing holes and panel joints. Either detail level treatment with liquid applied systems or full surface coating is assessed.
Where leaks start
Outlet connection. The most common source of flat roof leaks. The outlet must be joined to the waterproofing with a flange and the membrane taken down inside it. A second stage drain is needed to release water trapped under the finish.
Parapet base and top. The waterproofing must run at least twenty centimetres up the parapet and be mechanically terminated at the top. The coping must fall towards the roof and carry a drip edge.
Chimney and vent penetrations. Every penetration is a risk. Penetrations are sleeved and sealed around with detail tape.
Expansion joints. Structural movement joints must continue through the waterproofing. A membrane run straight over a joint tears in the first season.
Laps. A thin, continuous bead of bitumen should appear at the edge of every lap. No bead means the weld is incomplete.
Order of work
Surface cleaning and repair, checking falls, primer, waterproofing layer, detail work, flood test and only then the upper layers. If the test is skipped, the problem surfaces after the finish is complete and the cost multiplies.
Frequently asked
Can new waterproofing go over the existing layer
Sometimes, but the existing layer must be checked for trapped water and adhesion. A new coat over a membrane holding water underneath will blister.
Can work be done in winter
Bituminous membrane becomes brittle at low temperature and liquid systems do not cure below dew point. A suitable weather window is needed, or heating measures.
What changes for a roof garden
A membrane certified as root resistant is mandatory. Drainage and filter layers are added and the load calculation is revisited.
If your roof is leaking, the source should be identified before any stripping begins. An on site inspection prevents unnecessary demolition.