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Rafters carry the roof from the external walls to the ridge.
Rafters carry the roof from the external walls to the ridge.
The size of the rafters depends on the distance they need to span.
The size of the rafters depends on the distance they need to span.
Smaller rafters are cheaper, so your house was built with smaller rafter supported halfway along its length.
Smaller rafters are cheaper, so your house was built with smaller rafter supported halfway along its length.
That support is called a purlin and it spans across the width of the house, carried by the party walls.
That support is called a purlin and it spans across the width of the house, carried by the party walls.
When we convert your loft, the purlins are often removed and the rafters are usually replaced.
When we convert your loft, the purlins are often removed and the rafters are usually replaced.
Rafters to the front
Rafters to the front
When we convert the loft we may have to remove the purlin or else it would remain sticking out into the room, and this depends on the position of the wall to the front of the room -
- If the wall is further inwards than the purlin then the purlin is kept.
- If the wall is beyond the purlin then the purlin is removed and a new bigger rafter is usually needed to carry the weight of the roof.
The new thicker rafter sits against the existing thinner rafters, and is carried by the structure of the vertical wall.
Why is this important?
Why is this important?
So it is the span from this vertical wall to the ridge beam which sets how thick the new rafter needs to be, and the position of the wall sets the thickness of the new rafter.
If the design is tight, then it can have consequences for the position of doors and rooflights, and there is a maximum span depending on the angle of the roof.
Rafters to the rear
Rafters to the rear
Most of the rafters will be removed for the new dormer.
We don't need to put new rafters here, we just need to retain parts of the rafters which are already there -
- From the ridgeboard to the new dormer roof
- At the eaves where a margin of tiling needs to be retained
- To the sides in some circumstances -
- - No parapets
- - Building away from the boundary line to neighbours
- - The home is higher than the neighbour’s
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