Rotten joist in garage roof - what to do !!?!

[Peebs]

Hi guys

This is my first post, so be kind

I have a brick-built flat roofed detached garage in my back garden. Its about 18ft by 18ft. Its roof is made up from 9 by 2 inch rafters slung across the gap, with boards nailed on top, and felted. On the outside, there is a fascia board (plywood) nailed all around the edge of the roofing. The actual roof was re-felted about 2 years ago by my house's previous owner.

I noticed the other day that the paint on the fascia board had started peeling. Further inspection revealed that it was rotten through. Even worse, when I removed it the joist it was directly nailed to is rotten in the middle at the bottom for about 6 feet of its length at about 2-2.5 inches in height.

So what is best? Do I have to remove the joist completely, or will cutting out the worst (this 6 foot by 2 inch by 2 inch section) and replacing it be enough? I have some wood hardener, wet rot treatment & preserver. Is this adequate to treat the surrounding area if I do not completely replace the joist ?

We plan to convert the garage into a games room in the coming months. With this in mind, I have thought of getting a local roofing contractor to look at the roof. However, I expect them to say that I have to replace it so they could get some work out of it

I suppose that a portion of the roof would be have to be stripped back so they could replace the joist. Does anyone have any idea of this cost? I am in Nottingham.

Thanks in advance

 

[DAZB]

If you only have a small section of rot like you say ( 2-2.5 inch from a 9 inch timber) then it is not really going to have a detrimental effect on the strength of the roof especially since it is never going to have regular foot traffic on top .I would probably buy an 8 ft X 4 ft sheet of 18mm exterior plywood and have it cut into 9 inch strips and then just sandwich the joist both sides with a full run of the ply making sure you cover the six foot rotten section with a full piece of the ply (don't put a joint over the rotten part) and screw the whole lot all along the length with 2 - 2 1/2 inch screws. Of course the original problem that caused the rot will hopefully have been solved with the new roof fitting so the rotten joist will dry out and you can chop out the rot and splice a piece in for it to have a solid fixing point if you intend to board the ceiling later for your games room. Alternatively you can add a new joist alongside the rotten timber by chopping out the bricks alongside the rotten joist and set another new joist next to it and bolt them together along the length.You may have to remove some fascia boards on the outside too in order to slot the joist in and then back to the right position but they will refix. I myself would go with the first option because it will definately do the trick and be easier to do.

 

[Peebs]

Thanks! My builder is coming round tomorrow to survey the building for the games room conversion, so I'll have a word with him about this then..