Tiled Roof Underlay Problems

[erskine]

I have noticed this morning while in the loft that some of the plywood underlay on my roof has 'sprung' down from the joists. The felt and tiles above the boards are intact, but wonder how big a problem this is?

In the past I have secured these panels back into position by bracing them with timber supports secured between the joists.

 

[alan333]

My new-build house (year 2000) doesn't have any sarking/boarding, so ALL the underfelt is loose between trusses. You'll only have a problem if there's a lap joint where it sags IMHO. Lots of 1970's houses were built with fibreboard as sarking, and lots of them have bits with the fibreboatd sagged down.

 

[erskine]

Many thanks, Alan. That puts my mind at ease. I had visions of the roof coming in on the first strong wind.

BTW, what is a lap joint?

 

[alan333]

BTW, what is a lap joint?

The rolls of underlay felt won't run continuously along the roof, there will probably be laps each time a roll finished and they started another. If one of your saggy areas is under a felt lap joint then prop it back up as you did before, thus stopping the water from running into the loft.

Check that your rafters have some form of sway braces. These are strips of wood which were fitted, usually diagonally up the roof, and held the rafters in place until the roof boarding was put on. They shouldn't have been removed but strange things happen. If your boarding is sagging then these will hold the rafters in place such that they're not relying on the tile battens/counter battens.

The correct way to put your roof back to it's original is to strip off all the roof down to the rafters, fit new boarding which doesn't shrink, and put the roof back on. It'll need new felt, battens, probably tiles, and new valley gutters etc (if you have them).

I've no idea whether a structural survey would show the roof to be faulty with saggy boarding, but I see no reason why your roof would have any less integrity than mine. Unless mine was fitted with some fancy non-shrinking underfelt or something, which I'd think unlikely.

 

[erskine]

Many thanks for that. I will check the laps as you describe. I dont remember any being joined at the saggy boards, but did not have a good look this morning.

And I do know that there are some sway braces in place. I did notice that. That was the type of thing that I did to the boards previously. Simply fitting battens under the boards to press them back into place once more.