Water penetrating 9”wall into lean-to loft

[michael]

In a similar fashion to Keithj, Keith b and others, I too appear to have rain water coming through a 9” brick wall, in my case into the loft space of a felted and slated ‘lean-to roof’.
The loft of this lean-to is about 10m long from end to end and at its highest point against the lean-to wall there is sufficient height within the loft to kneel upright.
My problem is the roof rafters which sit partially within the wall appear to be absorbing water from the wall during heavy rain fall. The top 2 or 3 feet of the rafters become completely saturated resulting in water dripping onto the loft insulation and in some cases running down to the eaves. You can imagine all the patches of brown water staining that have developed on the ceilings of the rooms below.
I had a professional roofer come round to look at the problem and having examined the roof from inside and out and checking all the lead flashing he could not see how the water was getting in. In his opinion, the roof, the lead flashing and the wall all appeared to be in perfect condition bar the water saturation of the rafters.
The other side of the wall is exposed to the elements but again, it is all in very good condition. There are 12 courses of brickwork above the highest part of the roof and these courses were re-built about three years ago using NHL 3.5 hydraulic lime.The rafters and roof were not disturbed during this refurb but the lead flashing was was reset into the wall. The top of the wall is all capped with those rounded off capping bricks (not sure of the technical term) and all the lime pointing is sound and solid.
Does anybody have any ideas on what the problem might be?

 

[Penners]

I'm only able to state the obvious, I'm afraid. If it's the top ends of the rafters that are sodden, then I can't see any other explanation than one of the following:-

1. Water is getting through/around the flashing and running down the wall on to the rafters.

2. Water is getting through the wall from the other side.

3. Water is getting into the centre of the wall and soaking down.

In view of your and your roofer's failure to identify the explanation, I would tend to favour 3. above, because it's the most difficult to spot. You state that the flashing was re-made when the upper courses of the wall were recently re-laid - are you confident that the flashing was turned vertically upwards inside the wall, behind the outer course of bricks? If it wasn't, then it's not impossible that water is creeping between the lead and the layer of mortar on top of it, then seeping down inside the wall.

 

[michael]

There are two layers of lead flashing. The first layer is mortared into the brickwork one course above the slate roof, folded down flat to the wall and flat onto the roof extending over the slates by about 4”. The second layer of flashing is mortared into the wall in the course above the first layer of flashing. This is tightly folded down to overlap the vertical face of the first layer of flashing. I don’t think either of these flashings extend more than 1inch or so into the depth of the wall but the mortar holding the flashings all looks very sound. I don’t believe this is the problem as the wall inside the loft even up under the flashing feels cold but relatively dry

I think however you may have a point Penners with regard to both your options 2 & 3. I suspect that rain water driven by wind is saturating the lime joints on the outer face of the wall and these joints are effectively acting as a wick sucking up the water particularly on the other side which is more exposed. The outer face on the other side of the wall did on inspection, look rather wet after rainfall but later dried out when the rain stopped. Because the wooden rafters are perhaps half way into the 9” wall, I wonder if they too are acting like a wick and soaking up the water from the lime on the outer and center parts of the wall?

I’m considering getting a surveyor to look at the problem but I’m a bit reluctant to fork out £300 for his time only to be told what might be the obvious or that there is no easy solution. I’m sure this very wet and windy February has a lot to do with it. I am concerned however that this regular saturation of the timber could eventually lead to wood rot and a weakened roof.

 

[Penners]

Your theory about water penetration from the opposite side of the wall sounds like a distinct possibility. And the deeper that the rafter ends penetrate into that wall, the more likely that could be. The cut ends of the rafters would be end-grain, angled upwards - an ideal attitude for soaking up water.

However, I still have a niggling concern about that flashing. If the top edge only penetrates an inch or so into the mortar joint, this is not best practice. It should ideally penetrate the full depth of the brick and then kick vertically upwards behind the brick.

 

[88v8]

And depending on the slope, 4" is not necessarily a sufficient lap onto the roof.

Ivor (who is still not sure how water is getting down his chimney)

 

[michael]

If the flashings are at fault then wouldn’t I find water running down the wall or at least damp brickwork between the rafters? It only seems to be the rafters that are getting wet.

One other question Penners. I’m no builder, so I respect your obvious expertise. I was interested in your comments regarding the depth that the flashing should be in the wall.
If as you say, it should ideally penetrate the full depth of the brick and then kick vertically upwards behind the brick I would be interested to know how one would go about removing and reinstalling new flashing should it need replacing in the future without seriously disturbing the wall?

