Loft conversion floor joists

[jackboy29]

I am about to start a loft conversion in my 1950's semi and have a question about the new floor joists i will be putting in.

Currently there are 100mm ceiling joists and i will be running 200mm floor joists alongside. The property has brick built cavity walls. Presently the ceiling joists run over the internal leaf through to a wall plate on the external wall. I would like to sit my new joists on the wall plate also, but due to the increase in height, being 200mm instead of 100mm, these would need to be chamfered to fit under the pitch of the roof. I can ask my SE to check if ok but seeing as there is also an internal leaf to the cavity wall can i run the joist from the external wall plate and pack the internal wall up, say with mortar maybe?, to ensure the joist sits on both the wall plate on the external wall and also sits on the internal wall? This waould ensure the intergity of the joist is not too weakened by the chamfer.


thanks

 

[chickenlips]

That would depend on what pitch the roof is on how much a chamfer would be needed to be taken off joist.

The joist would want to shear off at the wallplate so the internal wall would really play any part in stopping that.

You can get Enginered Joists the can be manufactured with the correct angle to suit your roof. Have a look at open web joists or I beams if you haven't already purchased an items. Both types of joists will be able to span from wall plate to wall plate in 1 length, using the internal wall as a support (providing its load bearing)

Also it would be best to pack up the internal wall with a timber plate so the joists sit on that. Im guessing that wall is a loadbearing wall then ?

 

[tony1851]

In practice, you will find that there is no structural issue with chamfering the joist-ends to fit under the slope of the roof.
This doesn't matter whether your wall-plate is on the outer skin or the inner skin (generally the w.p.is on the inner skin).

Chamfering at the end does not affect the bending stress in the beam, nor does it affect the deflection. The only thing it does is to increase the shear stress at the end of the beam; BUT; shear stress is never an issue with domestic floor joists because the loads are so low.

Also, remember that because your joists are going right into the eaves, where headroom is nil, your floor will not be loaded to anywhere near the maximum required by regulations. I would guess that for 2 or 3 feet in from the eaves, there will be hardly any load on the floor. So chamfering will be OK.

 

[chickenlips]

I agree never usually an issue with a domestic floor.

Although a loft conversion can mean putting extra loads onto the floor close to the eaves. If the purlins supporting the roof are to supported through a structural stud wall built off the floor. Usually the will be close to the eaves where shear can become an issue.

 

[tony1851]

Although a loft conversion can mean putting extra loads onto the floor close to the eaves. If the purlins supporting the roof are to supported through a structural stud wall built off the floor.

Would a purlin be supported off the floor? I think the purlin is a beam which spans between the load-bearing walls and should impart no load to the floor.

Having said that, most people build a small stud wall directly under the purlin to close the eaves space off, and that wall IS supported off the floor. But its weight is quite low compared with the live load which is excluded from that part of the floor behind the wall.

 

[chickenlips]

If the conversion is an old roof where there have been horizontal collars supporting the purlins which then need to be removed. The purlins are then given support by building a structural stud wall off the floor.
I guess it depends on how it is designed and weather new purlins are specified.
I have done alot like this, seems to be common practice up here to leave existing purlins in and support them off floors.

The problem is loft conversions are generally done on PD and dont require full drawings, so inevitably scenarios such as the one I describe happen often. (well for me anyways)

 

[jackboy29]

Thanks for the replies. I emailed over a drawing to the SE and he said the same as has been said on here. The beams are as large as they are to prevent deflection and chamfering the end is ok as the force needed to sheer off the end is much greater than the actual weight going onto the joists. He sent me a simple calculation in case the BCO questions it but basically it is still well within acceptable limits.

Cheers