If you live in a house built in the last few decades you will almost certainly have a roof constructed of trussed rafters. This has major implications if you intend having a loft conversion.
Above: Photo courtesy of Bryn Pinzgauer
Up until the 1960s it was the norm to construct a roof on site out of lengths of timber. This was the carpenters job to frame up the roof and would take typically a couple of days or more depending on the complexity. Then came along the roof truss. The roof truss utilises slender timber but fixed together in such a way to form a series of triangles so that the whole unit derives its strength from its geometry instead of each individual piece of wood acting alone. The trusses are put together in a factory and are delivered to site ready for placing on top of the walls. Felt and battens are then fixed over to receive the tiles. Even before the roof is tiled the incomplete house is now sheltered from the rain. This can be done in a day, with complex roofs (if properly designed) taking not much longer.
Apart form being quicker, roof trusses also free the designer from requiring internal loadbearing walls since roof trusses, because of their inherent strength, can span further and usually can span from outside wall to outside wall.
So if you want to take some internal walls out it is often easier with a house with trussed rafters but if you want a loft conversion, a traditional roof construction with a loadbearing internal wall if often easier. I say often but there are so many exceptions you must always ask an expert to check before making any alterations to your house such as taking walls out or altering them.
The attic space in a house with a framed roof is appealing to the eye and people often underestimate how complex it is to convert to a usable space. Conversely the lay person often thinks it is impossible to convert a roof constructed with roof trusses. Building Tectonics have designed many many roof conversions of all types and even though a framed roof is often easier we have derived techniques to overcome most structural problems inherent in a loft conversion. Removing any timbers from a roof has to be done with great care as you must assume every piece of wood is there for a reason. The open up the space to form a room in the roof you must convert the trussed rafter to a more traditional structure. This takes know-how.. The design process has to also consider where the new stairs are going to go, means of escape in fire, headroom, thermal insulation and many other things required by the building regulations. To find a good solution is pretty difficult and I have seen many howlers of a loft conversion where one or another factor has been overlooked ( we sometimes get asked to obtain retrospective building regulation approval where other have designed the conversion ).
Above: Photo courtesy of Bryn Pinzgauer
What makes a loft conversion impossible is lack of headroom. Other factors such as a timber framed house, roof trusses, hipped roofs and a small first floor landing usually make the project difficult but not usually impossible. As a guide, allow for about £18k for the loft conversion and £2k for the design and regulatory work. You do not usually need planning permission for a loft conversion as long as you are not extending or enlarging the roof space but I am sorry to make the usual caveat and say you have to check with you local planning authority to make sure.
I would also mention that roof trusses should be treated with respect and you often see them stacked on site in the most appalling way. The other problem that was quite common was the wood being attacked by mould which is much more serious when you have such thin members. These problems have now largely been overcome by increasing the timber sized a little and ensuring adequate ventilation to the roof space – oh yes that howling gale in your loft is essential so do not be tempted to block any eaves or ridge vent up. You have to rely on insulation material above your ceiling to keep your heat in and this is another major consideration in converting your loft space properly. We as designers do not advocate taking short cuts but not all builders understand or care enough and taht is why you must get your plans prepared by an experienced company and then find a builder who will follow them to completion.
Happy loft converting!