 

[Penners]

The flippant answer, Michael, is "with difficulty". However, that doesn't change the principles upon which flashing should be laid, I'm afraid. When you insert it horizontally between the two courses of bricks, you are potentially introducing the risk of water hitting the slightly projecting surface of the lead and seeping inwards into the joint. Hence the recommendation that the lead should be dressed upwards behind the brick before it is cut off.

However, the difficuly of replacing the flashing is certainly not anything that should worry you in your lifetime! Properly laid and dressed flashing should outlive you (and most certainly me!).

 

[MatthewC]

It's a tricky problem. A few questions:

Of what sort of bricks is the wall made?
Was there such a problem before the wall was re-built?
What is the pointing between the rounded coping bricks at the top? NHL3.5 also? And are these bricks the same type or are they engineering bricks?

Matthew

 

[michael]

Thanks for your interest Matthew. I’ve contacted the builder who did the rebuilding for me (a builder that only uses lime) and he’s going to come and have a look. I’ll try and provide a few more details and answers to your questions in the next few days.

 

[plasticpigeon]

That would mean that the flashing would have to be built in wouldn't it? And also the bricks would not be held together by mortar directly, but be completely separated by a layer of lead, a bit like a lead damp proof course, which is no problem I suppose, but near impossible to replace. I've not seen that before? Also that approach would be impossible with a stepped flashing.

 

[Keithj]

I would be surprised (but not ever so) if water were penetrating a sound 9 inch wall. We solved a 50 year old damp problem in the church last year. Our builder (bless him) stood outside in pouring rain and watched to see what was happening. He saw where the water was going in (nowhere near where it was appearing on the inside). Fixing the problem was easy after that.

Water will find its own level. It will appear elsewhere from where it enters. Look around!

Our Welsh hideaway, years ago, suffered from terrible damp, which a builder spent much of his time and my money failing to cure. Then a builder moved in next door (friendly chap - this was North Wales) and said "You must have damp problems, with that roof like that". I was suspicious at first, but he explained: we were half way up a mountainside; the wind blows up the mountain so sometimes rain goes upwards and under the slates. The fix was lead and more overhang on the slates. Cost me £200 and fixed it completely.

Look, and get an expert to look, too.

 

[DJH]

That would mean that the flashing would have to be built in wouldn't it? And also the bricks would not be held together by mortar directly, but be completely separated by a layer of lead, a bit like a lead damp proof course, which is no problem I suppose, but near impossible to replace. I've not seen that before? Also that approach would be impossible with a stepped flashing.

It is possible to build onto lead sheet if it's coated with two coats of a hard bitumen paint and the top coat sprinkled with sand while its's tacky. The paint protects the lead from corrosion which can be experienced in sustained damp conditions.

Prefabricated individual lead cavity trays, which have fold over ends to maintain their integrity when keeping out wet/damp, can be built into masonry by removing about 18" of it at a time, fitting the tray, rebuilding using judiciously placed pieces of slate in the joints to hold it up while the mortar cures thus allowing the next section to be removed safely without disturbing the new work.

Stepped lead cavity trays are easy enough for a proficient leadworker to fabricate and fit using the same methods.

Doug

 

[michael]

I had the builder come round and look at the problem today. As he reminded me the rafters do not sit that far into the wall and are in fact getting most of their support from a 150mm x 50mm treated timber lintel that is bolted flat to the supporting wall directly under the rafters. This is an installation that must have been undertaken by previous owners as a precaution to support a new slate roof incase the old rafter ends were rotting in the wall.

The builder also pointed out that all the lime mortar and brickwork around the rafters is relatively dry. The other issue he pointed out is just by looking at the colour of the brickwork, the wall above the roof, on the roof side of the wall looks a lot damper than the opposite side of the wall. I always thought that the opposite side of the wall was more exposed but that side is east facing whilst the lean –to roof is on the west side where most of the rain and 80mph winds have been driving in from. Despite some protection from the main building, the winds do swirl around on this side of the wall.

The builder also advised me that when the upper part of the wall was rebuilt the lead flashing was reinstated in the wall and laid flat onto the slates following exactly the same line as the original. From memory he thinks the flashing overlaps the slates by about 125mm.

