flat roof construction

[Fred]

Hi,

I have got a carport with a flat roof. One side of the carport is the brick wall of the house; the other side of the carport is a brick wall (a single brick thick). Between the two walls run wooden joists.

On the house side, the joists are pushed into holes in the brick work of the house. These holes have not been precision made; they are great big holes, so I presume there is nothing to stop the joists moving except the weight of the roof. I have tried to fill the holes with foam. Is this ok?

The flat roof is chipboard and rotting so I need to replace it. If I extended the roof at the same time, would it be best to slot the new joists into the house as has been done already, or would it be better to fit a wall plate or use joist hangers? It is possible that we would walk on the flat roof occasionally to clean windows, get to the gutter, etc so I would need something that would take a man's weight.

On the other side of the carport, the joists simply rest on top of the brick wall. So again, it seems that only the weight of the roof is holding it in place. My worry is that when the chipboard is removed, the joists will be free to move.

Should I fit something like this:

just screw some 4x2 as noggins between the joists?

On top of the joist there is a tapering piece of wood. A builder came to quote for the repair (though I haven't received his price yet) and he said it ought to be 2 inches high on the house side, reducing to zero on the other side, to give the roof a slight slope for drainage. I hadn't realised that before; I thought a flat roof was flat in all planes. The wood there at present is just half that: 1 inch tapering to zero.

What is the proper name for this tapering wood? I think he called them footings? Have I remembered that right?

Can these be bought off the shelf or do you make them yourself with a table saw, feeding the wood at the appropriate angle?

Whoever built the flat roof decided to make an "internal gutter". Rather than fit a gutter on the fascia, they notched an inch or two from the end of the joists to make a lower channel in the roof. I am sure this is responsible for the roof being as bad as it is. Replacing all the joists could be expensive. What is the best way to proceed? Let the new plywood just overhang the notch, or should I fill the notch for support? Would you just plane a plank to size and drop it in the notch, using 2-part filler or plastic packers to fill up any gaps?

How will I know if any joists need replacing? If they are crumbling (I think the end of one is), obviously it will have to go but is there any way to tell if rot has set into any of the others?

TIA

[Dave Plowman (News)]

IIRC, firring strips. Quicker at new build to set the joists level and add them. Within reason, the greater the fall of a 'flat' roof the better.

[Bob Minchin]

> Or just screw some 4x2 as noggins between the joists?

The builder is correct. The fall should be away from the house. Normally IIRC 1 in 80 minimum but I tend to use 1 in 40 to avoid any dips in the decking causing ponding.

Mounting joists into the wall was the norm but they should have been pointed in with mortar afterwards. These days a wall plate fixed in with chemical anchors is easier and better in my view. You can use hangers or just put the joists on top of the wall plate.

The tapering pieces are called firring strips and normally bought from the timber merchant or cut your own on a table saw. They are cut in pairs from standard timber sizes to your specified length. Don't expect B&Q to do this for you. Use a proper timber yard. You can nail these strips on top of your existing joists.

Internal gutter is bad news - no real need to fill in the notch. If you can push a small screwdriver into the timber with ease then it is on the way out.

Yes the joists will move when you take the old deck off but the new deck will hold them in place. Fitting straps or noggins is possibly OTT for a car port - up to you. Noggins can be useful to support joints in the decking.

The far ends should also be on a wall plate but adding one now could screw up your levels. Maybe fit noggins here to keep the joists in place and upright.

I think that covers all your questions?

Bob

[Fred]

Thanks I guess it is about 1:80 and he was recommending 1:40 like you.

Some of the holes were so big you could fit half a brick and some mortar in!

So the 18mm ply will be strong enough to overhang it unsupported for five or six inches?

About those joints: if I am lucky the joists will be 40 cm or 60 cm apart so the edges of the sheets will fall over a joist but if I am not that lucky and the sheets join over thin air, what then? Do you cut the sheet so that it finishes over a joist? I am guessing not, as it would be wasteful. Do you just support the join with some noggins, and if so how many noggins, is there a rule one noggin every x inches?

Or is it that the 18mm ply is so strong a little overhang is not a problem? I suppose the real danger of overhangs is see-sawing if you tread on one end when it's not screwed down?

Thanks, sorry there were so many ;)

[Man at B&Q]

Our flat roofs are all tongue and groove planking. I refurbed one and didn't waste too much.

Wickes do a good leaflet

that got me up to speed on how everything goes together.

MBQ

[Bob Minchin]

18mm Ply will be strong enough to support itself over 5-6"

All joints should ideally be supported by joists or noggins (if at right angles to joists) along their whole length. If the joints do not fall on a joist then cut the ply until they do. Unsupported joints are asking for premature failure of the new felt. A small amount of wastage on the ply is a small price to pay for properly supported substrate for the felt. It does not sound like the original carport was put up by anyone with experience but they should of put the joists in at 400/600 (16"/24") centres apart from an end one perhaps to suit required overall dimensions.

hth

Bob

[Fred]

Thanks. I have downloaded that now.

Which system did you use? I think torched-on felt is supposed to be the best but I think you've got to know what you are doing with that, so I would probably go down the bitumen adhesive route, or is that asking for trouble later?

[Fred]

Unsupported joints are asking

Don't these two statements contradict?

Don't worry, I think the joists are roughly 40cm apart and you are quite right, if they have to be trimmed to size, it will only involve the loss of a few inches, so it will not be as wasteful as I first thought. I will definitely cut all sheets so they finish over a joist and support all joins with noggins.

That's what I think too ;)

But all the houses have them, so they were put up by the originally builders when they built the estate.

One last question: having just found out how heavy 18mm ply is, what is the safest way to get it up and onto the roof? How many friends do I need: one at each corner: two on the ground and two on the roof?

Thanks again.

[Man at B&Q]

Large one over the garage & car port was done by professionals with torched-on.

I recently did the porch myself and used the Wickes two layer self adhesive system. No problems so far.

MBQ

[Dave Plowman (News)]

If a normal small porch - ie just about door sized - I'd have used zinc. Which would see you out. ;-) It's an easy material to work with if you have basic metal working skills.

[Man at B&Q]

It's a bit longer than that, nearly 5m as it also covers a window.

MBQ

[terry]

Some thoughts.

1)Why is the roof rotting. Water leaks or condensation from below? Maybe add a vapour barrier on top of joists before new roof sheeting. How much more to use T&G plywood? 2) Where will the run-off go. Hopefully not onto somebody else's property? 3) To avoid joists from moving as the chipboard is removed maybe lengths of board can be screwed to bottom of joists. 4) Some sort of unusual 'gutter' suggest some previous need to not have water run off edge of roof? 5) A two inch drop is not much? I built a small shed 6 by 8 foot shed attached to an existing fence which in turn was part of our deck patio, adjacent house, and used a two inch drop (about the thickness of a two by four) in both planes. Good luck with project. PS. Calculate snow load; if that occurs in your area. And if necessary get up there and remove it promptly

[Fred]

It is chipboard and water is leaking through the felt at the joins.

No, I think it was the builders not wanting to spend money on guttering. My plan is to fir guttering to the new roof. There's no obvious reason not to.

No and that's probably added to the problem as it has allowed water to pool.

It's occurred everywhere this year hasn't it, so yes I will consider that. What are the rules about how much snow weighs and what you need to support it?