The Self Build Self Help Site Keeping Water Out

Keeping the Roof Membrane Water Tight

Although it is the job of the roof slates / tiles and lead valleys to keep the roof watertight, it is worth ensuring that the roof membrane is 100% watertight. During the process of felt and battening, scuffs, mishaps and refixing the membrane may cause the membrane to be holed. Sometimes the rain will come through where the battens have been nailed to the trusses.

It is not likely that a broken slate in the future would coincide with a hole in the membrane - but it is not impossible. And the remedy is not hard, especially if there is a spell of rain before the roof is slated / tiled. After the rainfall, check the trusses for signs of damp and, if the floor is completed, this will easily show where water has dripped from the trusses - as in the pictures below.

Diagram of structure of soffit

The bracing piece and truss web, in the middle of the photo, show the water mark left by rain leaking through the membrane where it is nailed to the truss.

Photo os soffit

The telltale sign of water dripping onto the floor.

(Click on photos for a larger view.)


A chimney is one way, if not done properly, of allowing water to penetrate the roof and seep into the blockwork. There is quite a specific requirements for the use of a lead tray spanning the chimney to prevent water from percolating down through the blockwork/brickwork, and for soakers and flashing to prevent water from penetrating off the roof. This is well demonstrated in our "Hints and Tips" pages on Forming a chimney stack .


Its inherent in plumbing that there should be no leaks so it seems a bit strange to include plumbing in this section. But even the best plumber in the world can have a leak from a joint. Make sure the plumber pressure tests all pipes before he finishes the first fix.

Installing shower units and baths

These, especially a shower, are extremely wet environments (now there's a surprise). The problem is that, if water gets where it shouldn't, then it doesn't easily get out and woodwork gets saturated and will rot. The worst situation is where its a small leak and is not noticed.

For shower units, plasterboard has often been replaced with a marine plywood, which is far more waterproof. This is an expensive alternative but Knauf have developed 'Aquapanel' which is yet a better and cheaper solution than marine plywood. As well as being designed to be water resistant, it is better for tiling - marine plywood will often suck the water from tile cement and reduces effectiveness of the bond - and it will tend to expand and shrink.

If you do use plasterboard then a wash with a diluted PVA glue where the tiles are to go is one good belt and braces measure. Make sure that a waterproof grout is used.

The next consideration is sealing around the edge of the shower tray and the bath edge. Once the shower tray is in, get a good amount of silicone sealant down between the tray and the wall. Once the tiles are on the wall, seal again with silicone sealant.

And observe the instructions that come with the shower cubicle. Their directions on where to put the silicone sealant are designed to ensure that, if water does get into the cubicle frame, it can get back into the shower tray, and not stay trapped within the frame or is released onto the floor.