Structural Considerations for Your Loft Conversion

Loft conversions drastically alter your home's structure. You must plan carefully and follow Building Regulations to guarantee the safety and stability of both your new loft space and the rest of your house.

New Floors and Beams

Your loft's current ceiling joists are usually too weak to hold the weight of a living space, along with its furniture and people. This means you'll need to make some changes to support the new floor.

• You'll need to install new, stronger floor joists. These typically slot in between your home's original ceiling joists.

• Existing walls can sometimes support these new joists, but only if they are robust enough and extend right down to a proper foundation.

• If your existing walls aren't strong enough, you'll have to add extra support, like steel or timber beams. Remember, these new beams also need proper support themselves and must be fire-resistant.

Existing Walls and Foundations

When your loft conversion adds new weight to your home, pay close attention to:

• Load-bearing walls: If an existing wall will hold up your new floor joists, it must run continuously from the loft right through your house to its foundation, or have enough support in between.

• Existing openings: For areas like 'through lounges' or other wide openings, you must verify that any beams (steel or timber) already spanning these gaps can handle the additional weight from your new loft.

• Existing foundations: Normally, a loft conversion won't drastically increase the burden on your foundations. However, sometimes the added weight can be significant. You need to assess if your current foundations are up to the task. If not, 'underpinning' (strengthening the foundations) might be required to boost their load-bearing capacity. Always consult a structural engineer or your building control body for expert advice here.

Supporting the Roof

The new walls you build inside your loft conversion do more than just create the boundaries of your new room(s). They are crucial for supporting both your existing roof and any new roof structures, especially if you've removed some of the original roof supports. Often, this means constructing short walls near the eaves (the part of the roof that overhangs) to shorten the unsupported stretch (or 'span') of your existing rafters. Keep in mind that other internal walls might also carry weight, and some may need to be fire-resistant for safety.

Dormers and Openings

To add new rooms, dormers, windows, or rooflights, you'll typically need to cut openings into your existing roof structure. Here's what you need to know:

• Dormers: A dormer is generally a timber structure featuring its own roof, side walls (often called 'cheeks'), and a front wall. You can support the cheeks by simply doubling up and bolting together existing roof rafters. For wider dormers, however, their cheeks might need to extend down to doubled floor joists, a beam, or even a party wall or external wall for proper support.

• Dormer Walls: Dormer cheeks, especially those near a boundary line, might require fire-resistant construction. This helps prevent fire from spreading between your home and a neighbour's. The specific level of fire resistance needed will depend on how large the dormer is and how close it sits to the boundary.

• Removing Rafters: When you cut out a section of your roof for a window, rooflight, or dormer, you must support the remaining parts of the cut rafters. You achieve this by fitting new timbers, known as 'trimmers,' across the top ('head') or bottom ('sill') of the new opening. For larger openings, these trimmers might need to be 'double trimmers' – meaning two timbers bolted together – to effectively transfer the weight to the existing rafters on either side. It’s also wise to reinforce these adjacent rafters by bolting an additional rafter of the same size and length alongside them, as they will now bear extra weight.