In conclusion and in agreement with most of you experts out there, he too suspects that despite the sound appearance of the flashing, with the high winds we’ve been experiencing, the rain may be finding its way under the flashing and that where the slates stand slightly proud of the wall, the water may be seeping under the flashing and over the edge of the slates onto the rafters. He suggests that we need to get up there, lift some of the flashing off the slates and see if the slates are wet underneath. We should do this when a few dry days are predicted so that if the slates are wet under the lead they can be left exposed for a day or so to dry out. He then suggests applying some of that grey lead sealant between the lead and the slate.

Against both of our better judgments we 'could' also apply a water seal to the brickwork above the roof on the wind blasted western side but this would seem to be going against the whole theory of lime.

Unfortunately, I think I’m going to have to get the scaffolders back in at great expense.

In the meantime a good number of my rafters remain quite sodden. Dose anyone think a dehumidifier would help? If so what size would I need? Loft is about 10m long by about 1.5m high (against the wall). The loft space is now quite well ventilated with air bricks in the supporting wall and roof vents in the slates so I suspect a dehumidifier would just be sucking moisture out of the outside air rather than the timbers. Those timbers that have become quite wet have small white beads of growth on them?

And finally, as promised, MathewC’s questions addressed :-
1.The bricks are red bricks , handmade, early 19th century
2.Difficult to say if the problem existed before the wall was rebuilt. There was always a bad condensation problem in that loft space with water dripping of both the rafters and the roofing felt. I hope that my recent measures to eradicate the condensation have worked i.e. the installation of three air bricks in the lean-to supporting wall, but because the winter has been relatively mild, the condensation has been less obvious i.e. no dripping from the roofing felt. Perhaps some of the condensation experienced in the past was also the leakage I’m now experiencing.
3.The coping bricks are similar to the other bricks in the wall i.e. with the exception of being rounded off, they too appear to be a relatively soft brick and not a hard engineering brick and the mortar used to rebuild the coping was 3.5 NHL like the rest of the wall and not 5.0. These bricks are however original to the wall. Twelve courses plus the coping seems to me an awful long way for the rain to seep down to saturate the rafters?

It’s still a possibility that the water is seeping in through the lime and brickwork but I guess I need to check out the flashing option first.

Can’t write any more. Too many gin &tonics down the hatch. Many thanks’ for your interest and help everybody. Any further comments most welcome, particularly any advice on dehumidification.

 

[Penners]

the rafters do not sit that far into the wall and are in fact getting most of their support from a 150mm x 50mm treated timber lintel that is bolted flat to the supporting wall directly under the rafters.....
....the wall above the roof, on the roof side of the wall looks a lot damper than the opposite side of the wall. I always thought that the opposite side of the wall was more exposed but that side is east facing whilst the lean –to roof is on the west side where most of the rain and 80mph winds have been driving in from.

The above details highlight the fact that, in order to get the best possible advice on problems from members of this forum, one needs to give them the fullest, most accurate information possible. Otherwise they will always be thrashing around in the dark, and may be pursuing possibilities that a bit more detail would have ruled out.

I don't intend this as criticism, Michael, just as a general observation.

As to your idea of a dehumidifier, speaking for myself I would not use one. Firstly for the reason that you have mentioned - the amount of air circulation in that area would mean that you were trying to dehumidify the county - and secondly because I think it may not be good for the timber to have its high moisture content sucked out too quickly. This can cause warping and cracking. I suggest that it's much better to eliminate the source of the damp and then let the wood dry naturally.

One more thought - if the wall into which the flashing is bedded is west-facing, this reinforces my view that driving rain may be soaking in along the top of the flashing, as well as under it. While you are applying your sealant, I would be inclined to run a bead of suitably coloured exterior sealant along the bottom edge of the line of mortar that sits on top of the flashing, too.

 

[MatthewC]

He suggests that we need to get up there, lift some of the flashing off the slates and see if the slates are wet underneath. We should do this when a few dry days are predicted so that if the slates are wet under the lead they can be left exposed for a day or so to dry out.

Do I see a small problem in this proposal?

The fact that the roof side of the wall is west facing sounds pretty relevant - the weather has been truly dreadful and anything south or west facing is bound to be suffering, so I do think that you are getting nearer to the cause of your problem. Another thought - what is the angle of the slate roof?

 

[Penners]

That's a good point Matthew. Generally, the minimum recommended pitch for a slate roof is 20 degrees. The lap is also an important factor. This increases as the pitch angle reduces - see this chart of recommended laps for the given pitch in an area of moderate weather exposure.

 

[michael]

Sorry Matthew. Been away and unable to address your question until now. I checked the pitch of the roof today. It seems to be about 32- 33 degrees to the horizontal. Just waiting for a week of dry weather to address the problem further.