A mouldy ceiling is caused by excessive moisture generation in the house and a humid roof space. The checklist on the previous page identifies things you can do inside, but if roof tiles are too porous, you have several choices. To sark the roof and install new tiles can cost thousands. Coatings for roof tiles aren't cheap, have a limited life and only cover the exposed part of the tile.

The alternative used here is to install the low-profile Envirofan RF7 (about $399). This is a plug-in, 24V unit controlled by a $40 rechargeable timer to circulate air each day while switching off at night. You'll need a power point installed in the roof space, but then you can fit the fan yourself. The fan moves nine cubic metres of air per minute, operates at 63db and is available in a range of popular roof colours.

Installing an Envirofan roof fan
Only get up on a roof if it is dry and rain isn't pending. Wet roofs are treacherous. Make sure you wear a securely anchored safety harness before you start work.

Here's how
Step 1 Remove a tile where the fan is to be installed, ideally fairly high up the roof but at least 2 tiles down from the ridge. To slip out a tile, slightly lift those above and overlapping at the sides and lift slightly so the lugs at the top can clear the batten. Also remove 2 tiles above the fan position.

Step 2 Where narrow tiles are used, adjacent tiles may need to be trimmed slightly so the fan can fit. This can be done by nibbling carefully with a pair of tile nippers or pliers. Fit fan with the straps pointing down into the opening.

Step 3 Push the upper flashing under the edges of the adjacent tiles. Bend the top of the flashing up a little to help protect against wind-driven rain blowing over the top in a violent storm.

Step 4 Reinsert the tiles above. You will need to partly lift the tiles above to get the lower tiles in.

Step 5 Dress the flashing down to match the tile profile, and, if necessary, cut to length at bottom so rainwater will run onto the tiles below.

Step 6 Back inside the roof space, screw straps to the battens, then plug the transformer, timer and fan into the power outlet.